Mercury on Sun - Rare Transit

A transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun.

Transits of Mercury with respect to Earth are much more frequent than transits of Venus, with about 13 or 14 per century, in part because Mercury is closer to the Sun and orbits it more rapidly.

Transits of Mercury can happen in May or November. The last three transits occurred in 1999, 2003 and 2006 ; the next will occur in 2016.

The term transit or astronomical transit has three meanings in astronomy:

The rest of this article refers to the first kind of transit.

Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits. The perihelion of Mercury's orbit precesses around the Sun at an excess of 43 arcseconds per century, a phenomenon that was explained in the 20th century by Albert Einstein's General Theory of Relativity. Mercury is bright when viewed from Earth, ranging from −2.3 to 5.7 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun is only 28.3°. Since Mercury is normally lost in the glare of the Sun, unless there is a solar eclipse it can be viewed only for short intervals before sunrise when it is near its maximum western elongation, or after sunset when near its maximum eastern elongation. At relatively high latitudes such as those of many European and North American population centres, it is even then near the horizon and obscured in a relatively bright twilit sky. However, at tropical and subtropical latitudes, Mercury is more easily seen because of two effects. (i) the Sun ascends above the horizon more steeply at sunrise and descends more steeply at sunset, so the twilight period is shorter, and (ii) at the right times of year, the Ecliptic intersects the horizon at a very steep angle, meaning that Mercury can be relatively high (altitude up to 28°) in a fully dark sky. Such conditions can pertain, for instance, after sunset near the Spring Equinox, in March/April for the southern USA and in September/October for South Africa and Australasia. Conversely, pre-sunrise viewing is easiest near the Autumn Equinox.