Local Circumstances Calculator (v1.0.2)
- The transit diagram, drawn to scale, shows how Mercury will traverse the solar disk as seen from the centre of the Earth (geocentric coordinates). The four positions of Mercury along the time-scale (straight graduated chord) are the exterior (1) and interior (2) contacts at ingress, the interior (3) and exterior (4) contacts at egress. The position angle of Mercury is measured counterclockwise along the solar limb from the direction of the north on the celestial sphere (N in geocentric coordinates from the center of the Earth and the short black line in topocentric coordinates at the observer’s location). N is of course the zenith at the observer’s location, that is when using the topocentric coordinates.
The curvilinear movement of Mercury across the solar disk relative to the zenith, the point directly overhead at the observer’s location, is also shown with its duration divided in thirty equal parts once computed for your location (from the green dot to the red dot). This is how you will see Mercury advancing on the solar disk when observing with the naked eye or a telescope on an altazimuth mount or just a basic tripod. Because of the diurnal motion of the celestial sphere, the sun’s disk rotates with respect to the direction of the zenith in the course of a day. The initially straight chord, representing Mercury’s trajectory with respect to the north point, is now transformed into a curved path. The grayed disks of Mercury indicate the invisible part of the transit during which the Sun is below the horizon.
When using an HTML 5 capable browser (Safari, Chrome or Opera) you can compute the local circumstances for any instant between ingress and egress using the time slider. Not only are the position angle, the Sun’s altitude and azimuth updated when you move the slider back and forth, the position of Mercury on the solar disk at the selected instant is also indicated in the diagram. The line along the solar disk indicates the direction of the celestial north.
- To find your local circumstances, first select the transit and then enter your location and time zone information in the Geographic Coordinates section. Then click on the "Compute Transit Circumstances" button. The predicted transit circumstances will appear in the table below that button.
To start with other default geographic or eclipse parameters you can append a query string to this web page URL. The parameters can be specified in any order or can be omitted. It should look something like :
- Transit is a text string specifying the transit’s date in the following format YYYYMMDD,
- Latitude is a number in decimal notation and is positive in the Northern hemisphere,
- Lng for Longitude is a number in decimal notation and is positive if East of Greenwich,
- Time Zone is a text string in the following format SHHMM,
- DST for Daylight Savings Time is a number (0 for Regular Time, else 1).
- The times and local circumstances of the transit from your location and selected time zone are given in the Predicted Transit Circumstances section. The information given in the first five lines is -
The information in the bottom line is -
- Transit event,
- The local date and time of the event (if the event occurs while the Sun is below the horizon, an asterisk (*) will appear after the hour),
- Alt - The altitude of the Sun, in degrees, above the horizon (a negative value means that the Sun is below the horizon and the particular contact will not be visible from the specified location),
- Azi - The azimuth of the Sun (0° = due north, 90° = due east, etc.),
- P - The angle between the celestial north and the contact point with Mercury on the Sun’s disk. In addition, for the maximum transit the minimum separation, in arcseconds, between the center of the solar disc and Mercury is also given,
- Z/V - The "o’clock" position on the Sun’s face of the contact point with Mercury (eg V=12.0 means that the contact point is in the "12 o’clock" position - ie the top of the Sun’s disk; V=3.0 means that the contact point is in the "3 o’clock" position - ie the right of the Sun’s disk; V=6.0 means that the contact point is in the "6 o’clock" position - ie the bottom of the Sun’s disk; V=9.0 means that the contact point is in the "9 o’clock" position - ie the left of the Sun’s disk).
The Sep value is the minimum separation between Mercury and the center of the Sun.
- Sunrise - The sunrise local time,
- Sunset - The sunset local time.
- This calculator does not account for atmospheric refraction, which makes a difference if the transit occurs close to sunrise or sunset,
- It is not possible to predict the exact value of ΔT in advance, although the extrapolated value should be good to better than 0.5 seconds.
The code contained that does the calculations has been released under the terms of the GPL Version 2.