An event of planet transit or passage across the disk of the Sun occurs when an internal planet, Mercury or Venus, aligns exactly between the Sun and the Earth. About thirteen passages of Mercury can take place in a century. The transits of Venus are far more rare. They usually occur in pairs separated by eight years, while two consecutive pairs are separated by more than a century.



The transit begins when the limb of Venus is externally tangent with the disk of the Sun - Contact I. After crossing the limb of the Sun, the planet's disk becomes internally tangent with the solar disk - Contact II. The period between these two phases is known as ingress. While passing in front of the Sun, there is a moment of minimal angular distance of the planet's disk from the center of the solar disk - Greatest Phase. When the disk of Venus reaches and touches the opposite limb of the Sun, the Contact III occurs. The transit ends when the two limbs are again externally tangent, Contact IV, and the planet's disk completely exits the Sun. Whereas the time to cross the limb is about twenty minutes, the full transit takes more than six hours. The diameter of the Venus disk projected on the Sun during the transition is about 1/32 of this of the solar disk.

2004 Transit of Venus Major events (Local Time with Daylight Saving)	Athens	Thessa- loniki
Contact I (ingress begins)	08:19:58	08:19:56
Contact II (ingress ends)	08:39:33	08:39:33
Greatest (minimal angle distance)	11:22:36	11:22:29
Contact III (egress begins)	14:04:19	14:04:06
Contact IV (egress ends)	14:23:34	14:23:23