A transit of Venus across the disk of the Sun is among the rarest of planetary alignments. The last transit occurred 120 years ago in 1882, while the next one takes place on 2004 June 08. Figure 1 shows the geocentric path of Venus across the Sun. The scale along the track gives the Universal Time1 of Venus's position at any instant. The planet moves westward with respect to the Sun and inscribes a chord through the Sun's southern hemisphere. Moving with an angular speed of 3.2 arc-minutes per hour, Venus takes about 6.2 hours to cross the Sun's disk.
crédit : http://sunearth.gsfc.nasa.gov
The transit begins with contact I when the limb of Venus is externally tangent with the Sun. It takes about nineteen minutes for the planet's disk to cross the solar limb where it becomes internally tangent with the Sun at contact II. The period between contacts I and II is known as ingress. At the end of ingress, the entire disk of Venus is seen silhouetted against the Sun's disk as it begins its slow six-hour passage across our star.
Contact III occurs when Venus reaches and first touches the opposite limb of the Sun. Another nineteen minutes elapse as the planet gradually exits the solar disk. Finally, the transit ends with contact IV when Venus's disk completely exits the Sun and the planet vanishes from sight. The period from contact III to IV is referred to as egress.
Table 1 gives the times of major events during the transit. Greatest transit is the instant when Venus passes closest to the Sun's center (i.e. - minimum angular separation). During the 2004 transit, Venus's minimum separation from the Sun is 627 arc-seconds. These contact times are actually for an observer at Earth's center. The contact times at other locations will differ up to plus or minus seven minutes. This effect is due to parallax since Venus's position will shift slightly2
depending on the observer's geographic coordinates on Earth. For the times of each stage of the transit from various cities around the world, see the tables at Transit Contact Times.
Universal Time or UT is the basis for the worldwide system of civil time. It is often referred to as Greenwich Mean Time although UT is actually based on atomic clock time rather than the Sun's mean motion. For more information, see All About Time Zones and Universal Time.
Venus's 58 arc-second disk may be shifted up to 30 arc-seconds from its geocentric coordinates depending on the observer's geographic position on Earth.
1 - Visibility of the Transit
2 - Weather Prospects for the Transit
3 - Modern Value of Venus Transits