Size Doesn't Matter When You're This Hot!

May 4, 2015

artist depiction of life on mercury
Credit M. Procell

  This is “Looking UP! in southern Colorado,” from the Colorado Springs Astronomical Society. I’m Hal Bidlack, and there are lots of reasons to look up!

There is a planet in our solar system that is close enough to us, and bright enough, to be easily seen, yet very few people have actually seen it. Can you guess which one it is? 

It’s Mercury, the closest planet to the sun. And it is that closeness that makes Mercury so hard to spot.

Because it orbits the sun inside our own Earth’s orbit, Mercury never appears to get too far from the sun. But right now is one of the few times each year when Mercury is distant enough from the Sun’s blinding glare to be seen by observers on Earth.

So, to see it, we have to have a clear view of the western horizon (a challenge to be sure to those with Pikes Peak in the way), very shortly after sunset. The most brilliant light you will see in the west is the planet Venus, but if you time it just right, very shortly after sunset, you may be able to spot the not quite so bright, but still pretty bright Mercury very low in the western sky.

Mercury is the smallest of the planets, so if you weighed 150 pounds on Earth, you would only weigh 57 pounds on Mercury. There are two moons in the solar system that are bigger than Mercury – Jupiter’s moon Ganymede and Saturn’s moon Titan. Our own moon is almost as big. Like our moon, Mercury is covered in craters.

The temperatures on Mercury are, well, mercurial. It can be as cold as    -279 on the side facing away from the sun and 800 degrees in the sunlight. Yet, even on the sunny side, there are pockets of water ice deep in the shadowed bottoms of craters where the sun never shines. Interestingly, Mercury is not the hottest planet. That honor goes to Venus, which is slightly farther from the Sun, but which has a thick atmosphere to trap and hold heat. Mercury has no significant atmosphere.

Mercury races around the sun, completing a year on Mercury in only 88 Earth days. But it rotates very slowly – a day on Mercury lasts 179 Earth days. So, if you get transferred to Mercury, ask for your vacation in days, not years.

If you’d like to take a closer look at the Mercury or any of the other wonderful and amazing things in the sky, please visit KRCC.org or CSASTRO.org for a link to information on our monthly meetings and our free public star parties! 

This is Hal Bidlack for the Colorado Springs Astronomical Society, telling you to keep looking up, Southern Colorado!