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By Lesa Moore

The closest planet to the Sun is an arid, airless world. Its cratered surface is often mistaken for that of the Moon. Temperatures range from 415 degrees C on the daytime side to -175 degrees C on the dark side. Mercury's orbit is eccentric to the extent that its distance from the Sun varies between 46 and 70 million kilometres. Its orbit is also inclined 7o to the ecliptic. Of all the planets, only Pluto's orbit is more eccentric than Mercury's.

A sidereal day (the period for one rotation on its axis) is 58 days, so Mercury spins rather slowly. It whips around the Sun, though, in just 88 days. A consequence of this is that its appearance and observability change over short periods of time (see below). Combining its spin and its revolution around the Sun gives a Mercurian day (sunrise to sunrise) of 176 Earth days.

If you were sitting on Mercury (with your SPF400 sunblock) at perihelion, you would notice retrograde motion of the Sun (it would travel east instead of west for a few hours). This is because Mercury is travelling fastest at perihelion, and its motion round the Sun at this time is faster than its rotation. The Sun would also appear 50% larger at this time, from Mercury, than at aphelion.

The atmosphere on Mercury is negligible, though it is ten times denser than the Moon's. It is only about a million millionths of the density of Earth's atmosphere, and its main constituent is sodium! The surface composition is similar to the Moon's, with a 7 percent reflectivity (albedo).

Mysteries about Mercury remain. An early puzzle was the advance of its perihelion which could not be accounted for by the motion of the other planets. Early this century, this was used as a confirmation of Einstein's theory of "General Relativity" (a theory of gravity). Mercury's density, however, is still unexplained. Being so small, 4880km in diameter, it is hard to account for its density being almost equal to Earth's. It also has a surprisingly strong magnetic field. These features lead us to believe Mercury has a large iron-nickel core, still partially molten, with an overlying crust of silicate rocks.


Three close passages of Mercury by Mariner 10 in 1974 and 1975 revealed surface details down to 100 metres across. Craters and impact basins abound, though half of the planet has only lightly cratered plains similar to the lunar maria. Mercury's maria don't stand out in photos like the Moon's, however, because the surface colours are similar all over Mercury. Features unique to Mercury are the irregular scarps, over 3km high, which cut through basins and craters in their path for several hundred kilometres. These could be the result of the planet shrinking as it cooled. Mariner 10 also searched for satellites but found none larger than 5km across. To this day, we do not have a complete surface map of Mercury, as the Mariner 10 images all showed the same face of the planet.

Observing Mercury

Whilst you will be unable to see any surface detail on Mercury with a telescope, it does show phases as it orbits the Sun. It first becomes visible after superior conjunction as an evening object gradually becoming higher in the sky over the following six weeks. During this period, its phase decreases from full to gibbous, but the size of its disk in telescopes becomes larger. At greatest elongation east, it shows a half phase, and from then on, it begins to drop lower in the sky. It remains bright even though its phase is now a crescent, as it is still coming closer to the Earth. Less than a month after greatest elongation, it reaches inferior conjunction, between the Sun and us. The cycle and phases unfold in reverse for the morning apparitions.

Being the innermost planet, it cannot exceed 28o in its apparent angle from the Sun. So it will always be seen in some degree of twilight glow. It is a naked eye object, however, and is not too difficult to find with the use of a current ephemeris.


Occasionally, at inferior conjunction, Mercury will appear to cross the face of the Sun. A transit can be safely viewed by solar projection, or with the use of a full aperture solar filter on your telescope. This is a rare opportunity to observe the dark side of the planet as a tiny black dot against the enormous disk of the Sun. The next transit of Mercury occurs in 1999.

Article supplied by Lesa Moore, Senior Guide, Koolang Observatory (1999) and reproduced by permission.

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