Key to Catalog of Transits

By Fred Espenak

Introduction

Catalogs of transit circumstances include the following data. The calendar date1 and geocentric Universal Time2 of the four transit contacts3 and the instant of greatest transit4 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 arc-seconds. Finally the transit series5 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.
    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:
    • 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).

    • Contacts I and II define the phase called ingress while contacts III and IV are known as egress.
    4 Greatest transit is defined as the instant when the planet passes closest to the center of the Sun as seen from the center of Earth.
    5 Transits recur in cycles much like the Saros cycle for eclipses:
    • 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.


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 (arc-seconds) 
             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 (Willmann-Bell, 1989).

The value for delta-T was determined as follows:

    1) pre-1600: delta T was calculated from empirical expressions by Stephenson [1997]
    2) 1600-present: delta T was obtained from published observations
    3) future: delta-T 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"



Seven Century Catalog of Mercury Transits

Six Millennium Catalog of Venus Transits

Transits of the Sun

Eclipse Web Site