Transits of Venus across the disk of the Sun are among the rarest of planetary alignments. Indeed, only six such events have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874 and 1882). The next two transits of Venus will occur on 2004 June 08 and 2012 June 06.
The principal events occurring during a transit are characterized by contacts. The event begins with contact I which is the instant when the planet's disk is externally tangent with the Sun. The entire disk of the Venus is first seen at contact II when the planet is internally tangent with the Sun. During the next several hours, Venus gradually traverses the solar disk at a relative angular rate of approximately 4 arc-min/hr. At contact III, the planet reaches the opposite limb and is once again internally tangent with the Sun. The transit ends at contact IV when the planet's limb is externally tangent to the Sun. Contacts I and II define the phase called ingress while contacts III and IV are known as egress. Greatest transit is the instant of minimum angular separation between Venus and the Sun as seen from Earth's geocenter.
Figure 1 (Low Res or High Res) illustrates the geocentric observing geometry of each transit across the Sun (celestial north is up). The 2004 transit crosses the Sun's southern hemisphere while the 2012 event crosses the northern hemisphere. The position of Venus at each contact is shown along with its path as a function of Universal Time. Each transit lasts over six hours. The apparent semi-diameters of Venus and the Sun are 29 arc-seconds and 945 arc-seconds respectively. This 1:32.6 diameter ratio results in an effective 0.001 magnitude drop in the Sun's integrated magnitude due to the transit. Geocentric contact times and instant of greatest transit appear to the left corners of figure 1 (Low Res or High Res).
The global visibility of the 2004 transit is illustrated with the world map in Figure 2 (Low Res or High Res). The entire transit (all four contacts) is visible from Europe, Africa (except western parts), Middle East, and most of Asia (except eastern parts). The Sun sets while the transit is still in progress from Australia, Indonesia, Japan, Philippines, Korea, easternmost China and Southeast Asia. Similarly, the Sun rises with the transit already in progress for observers in western Africa, eastern North America, the Caribbean and most of South America. None of the transit will be visible from southern Chile or Argentina, western North America, Hawaii or New Zealand.
The horizontal parallax of Venus (~30 arc-sec) introduces a topocentric correction of up to ±7 minutes with respect to the geocentric contact times for observers at different geographic locations. Topocentric contact times (Universal Time) and corresponding altitudes of the Sun are presented for over one hundred cities in Table 1A (international) and Table 1B (USA).
The global visibility of the 2012 transit is illustrated with the world map in Figure 3 (Low Res or High Res). The entire transit (all four contacts) is visible from northwestern North America, Hawaii, the western Pacific, northern Asia, Japan, Korea, eastern China, Philippines, eastern Australia, and New Zealand. The Sun sets while the transit is still in progress from most of North America, the Caribbean, and northwest South America. Similarly, the transit is already in progress at sunrise for observers in central Asia, the Middle East, Europe, and eastern Africa,. No portion of the transit will be visible from Portugal or southern Spain, western Africa, and the southeastern 2/3 of South America.
Note that due to the International Date Line the Western Hemisphere will see the transit on June 5.
The horizontal parallax of Venus (~30 arc-sec) introduces a topocentric correction of up to ±7 minutes with respect to the geocentric contact times for observers at different geographic locations. Topocentric contact times (Universal Time) and corresponding altitudes of the Sun are presented for over one hundred cities in Table 2A (international) and Table 2B (USA).
Transits of Venus are only possible during early December and early June when Venus's orbital nodes pass across the Sun. If Venus reaches inferior conjunction at this time, a transit will occur. Transits show a clear pattern of recurrence at intervals of 8, 121.5, 8 and 105.5 years. The next pair of Venus transits occur over a century from now on 2117 Dec 11 and 2125 Dec 08.
Edmund Halley first realized that transits of Venus could be used to measure the Sun's distance, thereby establishing the absolute scale of the solar system from Kepler's third law. Unfortunately, his method proved impractical since contact timings of the desired accuracy are impossible due to the effects of atmospheric seeing and diffraction. Nevertheless, the 1761 and 1769 expeditions to observe the transits of Venus gave astronomers their first good value for the Sun's distance.
The planet Mercury can also transit the Sun. Since Mercury orbits the Sun more quickly than does Venus, it undergoes transits much more frequently. There are about 13 or 14 transits of Mercury each century. All Mercury transits fall within several days of 8 May and 10 November. 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. However, the probability of a May transit is smaller by a factor of almost two. Mercury's slower orbital motion at aphelion makes it less likely to cross the node during the critical period. November transits recur at intervals of 7, 13, or 33 years while May transits recur only over the latter two intervals. The next two transits of Mercury are on 2003 May 07 and 2006 Nov 08. For details on the first event, see:
The upcoming transits of Venus offer a unique and rare opportunity to develop and test new techniques which can be applied to the detection and characterization of extrasolar planets. Additional predictions for these events will be published and updated in the coming months at:
Transit predictions were generated on an Apple iMac computer using algorithms developed from Meeus  and the Explanatory Supplement .
All calculations and diagrams presented in this section are those of the author and he assumes full responsibility for their accuracy.
Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac, 1974, Her Majesty's Nautical Almanac Office, London.
Maor, E., 2000, June 8, 2004--Venus in Transit, Princeton University Press, Princeton.
Meeus, J., 1958,"Transits of Venus, 3000 BC to AD 3000", J.B.A.A., 68, 98.
Meeus, J., 1989, Transits, Willmann-Bell, Inc., Richmond.