On Wednesday, 2006 Nov 08, Mercury will transit the Sun for the
first time since 2003. The transit or passage of a planet across
the face of the Sun is a relatively rare occurrence. As seen from
Earth, only transits of Mercury and Venus are possible. There are
approximately 13 transits of Mercury each century. In comparison,
transits of Venus occur in pairs with more than a century
separating each pair.
The principal events occurring during a transit are conveniently
characterized by contacts, analogous to the contacts of an annular
solar eclipse. The transit begins with contact I which is the
instant when the planet’s disk is externally tangent with the Sun.
Shortly after contact I, the planet can be seen as a small notch
along the solar limb. The entire disk of the planet is first seen
at contact II when the planet is internally tangent with the Sun.
During the next several hours, the silhouetted planet slowly
traverses the brilliant solar disk. At contact III, the planet
reaches the opposite limb and once again is internally tangent with
the Sun. Finally, 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.
Position angles for Mercury at each contact are measured
counterclockwise from the north point on the Sun’s disk.