This coming summer in 2018, Mars will be spectacular in our night sky as it outshines all the stars and planets except Venus. It won’t be quite as bright as it was in 2003, but nearly! In honor of this, the next few discussions will be about various topics connected to Mars.
To begin, let’s look at the motion of Mars in the sky. Of course due to the rotation of the Earth, Mars rises in the east and sets in the west just like all of the other astronomical objects in the sky. But when we look to how Mars moves relative to the background stars, that is when things get interesting. If you look at Mars from one night to the next, you will find the planet a little further east with each viewing. So relative to the background stars, Mars appears to move from west to east from one night to the next and this typical motion is called prograde motion. About every 2 years, though, there are a couple of months when Mars seems to change direction and move east to west relative to the background stars. This “non-typical” motion is called retrograde motion. This retrograde motion was very confusing to early sky watchers who initially thought it represented an angry god or some other mystical meaning. Later, it was believed to represent the actual motion of Mars in its orbit (we’ll discuss this more next time).
Ultimately, understanding this retrograde motion was arguably the origin of modern astronomy and science. And conveniently, in addition to Mars being very bright this summer, Mars will undergo retrograde motion from June 26 to August 27. Be sure to watch the night skies and keep an eye out for it!
Follow your curiosity to the Fred G. Dale Planetarium at Wayne State College.