A Transit, or passage of a planet in front of the Sun, could be considered a special kind of eclipse. Unlike the events we normally call eclipses, though, a planetary transit does not block a very large portion of the Sun's light.
From Earth, we can only see planetary transits of the inner two planets, Mercury and Venus. Planetary transits are very rare occurances. Mercury only averages about 13 per century. Planetary tranists of Venus usually occur in pairs with eight years separating the two events. However, more than a century elapses between each transit pair. The first transit ever observed was of the planet Mercury in 1631 by the French astronomer Gassendi. A transit of Venus occurred just one month later but Gassendi's attempt to observe it failed because the transit was not visible from Europe. In 1639, Jerimiah Horrocks and William Crabtree became the first to witness a transit of Venus.
Because Venus's orbit is considerably larger than Mercury's orbit, transits of Venus are much rarer. Indeed, only six such events have occurred since the invention of the telescope (1631,1639, 1761,1769, 1874 and 1882). Transits of Venus are only possible during early December and June when Venus's orbital nodes pass across the Sun. Transits of Venus show a clear pattern of recurrence at intervals of 8, 121.5, 8 and 105.5 years.
On June 8, 2004, the next planetary transit of Venus will occur, our first chance to see one since 1882. The next time we'll see one will be in 2012.
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 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.
Geographic Visibility of 2004 June 08
The global visibility of the 2004 transit is illustrated with the world map in figure 2. 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.Viewing the Planetary Transit of Venus
Venus is not only larger than mercury, it's also closer to Earth, so its apparent diameter is nearly 1 arc-minute. It should be possible to see the planetary transit without optical magnification, but solar filter protection is a must. However, since Venus seems to be only 1/32 of the Sun's apparent diameter, a pair of binoculars or a small telescope at modest power will offer a much more satisfying view. Of course, any optical device used must have adequate filtration to ensure safe solar viewing. The visual and photographic requirements for observing a transit are identical to those for solar viewing. Observing techniques and equipment are similar to those used for lunar occultations.Full Size Images:
Image Credit: NASA/Goddard Space Flight Center
