The Unusual Inclination of Uranus

Remember way back in June when we talked about the strange inclination of Venus? Believe it or not, there is one planet that has a stranger inclination. Here, the Sun does not rise in the west and set in the east. But then, it doesn't really rise in the east or set in the west all year long. For a portion of its year, Uranus has a hemisphere completely illuminated and another hemisphere completely in the dark.

Uranus inclination is about 98°, which means that its rotational axis is nearly perpendicular to its orbital axis. Another way of saying this, is that Uranus rotates on its side. This means that for a portion of its year, the north pole of Uranus is almost pointed straight at the Sun and the northern hemisphere is almost completely sunlight while the south pole is pointed away from the Sun and therefore, the southern hemisphere is dark. Vice versa, halfway around its orbit, the south pole is pointed towards the Sun and the north pole away.

This phenomena happens on Earth, but not as extreme. There are two latitudes, one north of the equator and one south of the equator, where in the northern summer, at any location north of the northern latitude the Sun will never set and in the northern winter, the Sun will never rise. The same thing happens in the southern latitudes near Antarctica. These latitudes are called the Arctic Circle and the Antarctic Circle. If you have heard of "The Land of the Midnight Sun", this is in reference to the Arctic Circle. On Earth, these circles are at 66.5° North for the Arctic Circle and 66.5° South for the Antarctic Circle. If you recall from science class, that the Earth is tilted 23.5° with respect to the Earth's orbital plane, those two angles equal 90°. On Uranus, it is a little more complicated than that.

When the north pole of Uranus is pointed towards the Sun (or the northern "summer" on Uranus), any latitude north of 8°N will never see the Sun set and any latitude south of 8°S on Uranus will never see the Sun rise. Vice versa, in the southern "summer" on Uranus, any latitude south of 8°S will never see the Sun set and any latitude north of 8°N will never see the Sun rise.

Why does Uranus have such an extreme inclination? Much like Venus, it is believed that soon after formation, Uranus was hit by a large object that knocked it on its side, but because it has a much larger mass than Venus, it did not flip upside down, but rather just onto its side.