Key to Catalog of Transits
By Fred Espenak
Introduction
Catalogs of transit circumstances include the following data. The calendar date^{1} and geocentric Universal Time^{2} of the four transit contacts^{3} and the instant of greatest transit^{4} are found in the first six columns. The Sun's coordinates (Right Ascension and Declination) and the Greenwich Sidereal Time at 00:00 UT are given next. The minimum separation between the centers of the planet and the Sun is listed in arcseconds. Finally the transit series^{5} is given.
Footnotes

^{1} The Julian calendar is used for all dates up to 1582 Oct 04.
After that date, the modern Gregorian calendar is used.
Due to the Gregorian Calendar reform, the day after 1582 Oct 04 (Julian calendar) is 1582 Oct 15 (Gregorian calendar).
Note that Great Britain did not adopt the Gregorian calendar until 1752.
For more information, see Julian and Gregorian Calendars.
 Contact I  The instant when the planet's disk is externally tangent to the Sun (transit begins).
 Contact II  The entire disk of the planet is internally tangent to the Sun.
 Contact III  The planet reaches the opposite limb and is once again internally tangent to the Sun.
 Contact IV  The planet's disk is externally tangent to the Sun (transit ends).
 Over the seven century period of the Mercury catalog, transits can be organized into twelve series. The transits in any one series recur with a 46 year period (16,802 days). The series numbers have been assigned in chronological order with respect to the first transit in each series.
 Over the four millennium period of the Venus catalog, transits can be organized into six series. The transits in any one series recur with a 243 year period (88,757 days). The series numbers have been assigned in chronological order with respect to the first transit in each series.
^{2} For most practical purposes, Universal Time (UT) is equivalent to Greenwich Mean Time (GMT).
^{3} The four transit contact times are defined as follows:
Contacts I and II define the phase called ingress while contacts III and IV are known as egress.
^{5} Transits recur in cycles much like the Saros cycle for eclipses:
Key to Catalog of Transits
Column Heading Definition/Description 1 Date Calendar Date (Gregorian) at instant of Greatest Transit. (Julian calendar is used before 1582 Oct 15). 2 I Contact I is the instant when the planet's disk is externally tangent to the Sun (transit begins). 3 II Contact II is the instant when the entire disk of the planet is first internally tangent to the Sun. The period from contact I to II is called Ingress. 4 Greatest Universal Time (UT) of Greatest Transit, which is Transit defined as the instant when the planet passes closest to the center of the Sun as seen from the center of Earth. 5 III Contact III is the instant when the planet reaches the opposite limb of the Sun and is once again internally tangent to the Sun. 6 IV Contact IV is the instant when the planet's disk is externally tangent to the Sun (transit ends). The period from contact III to IV is called Egress. 7 Minimum Minimum angular separation (arcseconds) Sep. between centers of the Sun and planet occurs at the instant of greatest transit. 8 Sun Geocentric Right Ascension (hours) of the Sun RA at greatest transit. 9 Sun Geocentric Declination (degrees) of the Sun Dec at greatest transit. 10 GST Greenwich Sidereal Time at 00:00 UT. 11 Transit Recurrence series of transit. Series Mercury transits recur after an interval of 46 years. Venus transits recur after an interval of 243 years.
Visibility of Transits
To determine whether a transit is visible from a specific geographic location, it is simply a matter of calculating the Sun's altitude and azimuth during each phase of the transit. The calculations can be performed on any pocket calculator having trig functions (SIN, COS, TAN). Armed with the latitude and longitude of the location, the transit catalogs provide all the additional information needed to make the calculations.
For a detailed description of how to calculate the Sun's altitude and azimuth, see Visibility of Transits. This web page also has links to Excel spreadsheets which can be used to calculate transit circumstances from any place on Earth.
Transit Predictions
Transit predictions are based on algorithms and elements published in "Transits" by Jean Meeus (WillmannBell, 1989).
The value for deltaT was determined as follows:

1) pre1600: delta T was calculated from empirical expressions by Stephenson [1997]
2) 1600present: delta T was obtained from published observations
3) future: deltaT was extrapolated from current values
All transit calculations are by Fred Espenak, and he assumes full responsibility for their accuracy.
Permission is freely granted to reproduce this data when accompanied by an acknowledgment:
"Transit Predictions by Fred Espenak, EclipseWise.com"