Looking Up: Mercury - Retrograding On A Curve

Sep 19, 2016

The innermost planet
Credit photo from the Mercury Messenger satellite / NASA

This week on Looking Up Tristan tells us the current travel path of the planet Mercury.  

If you are an early riser, one of the most elusive planets in our solar system will be putting on a show this week. The planet Mercury, the innermost of all the planets, is remarkably hard to see. 

Because of the orbital mechanics involved, Mercury, whipping around the Sun every 88 days at a distance of only 36 million miles, just never gets very far from the tremendously bright light of the Sun, relative to observers on Earth. Mercury simply gets swallowed up in the glare of the Sun almost all the time.

But every now and then, to include September 26, 2016, Mercury happens to be at its most distant from the Sun, relative to the Earth, at the same time the Sun is either rising, or setting, for us. And this September 26 is the best day of the entire year to observe Mercury. It will rise in the morning sky about 5:30 AM, nearly a full hour before the Sun pops up over the horizon. So look to the east for a bright diamond in the sky, starting around 5:30. You’ll have until the Sun starts to come up, and its tremendous light swallows up Mercury again.

And Mercury has a very strange orbital period of days and nights, so to speak, due to being in what’s known as a 3:2 resonance. That means Mercury rotates on its axis three times for every two times it goes around the Sun. Put another way, if you stood on the right spot on the surface of Mercury, a bad idea, staring at the Sun, a really bad idea, and you did it for one Mecurian day, you would actually be able to see the Sun rise, travel partially across the sky, then turn around and set from the same place it had risen. Happily, for Earthlings, all you need to do is get up early and look to the east.

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! 

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