Sunday, October 30, 2011

The Phases of Venus

Venus is the second planet from the Sun, and makes for a satisfying viewing target. Though it is covered with clouds at all times, it is not devoid of detail.

The planet Venus shows phases, like the Moon. In fact, the most common comment I hear when showing Venus through my telescope is "It looks like the Moon!"

During the daytime, it can be seen as a ghostly moon-like object. It takes great care to keep the telescope from viewing the Sun. If this can be managed, Venus shows up well during the day. The trick is to position the telescope away from the Sun, on the same side of the Sun as Venus, by using the shadows on the telescope without looking through the scope (and probably covering the end of the scope to keep the sunlight out!) Then, once you're sure there's no Sun in the scope, look through and sweep away from the Sun. Practice sweeping in the correct direction before your eyeball is at the scope.

At night, you don't have to worry about this. If anything, you may find Venus is a bit too bright. A color filter or neutral density filter can help cut the light down a bit to make it more comfortable to view. Careful and patient use of a variable polarity filter can reveal murky details in the clouds of Venus.

Venus shows a full set of phases, from new and thin crescent phases to a Full Venus.

It can be a lot of fun to watch Venus from week to week as its phases change. It goes much more slowly than the Moon, overall, but there are some phases that seem to shoot past, and others (like full) that seem to last forever.

Ordinary binoculars won't show the phases clearly, but when Venus is away from "full" it'll sure look like something's wrong. Crescent phases can take on all sorts of interesting shapes in binoculars, sometimes looking like a line, other times looking just mis-shapen.

For the early Greeks, Venus has two names depending on whether it was a morning star or an evening star. As a morning star it was Phosporus, it was called Hesperus as an evening star. In the sixth century B.C. Pythagoras recognized it as a single object, which was named Aphrodite and later given the Roman name Venus as the goddess analogous to Aphrodite.

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