Catch a glimpse of Venus before it bids farewell

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If you get up before the sun this week, you will get your last chance for a while to see Venus as a "morning star."

Sometimes things in the sky happen very slowly, so that we aren't aware of them. Venus has been a "morning star" since March 27, when it passed between the Earth and the sun. It reached maximum brilliance on April 30, and reached greatest elongation west – its greatest distance from the sun in the sky – on June 5. Since then it has been slowly fading in brightness and sinking lower in the morning sky, so slowly that you probably haven't noticed.

If you haven't been up before sunrise recently, you will be surprised at how close Venus is to the horizon as the sun rises, and within the next week or two it will become too close to the sun to be observed. It will be at superior conjunction, right behind the sun, on Jan. 12, 2010. It will reemerge from behind the sun in March as an "evening star."

Venus will remain in the evening sky until Oct. 29, when it again passes between the Earth and the sun. [This video explains the geometry of all this.]

Venus is popularly known as an "evening star" or a "morning star" even though it is not a star, but a planet. This terminology is a holdover from the days when all the planets were regarded as "moving stars" before their true nature as bodies orbiting the sun was known.

Because Venus' orbit is closer to the sun than the Earth's, it goes through phases just like our moon, though you need a small telescope to see them. At present, because Venus is on the far side of the sun with respect to the Earth, it appears very close to the full phase, since the side of Venus facing us is fully illuminated by the sun. After it passes superior conjunction on Jan. 12 it will begin to shrink in phase, reaching the "half Venus" phase on Aug. 18 next year. This is also the date on which it is farthest east of the sun in the sky.

Between Aug. 18 and Oct. 29, it will be a waning crescent, at the same time becoming much larger in apparent size as gets closer to Earth and then passes between Earth and the sun on the 29th.