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Tilts and Seasons Earth has an axial tilt of 23.5°. Because of this, Earth has seasons. In the temperate and polar zones, this means a summer, fall, winter and spring. In the equatorial zones, the temperatures are too equal, so it has just a dry and wet season. At the poles, there are two days a year -- six months of daylight and six months of nighttime. For regions a number of degrees from the poles equal to or less than the axial tilt, a planet experiences perpetually day and perpetually night for a fraction of its year. Outside this area, the planet experiences normal day and night cycles. Mercury has the lowest axial tilt for any planet at 0°. On a planet like Earth this would mean perpetually equinox (fall or spring) -- no seasons at all. But Mercury does have different amounts of heat falling on various longitudes. Longitudes? Mercury has a large orbital eccentricity, 0.20. That means the Sun is 40% closer to Mercury than when it is furthest away. That also means that Mercury receives twice as much light (1.4 * 1.4 = 1.96) when closest to Sun compared to furthest. For the orbital eccentricity to become a factor, Mercury's day must be long and it is. Mercury's solar day (the time it takes for the Sun to make one complete turn around the sky as seen from Mercury) is equal to two Mercury years. So on one side of the planet, the Sun is fattest at high noon while being small at both sunset and sunrise. While on the other side, the Sun is small at high noon, while being big at sunset and sunrise. Obviously, the Mercurian day is going to be hotter in the first scenario than the second. Venus has the greatest axial tilt as 177.36°. A tilt of 180° is a half turn. Or put another way it is like turning a ball upside down. This make makes Venus rotate backwards, the only planet to do so. Aside from rotating backwards, a tilt of 177.36° is like a tilt of 2.64°, making Venus similar to Mercury in one way. Venus is similar to Mercury in another way. Venus has a very long day. Its sidereal day is 243 Earth days. It is the only planet with a sidereal day longer than its year (224.68 Earth days). Since Venus rotates backwards, the Venus solar day is shortened to 116.8 Earth days, and makes the Venus year equal to 1.93 Venus solar days. Also, with Mercury, its sidereal day is 2/3 its year (55 Earth days), which makes the long solar day mentioned above. Mars has an axial tilt of 25.19°, along with its 25 Earth-hour day, further makes Mars the most Earthlike planet. Uranus has an axial tilt of 97.86°. Uranus is a world on its side, and as a result has the most extreme seasons, with the sun almost overhead during summer at Uranus' north pole. Triton, the large moon of Neptune, has the highest orbital inclination to the plane of its primary -- 23°. (Actually the orbital inclination is 157°, like 23° but with Triton orbiting the other way -- retrograde.) The norm is less than 1°, with only our Moon having an exceptional 5°. This orbital inclination of 23° combined with an axial tilt of 29.60° from Neptune itself can produce weird seasons. These two numbers can combine or cancel each other out during a cycle that lasts one Neptunium year. Forward to Weather |