Astroprof’s Page

Mercury Stationary

Published on Feb 13, 2007 at 4:16 pm. 1 Comment.
Filed under planets, skywatching.

I have a lot of resources to keep up with what is happening in the sky. In fact, I have so many that I often can’t keep up with them all! But, for about 20 years, my favorite has been the Observer’s Handbook, published each year by the Royal Astronomical Society of Canada. So, today I looked in my 2007 Observer’s Handbook, and it said that on February 13, at about 14:00 Universal Time (that’s 8am here in Texas), Mercury was stationary.

Huh? Mercury was stationary? Mercury zips around the Sun once every 88 days. It is in an elliptical orbit, and elliptical orbits speed up and slow down. Just a few days ago, February 10, it was at perihelion, the closest point in its orbit to the Sun. It moves fastest at perihelion. By my calculations, it should be moving at over 58 kilometers per second (130,000 mph for my American readers). Does “Mercury stationary” mean that it suddenly stopped in its orbit? No, that’s not it at all.

Rather, as Mercury orbits the Sun, from our vantage point on Earth, the planet seems to swing from one side of the Sun to the other. Back on January 6, Mercury was directly opposite the Sun from Earth. Mercury has since been moving a bit farther to the east in the sky each day. We call this prograde motion. Mercury got as far from the Sun as it was going to get on February 7. Now, it is starting to swing back to pass in front of the Sun, which it will do February 23 at about 05:00 UT (11pm on February 22 here in Texas). But, the Sun itself moves a little bit (nearly a degree) to the east in the sky every day relative to the stars. So, even with Mercury getting closer to the Sun for nearly a week, it has still been getting farther east in the sky. But, finally today, it got as far east as it was going to get. It is now moving toward the Sun’s position in the sky faster and faster as it moves towards inferior conjunction (the point between Earth and the Sun), so it now appears to be moving to the west in the sky. We call this retrograde motion. When it reaches inferior conjunction on February 23, though, don’t expect a repeat of the transit from November 8. This time, Mercury will actually appear to pass just north of the Sun in the sky instead of directly in front of it.

If you want to go see Mercury, then go out about half an hour after sunset and look towards the WSW sky. You’ll see a very bright object. That is Venus. Then, down to the right of Venus is a much dimmer object, about magnitude 1, and that is Mercury. The planet is actually most favorably positioned for observers in the mid northern latitudes this time around. To far north, and it is too low to the horizon. South of the equator, it will also appear lower on the horizon. But don’t delay too much. It is already getting lower on the horizon each night, and soon will be lost in the twilight glow. After that, you’ll have to wait until it is on the other side of the Sun and look for it in the morning sky just before dawn. It will be farthest from the Sun on March 21 or 22 (depending upon where you live). Those mornings will be the best time all year for southern hemisphere observers to go look for it, and it will be relative well placed for observers in the southern part of the northern hemisphere, as well.

-Astroprof

(Sky view created using Starry Night Pro software)