Wednesday, February 21, 2007

Venus in the Early Evening Sky

If on a clear evening you've been looking toward the West shortly after sunset, you may have noticed a bright start not far above the horizon. This "star" is the planet Venus, which at this part of its orbit around the sun is often called the "evening star". Venus is speeding towards the point of Greatest Eastern Elongation which this year will be reached on June 9. At Greatest Elongation (Eastern or Western) the angle between the line connecting the sun and the Earth and the line connecting the sun and Venus is greatest of all year. This diagram shows it better than any lengthy explanation can do. You can also follow the positions of the planets respective to the sun at this CGI page (use the controls to enlarge if needed).

As a result, at Eastern Elongation, the planet can be seen shortly after sunset, in the Western sky, fairly low over the horizon. Conversely, at Western Elongation the planet rises shortly before sunrise (in the Eastern sky) and then it's known as the "morning star".

Like the other inner planet, Mercury (but Mercury is so close to the sun, it's very hard to spot), Venus shows clear phases (much like the Moon) and at Greatest Elongation the planet is at half phase.

Whilst we're clearly still some time away from Greatest Eastern Elongation, I've already taken a couple of preliminary peeks at Venus, using my 150 mm reflector telescope. Unfortunately, a quick set-up from my front-porch yielded a very poor image, even in good seeing conditions: the planet is low over the horizon and in an urban area that means an amateur astronomer is peering over rooftops and through the heat and light pollution of the city (well, town in my case). Whilst I could clearly discern the disc, the magnified swirls of rising warm air made it look more like a flaming sun than a planet!

But at the earliest possible opportunity, I'm taking my trusted scope out to the seafront where the clear horizon causes light and heat pollution to be much less of a problem. Fingers crossed!


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