What is a Retrograde Orbit?

"What is a retrograde orbit, and what effect does it have on us?" - Question posed by Colin.

The word retrograde literally means 'moving backwards'. In astronomy, it has two popular uses. One in the term retrograde orbit and another in the term retrograde motion. I'll discuss these two, and then talk about what effect it has on us afterwards.

Retrograde Orbits

All of the planets in our solar system orbit the Sun in the same direction (anti-clockwise, if you're viewing from a position floating somewhere above the Sun's North pole). These general-consensus orbits are known as 'prograde' orbits (literally 'moving forwards'). This direction was, according to science's best theories for the formation of our solar system, set in place right at the beginning, when the cloud of gas and dust from which we were born first started to collapse and rotate. Most of the moons in our solar system trundle along on prograde orbits around their parent planets for the same reasons, but a handful are on retrograde orbits.

This simply means that, when viewed from our imaginary vantage point 'above' the solar system, these moons orbit their parents in a clockwise direction, instead of the more mainstream anti-clockwise choice.

Objects known to be following a retrograde orbit around their parents include Triton, Neptune's moon, and the largest retrograde orbiter we know; Charon, Pluto's largest satellite*; and a number of smaller satellites of Saturn and Jupiter.

On a similar theme, most of the objects in our solar system spin in the same (prograde) direction (again, anti-clockwise as viewed from 'above'), but there are a number of objects with retrograde spin. Venus is the most obvious of these, with a very slow retrograde spin.

Where do retrograde orbits and spins come from?

Current theories for the origin of retrograde orbits include gravitational capture some time after the initial formation of the bodies, and in some cases following violent collisions. Retrograde spins are most likely the result of collisions - unimaginably massive ones in the case of Venus and especially Uranus (which rotates almost perpendicularly to the plane in which the other planets rotate).

Retrograde Motion

This is a trickier one to get your head around, and describes what we see from our vantage point on Earth when we look out at the other planets. Although they're orbiting at a more or less constant speed, from our point of view they sometimes seem to stop, turn around and go backwards in the sky for a while, before turning around and carrying on in the usual direction. The part where the planet is moving, from our point of view, backwards in the sky is what is known as 'retrograde motion'.

It's difficult to describe why this happens, but it's all to do with our relative motion as we overtake*** the other planet in our orbit. Here's Brian Cox explaining it in his Wonders of the Solar System TV series:

What effect does this have on us?

The biggest effect that any of this retrograde stuff has on us is that it gives various peddlers of woo yet another opportunity to stand on their soapboxes and froth at the mouth. Here are some popular-but-wrong things said by such people:

Astrologers would have us believe that if a planet is retrograde then the opposite of what they usually stand for will be true. For example, if astrology would like us to believe that having a particular planet in your chart is good for your finances, then when it's retrograde you should keep a wary eye on your bank account. Thankfully, astrology is mostly bunk****.

The origin of the solar system:
Some people would like us to believe that the handful of objects that orbit the 'wrong' way actually casts doubt on the prevailing model of the formation of the solar system. This is often done so that they can insert their own 'theory' into the 'gap'. Such theories invariably involve gods or other forms of magic. I'm yet to see any actual evidence in support of these ideas, but I have seen copious quantities of spittle.

Our own self-importance:
There are still people who believe that the Earth is the centre of not only our solar system but the entire universe, and use the apparent retrograde motion of the planets as evidence for this. The unfortunate thing for this type of person is that the heliocentric***** model of the solar system actually makes explaining retrograde motion much less complicated than any proposed geocentric****** models do. In your face, woo!

The flippant answer to the question of what effect does retrograde motion have on us? might be
A more thoughtful, philosophical response would be that it is yet another interesting effect of the laws of nature that gives us an opportunity to think about and discuss the universe around us.

Take your pick, and I'm always here if you fancy the second one.

* To be honest, they're almost a binary dwarf-planet system.
** Known as 'superior planets'.
*** Or undertake, I suppose.
**** The few bits that aren't bunk are actually astronomy dressed up in special clothing.
***** 'Helio' refers to the Sun, so heliocentric means 'Sun-centred'.
****** 'Geo' refers to the Earth, so geocentric means 'Earth-centred'.