Astronomy Picture of the Day

NASA ^ | 5/11/08 | Tunç Tezel (TWAN)

[sig226]

Retrograde Mars
Credit & Copyright: Tunç Tezel (TWAN)

Explanation: Why would Mars appear to move backwards? Most of the time, the apparent motion of Mars in Earth's sky is in one direction, slow but steady in front of the far distant stars. About every two years, however, the Earth passes Mars as they orbit around the Sun. During the most recent such pass over the last year, the proximity of Mars made the red planet appear larger and brighter than usual. Also during this time, Mars appeared to move backwards in the sky, a phenomenon called retrograde motion. Pictured above is a series of images digitally stacked so that all of the stars images coincide. Here, Mars appears to trace out a loop in the sky. Near the top of the loop, Earth passed Mars and the retrograde motion was the highest. Retrograde motion can also be seen for other Solar System planets.

[sig226]

[svcw]

I love the fact everything is labeled in space.

[alloysteel]

Retrograde motion? We are just passing Mars on the inside track as we speed around the Sun on a much shorter radius.

Planets closer in have to travel in their orbits at a much different speed just to keep from being drawn into the Sun. Something about conservation of energy and the sum of the vectors acting upon the object.

[RightWhale]

That was a great show last fall. We had enough clear nights to follow Venus and Mars through the whole retro motion.

[Squawk 8888]

The key is that the shorter the radius of the orbit, the higher the velocity needs to be. Think of orbiting as throwing yourself at the ground and missing- the further you are from the surface, the lower the speed needed to clear the horizon.

[Lancey Howard]

I love the fact everything is labeled in space.

You can thank Adrian Monk for that.

[Saundra Duffy]

I took Astronomy in college; got an A; and we studied retrograde motion - BUT I STILL DON’T GET IT!!!

[Levante]

Wow, looks like the aliens are into garbage collection!

[Delta 21]

Spirograph in the sky. Cool!

[Delta 21]

Its all relative.

[xJones]

I took Astronomy in college; got an A; and we studied retrograde motion - BUT I STILL DON’T GET IT!!!

You are a voice crying in the wilderness for so many of us.

[xJones]

Its all relative.

Yes, but whose relatives? Einstein's?

Now if it was one of MY relatives Mars would doing back flips instead of perceived retrograde horizontal plane motion......

but I digress. Nevermind.

[Revolting cat!]

Hollywood Freeway at night.

[plsvn]

I took Astronomy in college; got an A; and we studied retrograde motion - BUT I STILL DON’T GET IT!!!

It's easy, think of it this way... You're on the expressway, you're (Earth) in the left lane traveling faster than a car in the right lane (Mars). When you're a great deal behind the car in the right lane, it looks like that car is traveling forward just like you. But when you come up beside the car, it looks like the car is traveling backwards (yet both cars are obviously moving forwards, you're just going a little bit faster). And when you get sufficiently ahead of the car in the right lane, you no longer have the illusion it's going backwards.

Now pretend the road you're traveling is circular, and the analogy hopefully translates even better to what happens when Earth "laps" Mars every couple of years.

[Ciexyz]

Thanks for the explanation.

[sig226]

Under the right conditions, the Hollywood Freeway gives the illusion that the cars are moving. I used to live next to that Godawful road.