Looking Up: A Spot That's Hard To Spot

Jul 9, 2018

Nope, not the moon - it's Mercury with a splash of color.
Credit NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington / nasa.gov

It's a good week to try and find the closest planet to our sun.

Often times, the brightest objects in the sky are our fellow planets. Jupiter, Saturn, and especially Venus blaze in the night sky. But the most elusive of all the planets to see might well be one that isn't farthest from the Sun, but rather is closest, the remarkable planet Mercury.

Mercury is very hard to spot, because it is so close to the Sun. It is only one third as far from the Sun as Earth, and so, from the point of view of Earth, it never seems to be too far out from the Sun. In fact, I actually saw Pluto before I was able to see Mercury. But oh happy day! The night of July 11th, Mercury will be almost as far from the Sun as it ever gets, so it's prime time to spot this elusive sun skimmer. Look for a spot of light down and to the right of the much brighter Venus, just after the Sun slips below the horizon. 

This picture of Mercury was taken by NASA's MESSENGER spacecraft.
Credit nasa.gov

You'll be able to say you saw the hard to spot planet whose year is only 88 Earth-days long, and weirdly, its day is 58 Earth-Days. Mercury is in an odd 3:2 resonance with the Sun, so it has three days every two years, so to speak. Mercury is hot, but it's actually cooler than the somewhat more distant Venus, due to Venus' s think atmosphere trapping heat. And perhaps oddest of all? We think we've spotted water ice on Mercury, at the north and south poles, hidden in the shadows of crater walls that never see the Sun. Pretty cool, I mean cold, right? 

The MESSENGER spacecraft transmitted to Earth the first high-resolution image of Mercury by a spacecraft in over 30 years, since the three Mercury flybys of Mariner 10 in 1974 and 1975.
Credit NASA / nasa.gov

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