Space Cam: Axes are Awesome

As most people probably know, the seasons on Earth are caused by the way it orbits the Sun. This has little to do with the distance between the two bodies—in fact Earth is closest to the Sun in January, height of the Canadian winter—and a lot to do with the tilt of Earth’s axis of rotation. Since the axis of Earth’s rotation is about 23° out of alignment with its orbit around the sun, the amount of sunlight reaching the southern and northern hemispheres changes throughout the year. During each hemisphere’s winter, that hemisphere is tilted away from the sun, receiving less energy. This is a fairly-well-known phenomenon so it will not take up any more of this article.

Instead, it is time to take a closer look at some much more interesting axes. Before that, there are two words that need to be defined: prograde and retrograde. “Prograde motion” is motion that is in the same direction as something else. In the case of the solar system, all of the planets—along with almost everything else—orbit the Sun counter-clockwise when looking down at the Sun’s north pole. Since the Sun also rotates counter clockwise from this perspective, the planets are said to orbit prograde. “Retrograde motion” is the opposite, where a rotation or orbit is opposite to another rotation.

As mentioned, all of the planets orbit the Sun prograde. Most of them also rotate prograde; when looking down from the Sun’s north pole, the planets rotate counter-clockwise, the same as the Sun. The Moon orbits Earth prograde as it rotates prograde. In other words, prograde is typical. There are plenty of exceptions, particularly among small moons and comets, but larger objects tend to be prograde. There are a few very interesting exceptions. The first one is Venus.

Venus has an axial tilt of 177°, meaning that it is only three degrees out from rotating exactly in-line with it’s orbit, the sun eternally almost directly over the equator. Only thing is, the orbit is upside-down because Venus rotates retrograde. Just as curious, this backwards rotation is incredibly slow; it takes Venus 243 Earth days to rotate once but only 225 to orbit the sun. There have been a number of theories over the years as to how this happened. The most pedestrian is that Venus was hit by a series of impacts that slowed down its prograde rotation, then started it slowly backwards.

A second theory is that Venus was created from the merger of two similar-sized, prograde orbiting bodies. If you imagine sitting on the proto-planet orbiting closer to the sun, you would be orbiting faster than the outer planet; this is an extremely important consequence of Kepler’s Laws that govern orbital motion. If you imagine yourself as stationary relative to the Sun, then the outer planet looks like it’s orbiting retrograde. When these two proto-planets merged, they maintained their rotational momentum. They continue to orbit prograde, but since the system was rotating retrograde before, it continues to rotate retrograde as a single mass.

The two above theories have largely fallen out of favour. Instead, it is now thought that Venus’ rotation is slowed down by tidal effects from the Sun. A slight retrograde torque caused by thermal effects in its thick atmosphere causes its slow, but existent, rotation.

Moving on to a completely different system, we come to Uranus. Once again, Uranus rotates retrograde, but with an axial tilt of 98°, the retrograde tag is the least interesting part. Uranus is less like a spinning top as it orbits the sun and more like a ball rolling in a circular path (although, admittedly, there is quite a bit of slip). Just as interestingly, Uranus’ ring system stays with the planet’s equator. This means that the rings of Uranus are broad-side for us to see from Earth, unlike the rings of Saturn that are much closer to edge-on.

Theories are abound as to why Uranus is so off-kilter, just as they do for Venus. One theory is the impact theory, similar to Venus. Computer simulations have shown that if a single impact turned Uranus sideways, then its ring would be orbiting retrograde to the rotation of the planet. The same simulations showed that a series of at least two or more impacts would cause the rings to orbit prograde, as is seen. Therefore, if Uranus’ odd rotation was caused by impact, there must have been at least two of them.

A second theory is that early in the solar systems formation, Jupiter and Saturn entered a 1:2 resonance where Jupiter orbited the sun twice for each orbit of Saturn. Resonances like these do really interesting things to orbits. In this case, it is theorized that the two larger planets could have transferred rotational momentum to Uranus, putting it on its strange path.

Whatever the reasons for our two retrograde-rotating planetary neighbours, the two are fascinating irregularities. Our solar system has only eight planets (nine if the Planet X hunt is successful). Yet in those eight planets, there is a tremendous diversity in all sorts of weird ways. Not only is this interesting in its own right, but it also raises the anticipation for all the discoveries to be made as exoplanets become ever-clearer in our telescopes.