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Astronomy/Morning sky observation

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Expert: - 11/23/2010


Question

11-23-2010_0510AM
I live in Southern CA; this morning I got my scope out to look at what I thought was Venus in the SE sky at about 45 degrees. What I saw looked like an eclipse similar to the attached clip-art jpg. I'm not well-versed in this area, but it is curious. What's going on?

Answer
Venus goes through phases, just like the Moon. A few weeks ago it was more or less in between us and the Sun, so its dark side was toward us. Now it is moving westward relative to the Sun, so we see it lit on the eastern side. Until it reaches greatest western elongation, a few weeks from now, it will display a crescent phase (thinner at first, then gradually fatter, so a "waxing" crescent).

You can view pictures and an animation of the phases at http://cseligman.com/text/planets/venusphases.htm Unfortunately, there isn't much explanation of how the phases are related to the orbital motion at the moment, but there is a page which discusses the "aspects" of the planets, at http://cseligman.com/text/sky/aspects.htm If you refer to the diagram of aspects for an "inferior" planet (which means, Mercury and Venus), at inferior conjunction the phase would be "new", at superior conjunction it would be "full", and at greatest elongation it would be "quarter". Between inferior conjunction and greatest elongation, it would be "crescent", and between greatest elongation and superior conjunction it would be "gibbous". Your image shows a crescent phase, which is correct for the current position of the planet.

(Incidentally, it was the fact that Venus has both crescent and gibbous phases that allowed Galileo to prove that it must orbit the Sun, instead of the Earth.)
    Questioner's Rating
    Rating(1-10)Knowledgeability = 10Clarity of Response = 10Politeness = 10
    CommentYour response came very quickly, and cleared up my confusion. Thank you!


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Courtney Seligman

Expertise

I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.

Experience

I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy.

Publications
Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

Education/Credentials
I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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