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Solar & Lunar Eclipses Terminology

This glossary will let you understand the terms used by eclipse chasers.

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Absolute Magnitude (symbol M)
  1. The brightness a star would have if it were at a distance of 10 parsecs in a void space, without interstellar absorption. The absolute magnitude is usually deduced from the visual magnitude, measured through a V filter, when it is written as Mv. If it is defined for another wavelength, it gets another index (U, B, etc). If the radiation on all wavelengths is included, it becomes absolute, bolometric magnitude, Mbol. The Sun has the absolute magnitude +4.8. Most of the stars have absolute magnitudes ranging between -9 (supergiants) and +19 (red dwarfs).
  2. The brightness a comet or asteroid would have if it were at a distance of 1 AU both from the Sun and the Earth and were completely illuminated by the Sun.
Accretion A process by which the mass of a body grows through matter accumulation under the form of either gas or small solid bodies, which collide with or adhere to the body. The bodies in the solar system formed through accretion; there are stars surrounded by accretion disks.
Albedo The fraction of the total light or other radiation which falls on a non-luminous body, such as a planet or a planetary formation, and which is reflected by it. Usually, the albedo is equal to the ratio between the light quantity reflected and the light quantity received. The albedo values range between 0.0 (0%), for a perfectly black area, which absorbs all incident light and 1.0 (100%) for a perfect reflector.
The planets or planetary satellites with dense atmospheres have greater albedos than those of transparent atmospheres or of no atmospheres. The albedo can vary from one surface point to another, so that a mean albedo is given for practical purposes. The natural surfaces reflect different light quantities in different directions and the albedo can be expressed in several ways, depending on the way in which the measurement was made: in one direction or on the average in all directions.
Altitude and Azimuth A way to express spherical coordinates. Normally in degrees.
Altitude is almost always from zero to ninety with zero at the horizon and ninety directly overhead.
Azimuth can be expressed in numerous ways. A common format is to use the compass points and degrees such as N30E for 30 degrees from the North towards the East.
Angular Size The size of something in sky is typically expressed in terms of degrees. You can think of the visible sky as 180 degrees. From the horizon to over head is ninety degrees.
For perspective, the angular sizes of the Moon and Sun are about one half of a degree.
Annular Eclipse A solar eclipse where the angular size of the Moon is smaller than the angular size of the Sun’s photosphere. The umbral shadow of the Moon does not touch the surface of the Earth, so that, at the middle of the eclipse, a ring or an "annulus" from the Sun’s disk remains visible throughout a narrow band on Earth. An annular eclipse can last for 12m 30s at the most.
Aperture The size of the light gathering lens or mirror. Typically expressed in millimeters or inches.
Aphelion The farthest point from the Sun’s center, on the elliptical orbit around it.
Apogee The farthest point from the Earth’s center on the elliptic orbit around it.
Apogee Eclipse An eclipse (of the Sun or Moon) which takes place when the Moon is at the apogee of its orbit. The solar apogee eclipses, when they are not only partial, are always annular. The maximum duration of an apogee solar eclipse is of 6h 15m (between the first and the fourth contact). The maximum duration of a lunar apogee eclipse, between the two exterior contacts of the Moon with the penumbra, is of 6h 18m (the maximum totality being of 1h 44m).
Apparent Diameter The linear dimension of a celestial body, such as it is seen by an observer, expressed in angle units; it is equivalent to the angular diameter.
Apparent Magnitude (symbol m) The luminosity of a celestial body as it is measured by an observer. Sirius, the brightest star, has an apparent magnitude of -1.46, while the weakest stars visible with the naked eye, in the most favorable observation conditions, have magnitudes of about +6.5. The stars of magnitude less than +23 are measured by professional observatories and those of magnitude less than +30 by HST (Hubble Spatial Telescope). If m has no index it means that there is a visual magnitude, if it has the bol index, there is a bolometric magnitude.
Apparent Position A position given by the coordinates calculated for a star, if it were seen from the Earth’s center, relative to the real equator and the real equinox, at a certain date. It includes the displacements from one heliocentric direction, given in a stellar catalogue, due to precession, nutation, aberration, proper motions, annual parallax and light gravitational deviation.
Ascending Node A point on the orbit in which a celestial body crosses the reference plane from south to north, such as the ecliptic’s plane or the celestial equator. The ascending node longitude is one of the elements of the Keplerian orbit of any celestial body.
Astronomical Unit (AU) A length unit, defined until recently as being equal to the average distance between the Earth and the Sun (or the large semiaxis of the terrestrial orbit). In the original definition, it was given by the third law of Kepler
n3a3 = k2 (m + mE)
where n is the average motion of the Earth, a the large semiaxis of the terrestrial orbit (in AU), m and mE are the masses of the Sun and Earth (in solar masses) and k the Gauss’s gravitational constant.
The astronomical unit is defined at present as the distance from the Sun of the particles without mass, which move on a circular orbit around the Sun, with the orbital period of one Gauss year, of 2π/k days of the ephemerides. The average distance between the Earth and the Sun is of 1.000000031 AU, where 1 AU = 149,597,870 km.
Baader Film Filter A commercial foil that can be used as a solar filter. Often used to create a filter for telescopes or binoculars.
Baily’s Beads The name given to the broken series of bright points at the very start or end of totality, when the Sun’s light crosses the valleys at the edge of the lunar limb.
The photosphere shining through the valleys along the edge of the Moon was described as a string of brightly lit beads by Francis Baily, an English astronomer during the early 1800s.
Barlow Lens A concave lens that elongates the focal length of an optical system.
Can be used in astrophotography as part of a "negative projection" system.
Beach A hotter and brighter spot in the solar chromosphere, visible in the Hα light and in the K line of calcium. The beach is the chromospheric equivalent of the facula on the photosphere, as it can be seen when an active region is near the limb. It is a remarkably strong magnetic field region.
Besselian Elements Polynomial values equating the movements of the Sun, Moon, and Earth against a flat plane for calculating eclipse specifics.
Butterfly diagram A graph on which the latitudes of the sunspots are presented depending on time. It shows how the spots migrate from high latitudes (30° - 40° north or south) to the equator (latitude of about 5°) during each solar cycle, according to Spörer’s law. The form of these distributions, when it is represented for both hemispheres, resembles the wings of a butterfly.
C1-C2-C3-C4 C1 - First Contact, Moon nibbles edge of Sun.
C2 - Second Contact, Moon covers Sun.
C3 - Third Contact, Moon done covering Sun.
C4 - Fourth Contact, Moon nibbling ends.
Cable Release A photographer’s tool for taking pictures without vibrating the camera on a tripod or rigid surface.
Calcium Line Filter A solar filter for viewing light centered around the 395 nanometer bandwidth, a deep violet color. As calcium looses energy near the surface of the photosphere it emits light at 393.3 nanometers and at 396.9 nanometers (the K and H lines). This light is at the near edge of the visual spectrum meaning that the views can be rather dark in the eyepiece.
Calcium line filters reveal details such as granulation and flares during the partial phases of an eclipse (and during normal clear days). They should not be used during totality.
Calm Prominence A long lasting solar prominence, which persists for several months and modifies only a little apparently. It has the shape of an arch, long of several hundred thousands kilometres, thick of several thousands kilometres and high of about 50,000 km. When seen projected against the solar disk it appears as a dark formation, called filament. The calm prominences are in the zone of the active regions situated towards the poles. Throughout the cycle of 11 years, part of them migrates gradually towards greater latitudes, forming a polar corona in the spot minimum. They rest migrates to the equator.
Calm Sun The Sun, in the neighbourhood of its activity minimum, during the cycle of 11 years, when the number of sunspots and active regions is the lowest. Then the activity is still present under the form of small luminous points in X radiation, prominences and some coronal formations.
Canon der Finsternisse The most famous catalogue of solar and lunar eclipses. Published in 1887 by Theodor von Oppolzer, the catalogue contains the elements of all solar and lunar eclipses between 1208 BC and 2161 AD.
CCD (Charge-Coupled Device) A device of coupling by charge; a silicon piece which contains a surface of many light sensitive diodes used to capture images. The photodiodes, arranged in a surface of lines and columns, charge when light falls on them. The charge quantity depends on the light quantity, which can be reported in time. These charges are displayed column by column, offering a signal analogous to the image on the surface, which is then converted under a digital form and stocked in the computer. Professional and amateur astronomers use the CCDs today on a large scale both, because they are much more sensitive to light than a photographic emulsion. The image can be almost immediately displayed at the end of the exposure, being ready for analysis.
Celestial Equator The big circle on the celestial sphere, situated in the terrestrial equator plane. Any point on the celestial equator is equally distanced from the north and south celestial poles. On the celestial equator the declination is zero.
Celestial Pole Any one of the two points relative to which the celestial sphere rotates every day. The celestial poles are, on the celestial sphere, right above the terrestrial geographical poles, and at 90° relative to the celestial equator. Because of precession, the celestial poles describe a circle around the ecliptic’s poles every 25,800 years.
Celestial Sphere An imaginary sphere, of an indefinite dimension, used as a basis to define the position coordinates of the celestial bodies. The sphere can have as center the Earth, the observer or any other point which plays the role of origin of a given system of coordinates. Seen from the Earth, the celestial sphere rotates against the axis that unites the celestial poles every 23h 56m 04s (the sidereal day), as a result of the Earth’s rotation. Two important circles on the celestial sphere are the celestial equator and the ecliptic. The angle between them is of about 23.4° and is known as the ecliptic obliquity. The celestial equator and the ecliptic intersect in the points of the spring and autumn equinoxes. The positions of the celestial poles and therefore of the celestial equator move gradually on the celestial sphere, due to a slow oscillation of the Earth in space, known as the precession.
Central Eclipse An eclipse during which the axis of the lunar shadow cone intersects the Earth’s surface (in the case of solar eclipses) or the axis of the terrestrial shadow cone intersects the lunar surface (in the case of lunar eclipses). The total and annular solar eclipses are usually central. They can also be not central; then, they are visible only from places situated at high latitudes.
Chromosphere A shell of the solar disk located just above the photosphere or brightest part of the Sun. The chromosphere is the transition from the photosphere to the corona. A region of the stellar atmosphere situated above its photosphere. The Sun’s chromosphere extends from the temperature minimum, of approximately 500 km above the photosphere basis, up to 9,000 km, where it meets the corona. Throughout the first 1,500 km the chromosphere is more or less continuous, but beyond that it breaks into indented spicules. The chromosphere temperature grows from 4,400 K at 500 km to almost 6,000 K at 1,000 - 2,000 km. A rapid growth of the coronal temperatures is registered at heights of about 2,500 km (the transition region), the exact height depending on the local magnetic field intensity. In the uppermost part of the chromosphere the density is the millionth part of its density at the basis. Immediately before or after a total solar eclipse, the chromosphere becomes visible either as a sickle or as a red diamond ring - due to the Hα emission - from which it also gets its name (meaning "colored sphere"). Besides the eclipses, it can be seen in Hα and in K line of calcium and, from space, in the ultraviolet emission lines. The presence of the chromosphere around the cold dwarfs is deduced from similar emissions.
Conical Shadow A shadow in the shape of a cone. The Sun being bigger than the Moon creates a conical shadow projected away from the Sun. The cone reduces in size and by the type it reaches Earth it is much smaller than the Moon.
Contact When the Moon appears to touch an edge of the Sun. First contact is the initial nibble of the Sun by the Moon. Second contact is the first diamond ring and the beginning of totality. Third contact is the second diamond ring and the end of totality. Fourth contact is when the last bit of the Moon is no longer seen against the Sun’s bright surface.
Corona A shell of the solar disk that extends deep in to the solar system. The corona (or crown) is the white/grayish glowing material seen around the eclipsed Sun. The corona is only directly visible during totality.
Coronal Condensation A part of the corona where the gas density and the temperature are higher than in its vicinity. The coronal condensations are visible on the solar limb, above the spots groups. The images in the X radiation and those supplied by the coronographs in white light bring forth such condensations consisting of structures under the form of nodes, which underline the corona magnetic field.
Coronal Hole A region of the solar corona of very small density, almost 100 times smaller than the active coronal regions. The coronal holes appear as voids (blanks) apparent on the X radiation images or at the limb, as an absence of the white light emission in the coronograph images. A large coronal hole is always present at each one of the Sun’s poles. The low latitude coronal holes appear a little before the solar minimum, growing in dimension for several months, merging even with one of the coronal holes at the poles. Other holes can contract or disappear. The magnetic field in the coronal holes has open magnetic field lines, which extend in space, along which the plasma flows to produce high-speed streams in the solar wind.
Coronal Line An emission line in the solar corona spectrum, provoked by the strongly ionised atoms. Ions like Fe 9+ (iron minus 9 electrons) and Fe 13+ (minus 13 electrons) give birth to the so called red or green coronal lines, at wavelengths of 637.5 and 530.3 nm, respectively. It was believed that these forbidden lines would be due to an unknown element, namely "coronium". The ultraviolet and X coronal lines are detectable in the stars also.
Coronal Loop A structure in the Sun’s corona, seen in images taken in the X or ultraviolet radiation, or in white light; it is shaped as an arch, sometimes with a height of up to 10,000 km, whose ends are beyond the photosphere. The big, thin coronal loops are associated with the calm corona (i.e. beyond the active regions), while the short, bright coronal loops appear in the active regions, making up coronal condensations or luminous points in the X radiation. The two ends of the loop are situated in photosphere regions of opposite magnetic polarity. This implies that the coronal loops are tubes of magnetic flux filled with hot plasma.
Coronal Mass Ejection (CME) A form of solar eruption that creates a severely distorted corona. A massive mass ejection from the solar corona at a speed of 10 - 1,000 km/s. The mass involved in such an event is of almost 1013 kg. The space coronographs show that a typical coronal mass ejection consists of a part of the gas that forms a loop or, rather, from a bulb in the corona, placed above a dark cavity. In the Hα light, an erupting prominence moves to the exterior in a dark cavity. An ejection produces a perturbation in the solar wind, preceded by a shock wave. The interplanetary probes that have met such perturbations have registered an increase of the solar wind speed and densities, as well as a rapid variation of the magnetic field. When these interplanetary perturbations meet the Earth, they create geomagnetic storms.
Coronal (Polar) Wedge A thin, radial formation in the white light solar corona, in the coronal holes from the Sun’s poles. The coronal wedges are best seen at the solar minimum. They can be also seen in the spectrograms of some ultraviolet lines, such as the line of strongly ionised magnesium at 36.8 nm.
Coronograph An instrument which permits the observation of the solar corona outside the solar eclipses. A French astronomer, Bernard Lyot, invented it in 1930. It is a kind of refractor telescope with the objective lens protected against the dust and the static charge, sometimes by means of a thin oil stratum. An occultation disk, placed in the first focus, makes an artificial eclipse. A lens situated right behind this disk makes an image of the objective on a diaphragm, through which passes the greatest part of the direct light coming from the objective. A third lens, situated behind the diaphragm, actually makes the image of the corona on the film or the detector. Only the observatories situated at high altitudes and with exceptional atmospheric conditions can use the coronographs with success. Even then, only the interior part of the E corona can be observed, although the K corona can be detected using the polarisation analysers. The space coronographs can observe the corona up to a distance of several solar radii, using electronic images instead of photography. LASCO (Large Angle Spectroscopic Coronograph) on the SOHO space probe, observes the corona on more than 30 solar radii.
Danjon Scale A scale to evaluate as exactly as possible the darkening degree of a total lunar eclipse, set up by André Danjon, a French astronomer. The 5 steps of the scale run from 0 (extremely dark, invisible Moon) to 4 (extremely bright, the eclipse having a very weak effect on the Moon’s visibility). The darkening at a lunar eclipse is determined to a great extent by the transparency of the terrestrial atmosphere, which is affected by clouds and the dust from the volcanic eruptions.
Declination (symbol δ) A coordinate on the celestial sphere, equivalent to the latitude on Earth. It is measured in grades north or south of the celestial equator, from 0° at the celestial equator up to +90° at the north celestial pole, -90° at the southern one, respectively. It is an equatorial coordinate.
Diamond Ring At 2nd and 3rd contacts when just a point or small sliver of the photosphere is visible it appears as a bright gem with the corona as the ring around the dark Moon. Hence the diamond ring effect. It is produced by the solar light, which passes through a valley of the lunar limb, causing the appearance of a bright point of a rare beauty.
Duration In eclipse chaser talk, duration almost always means the time between 2nd contact and 3rd contact. This is the totality time.
E Corona A part of the solar corona defined by the emission lines of the hot gases. These emission lines include the so-called forbidden lines of the strongly ionised atoms of iron, calcium and other elements. The E corona is thinner than the K and F coronas.
Eccentricity (symbol e) A measure of a conical orbit shape; it shows how much this deviates from a circle. If e = 0, the orbit is a circle. If e < 1, the orbit is an ellipse, if e > 1 it is a hyperbola, and if e = 1 it is a parabola. The orbits of the big planets and satellites are ellipses of small eccentricity. The eccentricity is one of the six elements that define a Keplerian orbit.
Eclipse The passage of the shadow of a celestial body over the surface of another.
Every year, the maximum number of solar and lunar visible eclipses is of seven. The minimum number is two, both being solar. The eclipses appear generally in pairs, a lunar eclipse being preceded or followed, in about two weeks, by a solar one at the opposite node of the lunar orbit.
The solar eclipses take place when the new Moon is close to a node of its orbit and on the same longitude with the Sun. At that moment either the shadow cone, or its extension, or only the penumbra surrounding them touches the surface of the Earth. For an observer situated in the shadow cone the eclipse is total, while for one placed in the cone extension it is annular.
The annular eclipses take place around the lunar apogee. An observer situated in the penumbra sees only a partial eclipse. A total or annular eclipse can be seen from a band with a width of 270 km at the most, around which, to the north and south, the much larger, partiality zone extends. The Moon’s shadow crosses the Earth from west to east with a speed of about 3,200 km/h. At a total eclipse the Sun’s disk is completely covered and the solar corona can be seen.
The total solar eclipses are rare in the same place on Earth, so that the astronomers usually travel great distances to see them. A solar eclipse can last up to 3 hours (between the first and the fourth contact). Totality has a theoretical maximum duration of 7m 31s, but it is usually much shorter.
A lunar eclipse can be seen from the Earth from any place where the Moon is above the horizon of that place; it takes place when the full Moon passes through the central, dark shadow of the Earth. The Earth’s shadow is much wider than the Moon that is why the lunar eclipses can last even four hours (between the first and the fourth contact). Totality can be of 1h 47m at the most.
Eclipse Chaser One who makes specific travel plans to be in the path of a total solar eclipse. A niche market for the travel industry often serviced by those who want to share the event with others as well as professional travel agencies.
Eclipse Coverage/Obscuration A measure of the Sun’s surface covered by the Moon.
Eclipse Glasses Solar filters fashioned into sometimes colorful eye glasses. Intended to be worn when looking at the partial phases of the eclipse.
Eclipse Magnitude The measure of an eclipse extension. At a solar eclipse it is the fraction from the solar disk diameter covered by the Moon. If the eclipse is total, the magnitude is replaced by the ratio between the apparent diameters of the Moon and Sun, which is of 1.00 or even more during the totality. The magnitude of a lunar eclipse is the fraction of the Moon’s diameter covered by the Earth’s shadow; at a total eclipse it can be greater than 1.00 (or 100%) because the Earth’s shadow is greater than the Moon. An eclipse magnitude is expressed either in decimal fractions or in percents; thus, at a partial eclipse it can be of 0.59 or 59%. It should not be mistaken for the eclipse coverage.
Ecliptic The path the Sun appears to follow in the sky relative to the constellations. The apparent trajectory of the Sun on the celestial vault throughout a year. The Sun’s movement along the ecliptic is actually the equivalent to the Earth’s motion on its orbit around the Sun. Thus, the ecliptic is actually the projection of the Earth’s orbit plane on the celestial sphere. Because of the inclination of the Earth’s rotation axis, the ecliptic is inclined with approx. 23.4° against the celestial equator, an angle known as the ecliptic obliquity. The ecliptic crosses the celestial equator at the equinoxes. Its name is due to the fact that the eclipses take place when the Moon is close to the ecliptic plane, meaning "disappearance" in Greek.
Edge Effects The shadows on the ground just before and after totality, when the Sun is over 50% obscured. When a tall shadow is cast on the ground, one side will appear sharp and distinct while the other side fuzzy and blurred.
Edge effects can also mean going to the edge of the path of totality in order to prolong the diamond ring at the expense of less totality time.
Emission Line A luminous part of the spectrum, marking a certain wavelength given by hot or excited atoms. The emission lines can appear superposed on a normal absorption spectrum, being produced by the hot gas around a star, or can appear by themselves in the spectrum of a nebula excited by the radiation from a hot star situated nearby. They permit the determination of the emittent gas composition.
Ephemerides Time (ET) A time scale which was used to calculate the orbits in the solar system, from 1960 until 1984. Its fundamental unit was the ephemerides second, which was defined so that the tropical year at the epoch 1900.0 should be of exactly 31,556,925.9747 seconds of ephemerides. The ephemerides time was inconvenient in many ways and in 1984 was replaced with the Terrestrial Dynamic Time (TDT), whose fundamental unit is the second SI.
Equatorial A type of telescope mount that tracks the movement of the stars as the Earth rotates.
Equinox One of the two points in which the annual apparent trajectory of the Sun (the ecliptic) intersects the celestial equator; or the data at which these things take place, namely around March, 21 (the spring equinox) and September, 23 (the autumn equinox). When the equinox type is not specified, then one has to do with the spring equinox (vernal). Around the equinoxes the day and the night are equal all over the globe. The equinox is not a fixed point, it moves due to precession and nutation. If only precession is taken into account, the point that results is known as the average equinox of the date. If nutation is also taken into account, then it is the real equinox.
Eruptive Prominence A solar prominence which was initially calm, but began to grow suddenly by several hundreds kilometres per second and could not longer be seen. Such prominences are often observed on the solar limb, in association with the coronal mass ejections. The equivalent phenomenon on the solar disk is also called eruptive filament or suddenly disappearance.
F Band The Fraunhofer line in the solar spectrum, situated at 486.1 nm and due to the absorption by hydrogen. It is also known under the name of the Hα line in the Balmer series.
F Corona The exterior part of the solar corona, illuminated by the solar light diffused or reflected by the solid dust particles. The same phenomenon also produces zodiacal light, much farther away from the Sun. The dust particles have several micrometers at the most and make up a disk stretched over almost 1 solar radius (700,000 km) from the Sun’s surface. Unlike the electrons, which are responsible for the K corona, the dust particles move relatively slowly. Thus, the light diffused by them has the same spectrum as the photosphere, its Fraunhofer lines (hence the name "F corona") included. The F corona is the most luminous part of the corona on 1.5 solar radii from the Sun’s surface.
Facula (plural: Faculae) Bright spots that form between solar granules as a result of magnetic field lines. A brighter and hotter spot on the solar photosphere, visible in white light and best seen near the solar limb, in contrast with its dark background. The faculae appear often immediately before the formation of a sunspots group and remain visible for several days or weeks after the disappearance of the spots. High latitude faculae (polar) also appear quite far away from sunspots, unlike the faculae of the sunspots, which are much more numerous at the beginning of a solar cycle. The faculae are somewhat hotter (with almost 300 K) than the surrounding photosphere. They are the premises of strong magnetic fields (0.01-0.05 tesla) and coincide with the luminous spots on the chromosphere and chromosphere network. Normally they can be resorbed in small luminous facular points, approx. 1,000 km wide and which last for about 20 minutes. There is often a connection between the faculae and the structure of the great sunspots, particularly the light bridges that are bridges of a luminous matter, which cross a sunspot.
Field of View A value expressed in degrees that describes a relative size for items viewed through telescopes, cameras, and binoculars.
Filament A long tongue of a relatively cold matter (10,000 K), suspended in the solar corona (2 millions K). The filaments seem dark in Hα light when they are seen projected on the solar disk; at the limb they look as what they actually are: prominences. The quiet filaments (the equivalent of the quiet prominences on the limb) can undergo gradual changes, but filament portions can move much faster, with speeds of several kilometres per second. The filament loops (a disk equivalent to the prominence loops) can sometimes be seen around the greatest flares. The equivalent of an eruptive prominence is the filament, which disappears, sometimes called also sudden disappearance.
First Contact The very start of the eclipse. When the Moon first appears to touch the surface of the Sun.
Flash Spectrum A valuable tool used to identify the elements that comprise the chromosphere and photosphere. Around the time of the diamond ring a flash spectrum is produced using a spectroscope or grated filter and a camera. Young observed it for the first time in Spain, at the solar eclipse in 1870.
Focal Distance (symbol F or f) The distance between a lens or a mirror and its focus. It is one of the main characteristics of a lens or mirror concave. The greater the focal distance, the higher the image scale is.
Fourth Contact The very end of the eclipse. When the Moon is no longer visible against the surface of the Sun.
Fraunhofer Lines Absorption lines and bands in the solar spectrum, observed for the first time in 1814 by Joseph von Fraunhofer, a German physicist and optician. In descending order from red they are A, a, B, C, D, E, b, F, G, H and K. The A and B bands are due to the absorption by the oxygen molecules in the terrestrial atmosphere and band a is due to the absorption by the terrestrial water vapours, but what remains results from the absorption in the Sun’s photosphere. The most important ones are lines D of sodium, lines H and K of the ionised calcium, the lines Hα, Hβ, Hγ of the hydrogen and band G, produced by the neutral iron and the CH molecules, all of them being characteristic of the stars of type F, G and K.
Gamma When analyzing a sequence of eclipses, as in a Saros series, Gamma is the smallest distance of the shadow axis relative to the Earth’s center. Gamma is greater than zero when the axis is north of the center, negative when south.
Grey Light The visibility of that part of the Moon not illuminated by the Sun. The solar light reflected by the Earth produces the phenomenon. It was explained correctly for the first time by Leonardo da Vinci.
H and K Lines Two Fraunhofer lines in the ultraviolet part of the spectrum, at 396.8 and 393.4 nm, respectively, due to the unique ionized calcium (Ca II). They are important for the spectra of the solar type and colder stars. The emission in the H and K lines is often found in the stars with a pronounced magnetic activity.
Hα or Hydrogen Alpha The most important line of hydrogen in the Balmer series. It has a wavelength of 656.3 nm, in the red part of the spectrum.
Hanle Effect A rotation of the spectral lines polarisation plane due to the magnetic field. It is used to measure the weak magnetic field of the solar prominences, which is of 10-3 tesla or over 10-2 tesla for the active prominences. The effect bears the name of the German physicist Wilhelm Hanle (1901 - 1993).
Heliopause A border of the heliosphere, where the solar wind pressure is in equilibrium with that of the interstellar gas. It is believed that it is situated at approximately 100 AU from the Sun.
HELIOS A series of German space probes, launched by NASA, to study the Sun and the interplanetary space. Helios 1, launched in December 1974, was placed in an orbit which took it to 45 millions km from the Sun at the perihelion, closer than any other preceding probe. Helios 2, launched in January, 1976, approached the Sun up to 43 millions kilometers.
Helioseismology The study of the solar interior through the observation of the oscillations at its surface. The solar oscillations take place both at a global and a local scale. The local oscillations become manifest through a small Doppler displacement in the Fraunhofer lines, in various zones of the solar disk. The waves period is on the average of 5 min, with maximum speeds of about 0.5 km/s. An oscillation model appears over several thousands kilometers and persists for 30 min. The oscillations are due to the superposition of several sonorous waves or p modes, which travel around the Sun between the surface and the less profound strata of its interior. The global oscillations are observed through the Doppler effect in the Fraunhofer lines from the Sun’s integral light. Such large scale oscillations are due to the p modulations, which travel from the Sun’s surface to the deepest parts of its interior; the oscillation period is of approximately 5 min, with a series of 4 - 8 min.
Heliosphere A region around the Sun, where the solar wind blows. It is considered that its ray is of 100 AU; it is bordered by the heliopause, outside which the interstellar gas exerts an equal pressure from the exterior. The shape of the heliosphere is unknown, but if there is a leakage of interstellar matter around it from a certain direction (an interstellar wind) the heliosphere should be like the terrestrial magnetosphere: spherical on one side, but with the shape of a long tail on the other side.
Heliostat A plane mirror on an equatorial mounting, driven to follow the Sun and reflect its light into a stable telescope. If the telescope is oriented parallel to the Earth axis, only the mirror is necessary. Nevertheless, the Sun orientation seen through the telescope will change throughout the day.
"Horizontal" Eclipse A lunar eclipse during which the Sun sets and the eclipsed Moon, which rises are above the observer’s horizon for a short time. The phenomenon is due to the atmospheric refraction, which makes the observed height of the bodies situated close to the horizon greater than the real one. At the horizon this motion reaches 36', which allows the "horizontal" eclipse to take place. Cleomede described the phenomenon at the beginning of our era.
Horizon Effects As the Moon’s shadow moves across the Earth it hits clouds and the atmosphere. Near the edge of the shadow the light from the Sun hits the clouds often before it hits the ground near you. This creates a form of sunset or sunrise look to the horizon. As the shadow is approaching the horizon grows dark and as it moves away the horizon lightens.
Herschel Wedge A prism that can be used in the construction of a white light telescope. Typically used as a diagonal replacement for refracting telescopes or in pairs. White light telescopes reduce the intensity of the light from the Sun by reflecting just a small amount of the light to the eyepiece. Solar details are much more distinct as a result.
Hershel wedges are often used as laser beam splitters.
Hybrid Eclipse A mixture of both total and annular along the eclipse path. These eclipse can be categorized in three classes:
  • A-T-A: At the beginning and end of the path the eclipse will be annular. Towards the center of the path the eclipse will appear total.
  • T-A: Total at the beginning of the path, and later the eclipse will be annular.
  • A-T: Annular at the beginning of the path, and later the eclipse will become total.
An eclipse is considered total if the shadow from the Moon extends beyond the center of the Earth in distance. Otherwise it is considered to be an annular eclipse. When the shadow just grazes the surface, but does not go past the center of the Earth then the event is considered a hybrid eclipse by eclipse chasers.
The term "hybrid" was first used to describe annular-total eclipses in van den Bergh’s seminal work on the periodicity and frequency of eclipses in 1954.
Hydrogen Alpha Filter Asolar filter that only allows the light of the color at 656 nanometers through. That color is the red seen in the chromosphere. Hydrogen alpha filters provide an enjoyable view of the Sun during the partial phases and under normal viewing conditions. They are not to be used during totality, a regular telescope or binoculars with out a filter work best during totality.
Intervalometer Photography tool for triggering exposures at predetermined times automatically.
Light Deflection The light deviation by the Sun’s gravitational field. Near the Sun’s limb it is of 1.75", radial from the Sun. This effect is taken into consideration at the reduction of the star positions from the middle position to the apparent one.
Limb Corrections Local eclipse circumstances are based on simplified geometry which produces results that are fairly accurate. However, the lunar limb profile is not a simple geometry. It is comprised of valleys and peaks. Valleys will shorten the time of totality while peaks will increase it. If you want to get the most accurate possible timing for the contacts, then the lunar limb corrections need to be applied. They are typically expressed as seconds to add or subtract from the calculated time.
Limb Profile The variable valleys and peaks along the edge of the Moon from the observers location.
Lunar Diameters A convenient, but not highly accurate, measuring method normally used to express the size of the corona in any given direction. A lunar diameter is roughly one half of a degree in angular size.
Lunar Eclipse The passage of the Moon through the Earth’s shadow cone, having as a result a darkening, which can be from a hardly detectable shadow during an eclipse through the penumbra up to a very dark phase, when the Moon enters entirely the Earth’s shadow. The lunar eclipses take place only at the full Moon phase, when the Moon is close to one of the nodes of its orbit around the Earth. When the Moon is somewhat to the north or south of the node, it does not cross the shadow centrally and consequently only a partial eclipse can take place. During the eclipses in the shadow, the Moon looks more or less dark, depending especially on the terrestrial atmosphere transparency. The Sun’s light refraction through the atmosphere on the eclipsed Moon produces sometimes a red colouring, while the darker eclipses may appear grey.
Coloured fringes can be seen around the shadow edge during the partial phases. A lunar eclipse is visible from the entire hemisphere where the Moon is above the horizon. The maximum duration of a lunar eclipse totality, when the Moon passes through the shadow centrally, is of 1h 47m.
Lunation A period of time between two identical Moon phases. It is the same as the synodic month.
Maunder Minimum The period between 1645 and 1715, when only a very few solar spot or aurora were observed. E.W. Maunder (and G.F.W. Spörer before him) concluded that it was a genuine decline in the Sun’s activity. This minimum can be emphasized also through the increased content of carbon 14 in the tree rings in that period, because the cosmic rays which produce carbon 14 reach the Earth in a greater number when there is a weak solar activity. A period of cooling of the terrestrial globe was also observed between 1550 and 1700, called the small ice age, which corresponds very well with the period including the Maunder minimum and the preceding minimum of Spörer. A drop of the solar production by 1% can explain this cold period.
Metonic Cycle A period of 19 years, after which the Moon’s phases repeat at approximately the same calendar data. The cycle was discovered by the Greek astronomer Meton, in the 5th century BC and is used in the construction of lunar-solar calendars. It is determined by the fact that 235 synodic months (lunations) take place in 19 tropical years.
Mylar Filter A solar filter made of the Mylar material.
Node An intersection of the ecliptic and the path of the Moon. The path of the Moon is inclined by five degrees relative to the ecliptic. One of the two points in which an orbit intersects the reference plane (or the fundamental plane), such as the ecliptic’s plane or the celestial equator’s plane. The line, which unites these two points, is called the nodes line. At the ascending node a body on the orbit moves from south to north against the reference plane; at the descending node it moves from north to south.
Objective Lens The light gathering lens of an optical system.
Occultation The passage of a celestial body in front of another one, usually the covering of a star by the Moon. Strictly speaking, a solar eclipse (when the Moon passes in front of the Sun) is a particular case of occultation.
Partial Eclipse Every solar eclipse contains a partial eclipse. That is when the Sun’s disk is partly obscured by the Moon. Partial eclipses are expressed in percentages of coverage. Any less than 100% is a partial eclipse.
Or a lunar eclipse in which the Moon does not enter the Earth’s shadow completely, so that a part of the Moon remains illuminated by the Sun.
Path In an eclipse chaser conversation, path means the track that the lunar shadow will make across the Earth. The path varies in width with the largest width being only about 100 miles. Being inside the path means that you will see a total solar eclipse.
Penumbra The outer part of a shadow when the light source is only partially obscured. The exterior region of the shadow left in space by a planet or satellite; an observer situated in the penumbra can witness a partial eclipse. The penumbra surrounds the narrower, dark shadow, in which the Sun is completely covered. The penumbra is darker the closer it is to the umbra. During a lunar eclipse, it passes through the penumbra of the terrestrial shadow before and after it passes through the umbra.
Penumbral Eclipse A lunar eclipse when the Moon passes easily to the north or south of the umbra and only through the less intense part of the exterior region (penumbra) of the Earth’s shadow, presenting, generally, only a negligible darkening. When the Earth’s atmosphere is loaded with dust (following volcanic eruptions, for example), the penumbra can be darker than usual.
Perigee A point on the circumterrestrial orbit, situated at the closest distance from the Earth center. When the Moon is closer to perigee it appears larger in the sky by a small amount resulting in longer totality durations.
Perigee Eclipse A solar or lunar eclipse which takes place when the Moon is at the perigee of its orbit. When they are not only partial, the solar perigee eclipses are total. The maximum duration of a solar perigee eclipse is of 5h 14m (between the first and the fourth contact). The maximum duration of a lunar perigee eclipse, between the two exterior contacts of the Moon with the penumbra, is of 5h 16m (the maximum totality summing 1h 40m).
Perihelion A point on the circumsolar orbit, situated at the closest distance from the Sun center. This results in shorter eclipses. Perihelion takes place in early January.
Photosphere The bright surface of the Sun, from which a great part of the energy is emitted under the form of visible and infra red radiation. The solar photosphere is a thin layer of approximately 500 km breadth. Its temperature decreases constantly from 6,400 K at its basis to 4,400 K at the temperature minimum, where it merges with the chromosphere situated above it. This temperature drop with the height is the cause of the darkening at the disk border. The photosphere has a texture similar to the rice seeds, called granulation, caused by the convection cells of the hot gas. Other characteristics of the photosphere are the sunspots, faculae and filigree structures, the legs of the magnetic tubes. Almost all the characteristics of the solar light visible spectrum originate in the photosphere, the dark Fraunhofer lines included.
Pin Hole A simple way to project the eclipse image. Use a piece of paper, put a small hole in it, and hold it so that it castes a shadow on the ground. Where the hole is located you should see an image of the partial eclipse.
Polar Brush A series of coronal spikes along the northern and southern poles of the Sun.
Polar Mount A type of telescope (or camera mount) that tracks the stars during the course of the night.
Precession An oscillation motion through which a body’s rotation axis describes a conical surface. The terrestrial axis undergoes a slow precession, due to the combined attraction of the Sun, Moon and planets. The terrestrial pole describes a complete circle on the celestial sphere in about 25,800 years; this circle has the radius of approximately 23.50°, i.e. the terrestrial axis inclination. The equinoxes describe a circuit of the ecliptic in the same time interval. Right ascension and stars declination change in time as a result of precession, so that the epoch at which these are given has to be specified.
Projection In eclipse chaser conversations a projection normally refers to a way of observing the Sun safely during the partial eclipse phases. The image of the Sun is projected onto a surface using some sort of optical device.
Prominence An eruption from the photosphere into the corona along the edge of the Sun. Appears the same color as the chromosphere, bright red. A formation with the aspect of a cloud, visible especially in the Hα light, situated in the solar corona, but colder and denser than the latter one. The prominences have temperatures of approx. 10,000 K, typical to the solar chromosphere and densities 100 times greater than the corona. They are often seen around the solar limb at total solar eclipses.
In the Hα light they can be seen projected against the Sun’s disk, being then called filaments. They are registered as calm prominences and active prominences, depending on their behaviour. The regions of active prominences have rapid motions and last only for several days, while the calm prominences last for at least a month (a solar rotation). Between the relatively cold matter of the prominence and the hot corona there is a transition region, where the temperatures vary between 15,000 and 1,000,000 K. Prominences follow closely the magnetic inversion line and they are maintained against the gravity by the magnetic fields.
Prominence - Loop Prominences that follow magnetic lines forming a loop as viewed from the Earth. The material is ejecting from one side and falling back on the other side of the loop.
Prominence - Spike A prominence that appears like a pole or spike.
PSE Partial Solar Eclipse
RA-Dec Right Ascension (symbol α) and Declination (symbol δ), polar coordinates in the sky.
Relative Number of Sunspots (symbol R) A certain measure of the number of spots on the Sun. It was introduced in 1848 by J.R. Wolf and it is also known as Wolf’s number or the relative number of Zurich sunspots. It is calculated taking into consideration both the total number of individual spots (f) and the number of spots groups (g), plus a k factor representing the observer and the telescope’s efficiency, R = k (10g = f). The k factor is equal to 1 even for Wolf’s observations in Zurich. The relative number of sunspots used today is based on the observations made at a network of 25 observation stations; the observatory in Locarno, Switzerland, acts as a reference station to keep the continuity of the Wolf numbers. The values are published by Sunspot Index Data center in Brussels.
Right Ascension (symbol α) A coordinate on the celestial vault, equivalent to the longitude on Earth. It is measured on the celestial equator in hours, minutes and seconds of sidereal time (sometimes even in grads) beginning with 0h at the spring equinox, counter-clockwise. Line Oh of the right ascension is the celestial equivalent of the Greenwich meridian. An hour of right ascension is equivalent to 15°, so that 24h of right ascension are equivalent to 360°.
It is an equatorial coordinate.
Saros Cycle Approximately every 18 years, 11 days, and 8 hours circumstances for an eclipse repeat (or 18 years, 10 days and 8 hours, depending on the number of bisect years). This cycle was used in ancient times to predict lunar and solar eclipses. Given an observed eclipse one can calculate into the future when others will occur.
Equivalent to 223 lunations, following which the Sun, Moon and Earth come back almost to the same alignment. The solar eclipses return after each Saros, but in different geometrical places. A Saros period is almost the same thing as 19 years of eclipses; the small difference makes the eclipses in a Saros series take place gradually more to the north or to the south, until, after approximately 70 eclipses (1,262 years), they do no longer reach the Earth’s surface.
Season of Eclipses The period when the Sun is close enough to one of the lunar orbit nodes so that an eclipse can take place. It lasts for 37 days for the solar eclipses and for almost 24 days for the lunar ones. These intervals repeat every 173.31 days. Two seasons of eclipses make up a year of eclipses.
Second Contact The very start of totality. When the Moon fully covers the surface of the Sun.
The second contact at a total solar eclipse can be immediately preceded by the appearance of Baily’s Beads, and by the diamond ring. At an annular eclipse, the second contact refers to the moment when the limb of the Moon leaves the limb of the Sun, so that the entire lunar disk is reflected on the Sun.
Shadow Darkness cone, in the direction opposite to the Sun, coming from a planet or a satellite, in which the Sun’s disk is completely hidden. The passage of a body through this shadow has as result its eclipsing (such as when the Moon crosses the Earth’s shadow). The dark shadow is surrounded by a penumbra, which is much larger and in which the solar disk is only partially hidden.
During the time of the partial eclipse nearer to totality, shadows will take on a strange appearance. Pinholes will look like little images of the eclipsed Sun and the edges of long shadows will appear uneven.
Shadow Bands Fast moving crescents caused by atmospheric scintillation that appear in the moments before and after totality. An optical phenomenon seen around the beginning of totality, at a solar eclipse, consisting of the slow motion of the weak contrast waves, of light and darkness, only a few centimetres wide, seen on the surfaces of coloured light on the ground. It is assumed that they are due to the differential refraction of what is left from the solar light in the cooling, upper atmosphere, around the total eclipse.
SOHO (Solar and Heliospheric Observatory) Solar observatory in space. ESA-NASA space mission, launched in December 1995, to observe the Sun in the visible and ultraviolet spectrum and to study the solar wind and measure the small oscillations on the Sun’s surface. It is on an orbit in the vicinity of the Lagrangian interior point (L1) between the Sun and the Earth, at about 1.5 millions kilometers from the Earth, in the direction towards the Sun, so that the Sun can be observed continuously, even outside the eclipses seen from Earth.
Solar Activity Collective name for all active phenomena on the Sun, sunspots, faculae, active regions, beaches, active prominences and flares included. The solar activity is closely associated with the magnetic fields, which seem to originate in the action of the solar dynamo. The solar activity increases and decreases within a cycle that lasts for approximately 11 years.
Solar Cycle The variation in the number of sunspots and of other forms of solar activity with a period of about 11 years. In each successive cycle the north and south magnetic polarities reverse, so that there is a magnetic cycle of 22 years. The lunar averages of the relative number of sunspots are about 6 in a year of solar minimum and of 116 at the solar maximum. There are variations in the number of spots, active regions and flares, as well as in the level of the various emissions (such as the ultraviolet or radio spectrums) associated with such regions. It is believed that the periodicity of 11 years is due to the solar dynamo action. The sunspots cycle may not have always acted like today, such as the existence of the Maunder minimum suggests.
Solar Eclipse The passage of the Moon over the Sun’s disk. The solar eclipse takes place only at the new Moon, when the Moon is close to one of its nodes around the Earth. They do not take place very month, because the new Moon is usually either to the north of south of the node, due to the Moon’s orbit inclination. The total solar eclipses are rare in one place on Earth because the Moon’s shadow falls only on a limited surface. The theoretical maximum duration of totality is of 7m 31s, but usually it is not longer than 3-4 minutes.
Around the totality a series of interesting phenomena take place, such as the shadow bands or Baily’s Beads. During the totality the solar prominences and corona can be seen. The solar eclipses, which take place around the lunar apogee, can be annular eclipses. A partial eclipse is visible from each side of the ground band, from where a total or an annular eclipse is seen. During the partial eclipses the light becomes hardly perceptible only if more then 90% of the Sun’s disk is covered.
Solar Filter A filter for your eyes, camera, or a telescope that will block out the harmful radiation of the Sun and only allow a very small amount of light through.
Solar Flare Sudden energy growth in the solar corona, which can last for several hours. The flares emit radiation in the entire spectrum, from gamma rays to radio waves. They also send out high speed particles (electrons, protons and atomic nuclei) at speeds of approximately 70% of the light speed, which reach the Earth in about 15 minutes, depending on their trajectory. The most energetic ones, when enter in the chromosphere produce flares and are visible in white light. The solar flares take place in active regions, with complex magnetic fields, the biggest ones being in the most complex regions. The greatest part of the energy is liberated in the first minutes, the impulsive stage, which can last for several seconds. The total energy liberated can reach up to 1027 J; there is not a well-defined low energy limit, 1021 J being the smallest ones.
The solar flares are classified in two ways: by their appearance in the Hα light and by the emission of soft X radiations. In Hα the name of subflares is given to the small events, for which the scale runs between 1 and 4 with the area growing. A luminosity code is also associated with them, which goes from weak (f), via normal (n), to bright (b). In the soft X radiations (0.1 - 0.8 nm) they are classified as C, M or X, according to the increased intensity, with subdivisions from 1 to 9.
In Hα the flare may began with the disappearance of a filament (namely of a prominence projection on the solar disk), luminous zones developing in bands on both sides of the magnetic inversion line. There a rapid development stage takes place, also called the flash phase. There are often tough X radiations and radio explosions in microwaves, forming a stage in precocious impulses, with soft X radiations, which grow gradually to the maximum several minutes later and are followed by a decline (the cooling phase). They take their energy from the energy stocked in the magnetic fields, although the exact mechanism is not very well known yet.
Solar Maximum A time period from a solar cycle, when the solar spots and other activity manifestations are most frequent.
Solar Minimum A time period when the solar spots and other types of solar activity are less frequent, although not necessarily absent.
Solar Telescope A special telescope made to study the Sun. Lenses or mirrors with very great focal distances are used, so that as many details of the Sun as possible can be observed. Many great solar telescopes, such as McMath-Pierce Solar Telescope, are situated at great heights, to reduce the atmospheric turbulence and avoid the cloud coverage. Even better conditions are obtained at the observatories surrounded by waters, such as Big Bear Observatory in California.
Spectroheliogram A picture of the Sun, taken in the light of a strong Fraunhofer line (or a part of the line). The most commonly chosen for spectroheliograms are the lines Hα and calcium K, which emphasise the chromospherical characteristics. A spectroheliogram is produced by means of a spectroheliograph; if filters are used to select the wavelength, then the result is a filtergram.
Spectroheliograph An instrument to photograph the Sun in the wavelengths of a strong Fraunhofer line. An image of the Sun produced by a telescope is focussed through a primary slit and the light from this passes through a grate or prism. The dispersed light, formed in this way, is then intercepted by a second slit, placed at the desired wavelength (for instance, a part of an Hα line) and the photographic plate is placed behind this slit. The primary slit is then shifted over the Sun’s image and the secondary slit is displaced to maintain its spectral positioning, producing a spectroheliogram on the photographic plate.
Spectrohelioscope An instrument with which the Sun can be observed visually at the wavelength of a certain spectral line. The principle is very much similar to that of the spectroheliograph, except for the slits (primary and secondary), which are shifted rapidly forward and backward over the solar image, so rapidly that the eye cannot see the movement.
Spicules The smaller spikes of the chromosphere. Eared jets in the superior chromosphere of the Sun, visible especially in the Hα light. The spicules evolve towards the corona exterior with speeds of almost 30 km/s, reaching altitudes of 9,000 km in about 90s. They last for almost 15 min and end their lives by the ionisation in the corona. They tend to form groups, which create the impression of hair tufts on the solar limb. Seen on the disk, in Hα, they look like a very fine granulation; they are luminous on the limb and bright on the disk, when observed in the red light part of the Hα line). The spicules form in the chromospheric network. At the center of the solar disk groups of spicules make up formations with the shape of rays, called rosettes.
Spörer’s Law The displacement of the solar spots’ average latitude to the solar equator during the 11 years solar cycle. The spots of a new cycle are generally situated at latitudes of 30 - 40° north or south, but during a cycle the average latitude drops to 50 - 10°, as the butterfly diagram shows. The law bears the name of G.F. Spörer, although it was set forth for the first time by R.C. Carrington.
Spörer Minimum A period between 1450 and 1550, when the Sun’s activity seems to have been unusually weak, according to the historical records of the solar spots and auroras observations and to the measurement of carbon 14 in the tree rings. The Spörer minimum and the similar, Maunder minimum, coincide with the period of the lower temperatures on Earth, known as the small ice age. It was discovered by the American John Allan Eddy (born 1931), who named it after G.F.W. Spörer.
Sublunar Point A point on the Earth where the Moon is exactly at the zenith, at a given moment.
Subsolar Point A point on the Earth where the Sun is exactly at the zenith, at a given moment.
Sun Spot A form of solar event that appears darker against the photosphere. Sun spots are commonly associated with prominences when near the edge of the Sun. A dark area on the solar photosphere, colder than the zone around it, associated with very strong magnetic fields (0.4 tesla). The spots generally appear in pairs or groups, the first and the latter ones having opposite magnetic polarities. The spot dimensions vary from small pores, of 300 km diameter, to groups stretching well over 100,000 km. The great spots usually last the most, namely over 3 months; some small pores can last for less than an hour. The spots are usually in the active centers, on both sides of the equator, between 40 and 50° northern and southern latitudes, appearing at greater latitudes at the beginning of a solar cycle and then moving towards the Sun’s equator, as the cycle evolves.
The well-developed spots have a darker interior - umbra, of almost 1,600 K colder than the photosphere and a more luminous exterior part - penumbra, which represents over 70% of the spot area and is with almost 500 K colder than the photosphere. All sunspots begin their existence as small, dark pores and can later develop into small spots without penumbra, disposed in pairs. In a developing group the spots become much greater and more separate in the first two days, reaching the maximum area and complexity in about ten days.
Large categories of spots groups can be defined. In the McIntosh scheme (which replaced the old Zurich scheme), a code of three letters describes the class of spots groups (single, pair, complex), the development of the penumbra at the great spot and the group compactness. The Mount Wilson scheme is used to describe the magnetic field structure, which can be simple (bipolar or unipolar, if there is a single spot) or complex. The spots, which give birth to flares, tend to be very complex in appearance and magnetic field structure.
Streamers In eclipse chaser descriptions of the corona the term streamers refers to the elongated sections typically seen along the equatorial regions.
Synodic Month Equivalent to a lunation, the time in which the Moon passes through a cycle of its phases; it has 29.53059 days.
Syzygy An alignment of three or more celestial bodies.
Terminator The separation line between the illuminated and the dark parts of a planet or satellite.
Terrestrial Dynamic Time (TDT) A name used in the beginning for the terrestrial time. It was introduced in 1984. In 1991 the word "dynamic" was eliminated; the other two scales are absolutely equivalent, except for the name.
Terrestrial Time (TT) A time scale used to calculate the precise geocentric positions of the bodies in the solar system. It is a continuation of the ephemerides time. TT was introduced in 1984, under the name of terrestrial dynamic time (TDT), but it was renamed in 1991. The fundamental unit of TT is the day of 86 400 s SI. TT is connected to the international atomic time (TAI), and is defined so that 1.0 January 1977 TAI should correspond to 1.0003725 January 1977 TT. This means that TT is advanced with 32.184 s relative to TAI.
Third Contact The very end of totality. When the surface of the Sun first appears from behind the Moon. At a total solar eclipse is the moment when the limb of the Moon coincides with the limb of the Sun and totality ends; or, at a total lunar eclipse, the point where the front limb of the Moon touches the eastern edge of the Earth’s shadow and totality ends. Immediately after the third contact, at a total solar eclipse, the diamond ring or Baily’s Beads can be seen. At an annular eclipse, the third contact refers to the moment when the front limb of the Moon leaves the limb of the Sun and ends the annular phase.
Time Line Normally refers to the eclipse times.
Time Zone Eclipse calculations are done using UT (Universal Time). The time zone is a numeric offset in hours for the local time.
Total Eclipse A solar eclipse when the Moon covers the visible solar disk completely; or a lunar eclipse when the Moon enters the Earth’s shadow completely.
Totality At a total solar eclipse it is the brief period when the Sun’s disk is completely covered by the Moon. At a total lunar eclipse it is the period when the Moon is completely immersed in the Earth’s shadow. At a solar eclipse the totality can last from only several fractions of a second to a theoretical maximum of 7m 31s, depending on the distance from the Moon to the Earth. Around the perigee the Moon appears bigger and needs more time to cross the solar disk. At a lunar eclipse totality can last up to 1h 47m, also depending on the distance from the Moon to the Earth and on its passage through the shadow.
TRACE (Transition Region and Coronal Explorer) A NASA satellite, endowed with an ultraviolet telescope of O.3 m diameter, made for the study of the solar corona and transition region, where the solar atmosphere temperature grows very much.
Tracking Mount A telescope or camera mount that tracks the sky during the course of a night.
Transit When an object passes in front of the Sun as seen from Earth, that object is considered to be in transit. Transits of Mercury and Venus are often interesting to eclipse chasers.
Tripod Any device that will hold a camera steady.
TSA (Transportation Security Administration) Due to the travel involved in eclipse chasing, most must use airlines and other public transportation that falls under the guide lines of the TSA. It is wise to learn in advance what things you can and cannot take with you plus how the security procedures in place may effect delicate hardware such as telescopes, film, and cameras.
TSE Total Solar Eclipse.
Two Line Elements The basis for orbital prediction programs.
Eclipse chasers also like to capture other transitory astronomical events and usually together. T.S. Kelso is the best source for current data.
U1-U4 Umbral contact times. Same as C1-C4 however often expressed in UT (Universal Time).
Umbra The darker part of a shadow where the light source is completely obscured.
Umbraphile A term coined by Glenn Schneider to describe eclipse chasers. Also the name of a software program created by Glenn for automated control of cameras allowing the operator the opportunity to observe the eclipse.
Universal Time (UT) Greenwich Mean Time or Zulu Time. The time in at the Greenwich Observatory near London England, where the zero meridian is located by common convention.
The most common standard time scale, the same with GMT (Greenwich Mean Time). The universal time is the solar average time at the Greenwich meridian. It is defined as the Greenwich hour angle of the average Sun, plus 12 hours, so that the day begins at midnight and not at noon. It is closely connected to the Greenwich mean sidereal time (GMST), because the mean sidereal day is a well-known fraction from the average solar day. Practically, UT is determined by a formula from GMST, which in its turn is determined by the observations of the star passages at the meridian. UT deduced directly from this kind of observations is noted with UT0 (which depends a little on the observation place).
Corrected by the longitude variation due to the Chandler oscillation, UT0 becomes UT1, which has a large application. Compared to the international atomic time (IAT), UT1 loses approx. a second per year against the former. The hour signals broadcast on the radio use another time scale, known as the universal coordinated time (UTC). This is actually IAT corrected by a whole number of seconds. The correction is applied, whenever necessary, by introducing an interposed second and UTC is always kept with a difference of 0.9s at the most against UT1.
Universal Coordinated Time (UTC) A time scale given by the hour signals, which lies at the basis of any civil hour. This standard time should be the universal time (UT), but the latter one contains unpredictable irregularities due to the non-uniform rotation of the Earth. Consequently, UTC is connected to the international atomic time (IAT), which is adjusted from time to time, through the so-called "second jumps", so that it does not move off more than 0.9s from UT. The well-known GMT is also present in UTC.
Welding Filter #14 A #14 Welder’s filter can be used for viewing the partial phases of the eclipse.
Year of Eclipses The time interval between two successive passages of the Sun through the same node of the lunar orbit, when eclipses take place. It lasts for 346.62 days, namely for two eclipse seasons. An eclipse year is shorter than a sidereal year, because the lunar orbit nodes regress to the west around the elliptic with almost 190 per year. 19 years of eclipses make up a Saros.
Zenith A point on the celestial sphere, situated exactly above the observer’s head. The direction to the zenith is perpendicular to the horizontal plane. the opposite of the zenith is the nadir. This definition refers only to the astronomical zenith because there can be two other types of zenith, namely 1: the geocentric zenith, situated in the continuation of the direction from the Earth’s center to the observer and 2: the geodetic zenith, situated on the direction of the normal to the geode, in the point where the observer is placed. All three types of zenith differ a little, because of the non-spherical shape of the Earth. Unless specified otherwise, zenith refers to the astronomical zenith.

Last page update on April 10, 2006.
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