Tag Archive: Uranus

UPrior to the invention of the telescope, five planets were known: Mercury, Venus, Mars, Jupiter and Saturn. Earth, of course, is a planet also, but this was not recognized until the acceptance of the Copernican model of the Solar System. A seventh planet, however, is visible without a telescope: Uranus.

Hubble false-color infrared image of Uranus (Source)

Hubble false-color infrared image of Uranus (Source)

Uranus was not recognized as a planet until telescopes became available because it is so dim relative to the classical planets. In fact, at magnitude between 5 and 6 it is not visible to most people today, simply because artificial light has made truly dark skies hard to find.

The oddest thing about Uranus is that its pole is almost in the plane of the ecliptic. In fact, which is the North Pole depends on how north is defined. On earth, the sun, and every other planet, the right-had rule reigns. If the fingers of the right hand are wrapped around the equator with the finger pointing in the direction of rotation, the thumb points north. On that basis, the North Pole of Uranus is on the wrong side of the ecliptic. On the other hand if the astronomical definition is used, that the North Pole is the pole on the same side of the ecliptic as the Earth’s North Pole, the planet is rotating backward, with the sun rising in the west.

Like the gas giants, Uranus has rings, which being equatorial are nearly at right angles to the Ecliptic*. Its weather is not well understood, and its seasons must be extreme. After all, its Arctic and Antarctic circles are almost at its equator, while its tropics of Cancer and Capricorn are very close to its poles.

Wouldn’t working that weather into a science fiction story be fun?

(If the word Ecliptic is new to you, it is the plane of the earth’s orbit.)



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 (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, 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 (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 (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 (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 (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 (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?