If you have noticed a bright object in the western sky after sunset over the past couple of months, that’s our beautiful “Sister Planet” Venus. It’s the third brightest object in the sky after the Sun and the Moon. And this month it is extra luminous, as it climbs up higher some 46 degrees east of our Daytime Star (at its “greatest eastern elongation” in astronomical terms).
Just how bright is Venus anyway? Well at peak brightness it can cast shadows on snow-covered ground and be seen reflected in ponds and lakes. And it can even been glimpsed in broad daylight against a clear blue sky if you know just where to look. Now that’s bright! Surprisingly though, Venus isn’t brightest when it’s full because then it’s on the opposite side of its orbit from the Earth and small in apparent size due to its distance from us. At either greatest eastern or western (in the morning sky) elongation, it is much closer and its apparent disk much larger, causing it to appear considerably brighter than when full. (Counter intuitive – right?)
But there’s more. As Venus draws closer in its orbit to the Earth, eventually passing between us and the Sun, it continues to get bigger and brighter – becoming a very large crescent that increases its brightness even more. In fact, so large is this crescent that it can be seen in binoculars. Many full-Venuses would easily fit on top of that crescent, such is the difference when close to us rather than when on the opposite side of its orbit.
Venus has been dubbed our “sister planet” because it’s nearly the same size as the Earth – but that’s where the resemblance ends! Its surface temperature is around 900 degrees F. (twice as hot as an oven) and the atmosphere is a toxic mixture of gases at some 90 times the air pressure here. Truly beautiful to behold in the sky – but only from a distance!
We mentioned the planet passing between the Earth and Sun, but it actually moves just north or south of the solar disk itself. On those very rare occasions when it does directly cross the Sun, a Transit of Venus occurs. The last one happened on June 5th of 2012 and was previewed here in Star Talk that month. The next transit won’t occur until December of 2117!
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing. His latest, Celebrating the Universe!, is available from HayHouse.com.