How long does a transit of Venus last?
A transit of Venus lasts from 3 to 7 hours; the exact time depends on the path taken across the Sun. The June 2004 and June 2012 transits of Venus will each last over 5 hours. The black drop effect At the beginning of a transit of Venus, when Venus has just entered the Sun’s disk, an odd optical phenomenon occurs; it is called the black drop effect. The light from the Sun is bent around Venus (it is refracted by Venus’ dense atmosphere), making Venus look stretched-out. This phenomenon also occurs towards the end of a transit of Venus, just before Venus leaves the solar disk. How often does a transit of Venus occur? A transit of Venus is very rare. On average, there are two transits of Venus every 125 years. Only five transits of Venus have been seen by human beings. Before the transit of Venus in June 2004, previously observed ones happened on December 4, 1639, June 5, 1761, June 3, 1769, December 8, 1874, and December 6, 1882. There have been only 52 transits of Venus across the Sun
