As seen from Earth, only transits of the inner planets Mercury and Venus are possible. A transit of Mercury can last for up to 8 hours, while a transit of Venus can slightly exceed 8 hours at its maximum. The first observation of a transit of Mercury was registered in 1631 by the French astronomer Pierre Gassendi. Even though the telescope, necessary to observe the transits, was in use since until approximately 1610, no one observed the transits of 1615, 1618 and 1628, because their occurrence was not known. The release of new tables with more accurate positions of the planets (the Rudolphine Tables of Kepler in 1627) made possible the prediction and observation of the transit of Mercury in 1631. The first transit of Venus was observed in 1639 by the briton Jeremiah Horrocks. As it turns out, Kepler had predicted a transit of Venus for 1631. However, despite the efforts of Gassendi, the transit was not observed because it occurred during nighttime in France (this fact was obviously unknown to Gassendi because of the limited accuracy of Kepler’s tables). Kepler, however, did not predict a transit for 1639. It was Horrocks himself who, on October 1639, when comparing Kepler’s tables with the older and inaccurate tables of Lansberg, realized that the latter were predicting a possible transit of Venus on November of the same year. Horrocks confirmed the prediction with his own calculations but the short time available caused that the event was not widely announced. Thus, only Horrocks, his brother Jonas and his friend Crabtree were aware of the occurrence with enough anticipation. Both Horrocks and Crabtree observed the transit and took some measurements but it seems that Horrock’s brother never observed it. Currently, the transits of Mercury are just a curiosity with almost no scientific interest. The same can be said of the transits of Venus. Among the few contributions that Mercury’s transits can make today is the study of Earth’s rotational velocity. When the circumstances of a transit observed in the past are known (i.e., contact times and observing site), one can eventually infer small variations of the length of the day along the centuries. The situation, however, was radically different up to the XIXth century. Until then, the transits of Venus, as suggested by Edmond Halley in 1716, had been used to attempt an accurate measure of the mean distance between the Earth and the Sun, namely the Astronomical Unit. Many scientific expeditions were organized to observe the transits of 1761, 1769, 1874 and 1882. But, in spite of all the efforts, several observational limitations, most notably the black drop effect, made impossible a sufficiently accurate determination of the Astronomical Unit through the method of the transits of Venus. During the second half of the XIXth century and along the XXth century, new methods were developed to determine the Earth-Sun distance, none of them implying the transit of Venus or Mercury across the Sun. The past strong scientific interest of these events was therefore lost. Currently, only phenomena derived from the transits such as the mentioned black drop effect are subject of observation and study. Shortly after the internal contact between the discs of the Sun and the planet (Mercury or Venus) something strange occurs. Instead of separating clearly from the solar limb, the disc of the planet seems to stick for a few seconds to the solar disc, and experiences a deformation in the appearance of a black drop. The same phenomenon repeats again just before the last internal contact. The effect of the black drop prevents an accurate registration of the times of contact between the disc of the planet and the Sun, and was the chief cause of the failure of the attempts to determine the Earth-Sun distance using Halley’s method. The main cause of the occurrence of this phenomenon is the turbulence of Earth’s atmosphere, albeit the solar disc being slightly darker at the edge than at its center (known as limb darkening) is also partly responsible. Here is a more complete explanation of the black drop effect. Know more about deltaT (ΔT) and leap seconds. Having a look at the solar activity cycle is interesting to know if there is a chance to have sunspots.
Planetary transits are far more rare than eclipses of the Sun by the Moon. On average, there are 13 transits of Mercury each century. Nowadays, all transits of Mercury fall within several days of May 8 and November 10. During November transits, Mercury is near perihelion and exhibits a disk only 10 arc-seconds in diameter. By comparison, the planet is near aphelion during May transits and appears 12 arc-seconds across.
The next Mercury transits will occur at the following dates : Saturday, November 13, 2032 :
Monday, November 7, 2039 :
Friday, May 7, 2049 :
Tuesday, November 9, 2052 :
Wednesday, May 10, 2062 :
Wednesday, November 11, 2065 :
2003 May 7 Mercury transit time-lapse
To compute the local circumstances of a Mercury transit, a calculator is also available for your own use. A time exposure calculator is there to help you choose your camera settings.
A rare phenomenon occurred on 2004 June 8: the planet Venus was in transit in front of the Sun. No one alive in the XXth and XXIst centuries had ever seen a transit of Venus: the last transit was in 1882 and was only partially visible in Europe. The observation of the 1882 Venus transit was the most well-known instance to measure the distance between Earth and Sun, that distance being used as the base unit for distances between all heavenly bodies.
The next Venus transits will occur at the following dates : Saturday, December 11, 2117 :
Saturday, December 8, 2125 :
2004 June 8 Venus transit time-lapse (courtesy of NSO)
To compute the local circumstances of a Venus transit (2004, 2012 and 2117), a calculator is also available for your own use. A time exposure calculator is there to help you choose your camera settings.
This application controls up to 4 USB, Firewire or Ethernet connected DSLR or CCD cameras during a Mercury or Venus transit, so that you can be free to concentrate on observing the event visually. This brief overview will let you know more.