Retrograde Motion | Universe Today

Written by Fraser Cain

All of the planets orbit around the Sun in the same direction, but from our position here on Earth, the planets can sometimes seem to go backwards in the sky, going in retrograde motion? What's going on?

This strange behavior of the planets used to baffle ancient astronomers, and they created elaborate models of the Solar System to explain how the planets could have retrograde motion. But their mistake was that they put the Earth in the center of the Solar System. Once they realized that the Earth was a planet, just like the other planets, the retrograde motion was understood.

Whenever we see a planet, we're seeing its position against a background of more distant stars. But the Earth is moving faster in its orbit that than the outer planets. As Earth catches up to a more distant object like Mars in the sky, it appears as if Mars is traveling backwards. In just the same way it looks like a slower moving car is going backwards compared to you. But it's really going forwards, just less quickly compared to you.

All of the planets formed out of the same stellar nebula that created the Sun 4.6 billion years ago. The conservation of angular momentum set the whole Solar System spinning in the same direction, which is why all the planets in the Solar System orbit the Sun in the same direction that the Sun rotates. And the planets all rotate in the same direction; seen from the north pole, they'rell turning in a counter-clockwise direction. There's one exception,though. Venus is rotating in a clockwise direction, seen from the north pole. Most of the planets' moons orbit in the same direction that the planets spin; however, there are a few moons that orbit in a retrograde direction. The best example of this is Neptune's moon Triton, which orbits in the opposite direction that Neptune spins.

We have written many articles about the retrograde motion of planets in the Solar System. Here's an article about Mercury in retrograde, and here's an article about Jupiter in retrograde.

If you'd like more information about orbits, check out NASA's real-time tracking tool.

We've also recorded an episode of Astronomy Cast about Neptune and its moon Triton, which follows a retrograde orbit. Listen here, Episode 63: Neptune.

Filed under: Astronomy

Tags: orbit, orbits, retrograde, retrograde motion, retrograde orbit