Sunday, October 23, 2011

Viewing Planet Mercury

Planet Mercury is the closest planet to the Sun. Its orbit around the Sun is entirely within the orbit of the Earth. This means that from our point of view, Mercury always appears in the sky near the Sun. Sometimes it is seen setting in the evening shortly after the Sun sets, other times it is seen in the morning just before the Sun rises. At other times it is invisible, lost in the Sun's glare.

The early Greeks had two names for Mercury, depending on whether it was seen as a morning star or an evening star. As a morning star it was Apollo, as an evening star it was Hermes. It was later recognized to be a single object, and the name Hermes, the messenger of the gods, was the precursor to the Roman name Mercury that we use today.

Mercury can be seen by eye as a medium-bright red-orange star near the Sun. Binoculars will show that it's not a star, but a very small disk-like shape that looks un-star-like. It looks much the same in a telescope, though under good conditions it will show a "phase" like one of the Moon's phases, most commonly a crescent, since it is usually seen when well away from the Sun. It doesn't show any detail in telescopes, it just appears as a small reddish-orange shape, its shade of color varies some because of Earth's atmosphere.

At times it moves very quickly across the sky. Its position relative to the Sun, and to the stars in the sky, can be seen to change rapidly from day to day. It makes an interesting project to keep track of Mercury's position in the sky on a star chart over the course of a month or two.

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