Tuesday, March 04, 2008

Venus if you will, please send an occultation us to thrill

The moon occults Venus in 2007
Image Credit: Alan C. Tough / Sky and Telescope

Tomorrow, (Wednesday March 5), people across much of the midwestern and southern United States have the chance to see the moon pass in front of the planet Venus in broad daylight. If you have binoculars, seeing this event will be fairly easy, but if you are patient and lucky, you may be should be able to see the moon and Venus in the daytime with just your eyes.

The planet Venus is usually bright enough to see in plain daylight (as are the other bright planets and, some claim, the brightest stars). The reason most people never see the planets during the day is that your eyes are fooled by the sky. If you look at a blank sky, your eyes tend to focus at a spot about ten feet away from your face, which puts any planets and stars in the sky out of focus, making them impossible to see against the bright blue sky. But if you know where to look and can focus your eyes properly, Venus is quite easy to see against the sky!

The easiest way to see planets in the daytime sky is to wait for the moon to wander nearby. Many people have seen the moon in the daytime sky, and looking at the moon focuses your eyes just right. So, if a planet is close to the moon in the daylight sky, your eye may be able to pick it out and stay focused. I've used this trick to see Venus in the daytime sky before, and it is quite amazing when it works. (I've also been told that looking at the sky from the bottom of a deep well or mineshaft helps you to see stars in the daytime, but I would not recommend falling down a well just for the sake of seeing stars in the daytime. A good whack on the head can accomplish the same end.)

The trick with tomorrow's event will be finding the moon in the sky. The moon will be about 25 degrees away from the sun -- that's only a little more than the width of your stretched hand (from pinky tip to thumbtip) at arm's length. The moon will also be very hard to see, because it will be a very skinny crescent. To see the moon without hurting your eyes due to the sun, you'll need to hide stand in a shadow. And, the moon will be between the sun and the horizon, so a building shadow will likely hide the moon, as well. You'll need a tree branch or lamppost or some other well-placed shadow.

An occultation means that the Moon will pass between Venus and the observer, so this is sort of like an eclipse of Venus. Venus will be behind the moon for over an hour in most places, so if you look during the occultion, you'll just see a crescent moon (like in the picture above, only even skinnier). Whoop-di-doo. No, the best views are either just before or just after the occultation, when you can see the moon and the brilliant light of Venus right next to it.

What time should you look? It depends where you live. Here is a website with a map and times of the start and end of the occultation. The times are in 24-hour format (e.g., 20:00 = 8:00pm), and in Universal Time (UT). Eastern Standard Time is the UT time minus five hours; so if the occultation starts at 20:25 UT, it starts at 15:25 EST (which is 3:25pm). Likewise, Central Standard time is the UT time minus six hours, and Mountain time is UT time minus seven hours.

The moon will be really, really hard to see without binoculars, because it will be very skinny. If you want to use binoculars, be very, very careful to make sure you don't point them anywhere close to the sun. Looking at the sun, even accidentally, through binoculars will instantly and permanently damage your eyes!

So, in short, if you have clear skies tomorrow afternoon and want a challenge, try going outside to see Venus and the moon in broad daylight. They will be hard to find, but seeing both in the daytime sky is something you'll remember for a long time to come!

For more information, read href="http://www.skyandtelescope.com/observing/highlights/16079237.html"Sky and Telescope's article on the occultation. Good luck!

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