I went to an Alaska Writers Guild meeting Tuesday night, and mentioned Friday’s post on the effect of orbital tilt. This led to a discussion of day length, and I realized that while I knew some planets had really weird day lengths, I wasn’t sure which ones. (I thought it was the inner planets, which turned out to be right.) So as long as I was looking the information up, I thought I’d share it.

Mercury

Mercury (Wikimedia)
Mercury turns out to be the planet whose days are longer than its years. For many years the planet was thought to keep the same side facing the sun all the time: one rotation about its axis relative to the stars for each revolution around the sun. We now know it rotates three times for each two revolutions around the sun, making its days a year and a half long. Luckily it’s a short year (88 Earth days.) Its tilt, by the way, is so near zero it is hard to measure. (Its closeness to the sun doesn’t help.)

Venus

Venus, Hubble photo
Venus is the really weird one. Its rotation is in the opposite direction from its revolution around the sun, so from the surface of Venus, the sun would appear to rise in the west! At perihelion the sun may actually appear to stand still or go backward in the sky. That is, it would if you could see the sun through the sulfuric acid clouds. A Venusian day is long, however: 116.75 Earth days. A Venusian year is 1.92 Venusian days or 224.65 Earth days long. The tilt of its axis is only about 3.4°.

Mars

Mars (Hubble)Mars is easily the least different from Earth when it comes to day length: 24 hours 39 minutes and slightly more than 35 seconds. This is more precise than is generally stated for the other planets, quite simply because Mars is the planet with human-piloted rovers on its surface, and to have daylight, these pilots must work on Martian days (or sols) even though they are located on Earth. (Pilot may not be quite the right word, given that radio communications take 4 to 20 minutes to get to Mars.) Its axial tilt is also similar to Earth’s: 25.2°. A Mars year is 1.8809 Earth years.

Jupiter

Jupiter (Hubble)Jupiter has the fastest rotation rate, and thus the shortest day, of any of the planets: slightly less than 10 hours. Why the vagueness? All we can see of Jupiter is the cloud tops, and those rotate at slightly different speeds at different latitudes. It is clear, however that Jupiter’s days are very short, especially compared with its year length of 11.86 Earth years. Its axial tilt is small, only 3.13°.

Saturn

Saturn (Hubble)Saturn, like Jupiter, rotates fast and the rotation seems to vary with latitude but is slightly more than 10 hours. The year, however, is over twice the length of Jupiter’s – 29.46 years. The axial tilt is relatively large: 26.73°, which is why the visibility of Saturn’s rings from Earth varies so much. Seasonality is probably weakened by internal heating and the large distance from the sun.

Uranus

Uranus (Hubble)Uranus rotates slower than the gas giants but still faster than earth, with a day length of 17 hours, 14 minutes. Its year is 84 Earth years long. It is a few years past an equinox (2007) and won’t reach another solstice until 2028. There is some question as to which is the north pole, since its axis is either tilted at 97.77° with normal rotation or 82.14° with retrograde rotation.

Neptune

Neptune (Hubble)Neptune has a day length of roughly 16.11 hours. Very roughly – Neptune has even more variation in rotation rate of the cloud tops with latitude than does Jupiter, with apparent rotation periods varying from 12 hours at the poles to 18 hours at the equator. Its tilt is a little larger than earth’s, about 28.32°, which should give it pronounced seasons, though not as pronounced as those of Uranus! It year is roughly 164.8 years.

All of this variation is just in our own solar system. What else may be out there?

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