Mercury and Venus at greatest elongation

¡SkyCaramba! Weekly astronomy blog for the week ending August 11, 2012

Mercury and Venus are about to put on one of their best shows all year together in the morning sky. The two planets are closer to the sun than Earth. So they never get to be up all night. There are points in their movement in the sky that they are at the farthest they will get from the sun. That’s called greatest elongation.

Venus is easy to spot. It’s always bright. Its atmosphere has highly reflective clouds. Even when most of the planet’s dark side is toward us, plenty of sunlight is reflected to make Venus easy to see.

Around August 5, you’ll see Venus near one of the hands of Orion almost at the feet of Gemini. Watch from morning to morning, and you’ll see the planet moving eastward among the stars. There’s actually an optical illusion I’ll try to help you see through. While it appears to be moving toward the sun, Venus is getting farther from the sun each morning until the 15th.

As Earth makes its way around the sun, the stars we see at night change. Imagine standing in the middle of a room and pointing a flashlight toward one of the walls. Turn around with the flashlight pointing in the same direction from you and the spot it makes will move around hitting different points of the wall and other objects in the room until it gets back to where it started. It’s kind of like that from night to night as Earth orbits the sun but without a flashlight shining on anything out there.

Now imagine someone else in the room walking around as you turn around with the flashlight. Where the person is in your field of view changes. Sometimes, it will be possible to see the person moving left in your field of view even though he or she is walking to the right. The same thing is happening in the solar system. Right now, as Venus is heading away from the sun from our point of view, the stars are moving faster.

Around August 15th, Venus will be at the feet of Gemini. It’ll be at the feet of Castor, specifically. If you’re not sure you can find it, look on the morning of the 13th. A thin crescent moon will be just above Venus. On the 14th, a thinner crescent moon will be below it. Also on the 14th, the moon will be somewhat close to Mercury. Look below the moon to the left for our solar system’s first planet. Below the moon and to the right is the star Procyon.

If you’re a little frustrated trying to positively identify Mercury, that’s nothing unusual. Mercury stays close to the sun, so you have to spot it right after sunset or right before sunrise. And you have to find a clear horizon. A few tall trees or buildings nearby will hide it.

If you can look at Venus and Mercury in a telescope, you’ll see two slightly less than full objects. The two inner planets go through phases somewhat like the moon does. Venus will be gibbous at greatest elongation. Mercury will be closer to full.

Enjoy the view. ¡SkyCaramba!