Thursday, March 18, 2010

Twilight Pair

Last night (Wednesday) I headed west out of town towards Kitt Peak National Observatory to work on an observing project before the moon climbs higher into the sky. It's natural light pollution would drown out my subject (more in a day or two on that one, I hope). But while I'm avoiding the fast brightening Moon, it was quite pretty in the twilight, hanging above our new "Evening Star" Venus, getting higher and brighter every day. They were side-by-side Tuesday night, but some high clouds inspired me to stay home, so missed that conjunction, but Wednesday, as nightfall descended, I took time out to take a couple pics of the pair.

As Venus 'rounds the sun, it is apparently getting higher in the evening sky during twilight. The presence of the Moon was just happenstance, and will pass Venus again in 4 weeks, that time the pair joined by innermost planet Mercury.

The coolest thing about the skinny crescent moon is not the sliver of it's surface directly lit by the sun, but rather the "Old Moon", which should be dark, but is lit by a mysterious glow. What causes it? It is pretty simple actually - we do, or rather, the Earth! At new moon or thin crescent stage, it is between us and the Sun, and if we were lucky enough to be standing on the lunar surface, the Earth would be a brightly lit "full" phase. In other words, when it is a new moon, from the Moon, there is a full Earth! And because the Earth subtends a considerable size from the Moon's surface (about 4 times the Moon's appearance from Earth), and also because the Earth's clouds, oceans and continents reflect much more light than the asphalt-equivalent reflectivity of the moon's surface, the dark side of the moon is lit up by earth shine, which we can easily see with our naked eyes. So while I can take a short exposure and show some of the craters along the terminator (sunrise line, where shadows enhance the Moon's relief), it always seems more impressive to me to expose a few seconds to show the earth shine.

The photos here were taken with the Canon XSi, the closeups with a Celestron 5", the wide shot with the kit lens set to 50mm focal length.

1 comment:

David A. Harvey said...

Good to see an Astro post again Dean - nice shots. Can't wait to hear about your project. Might it have something to do with zodiacal light?