Star Points for June 2009 by Curtis Roelle Planetary Seasons Vary Each year on our world marks another cycle of the four seasons. Fall, winter, spring, and summer are familiar to us all. Earth's seasons are caused primarily by the tilt of its rotational axis to the plane of its orbit. During Earth's yearly revolution, the sun's rays fall more or less directly onto given parts of the planet. This variation in solar radiation warms and cools our planet. In the northern hemisphere, summer is defined as the season occurring when the north pole is inclined toward the sun. When it is summer in the northern hemisphere, it is winter in the southern, and vice versa. A couple other planets in our solar system have seasons as well, but the seasons there are not at all like the ones we're used to. Furthermore, the causes for the changes are different. Mercury, for example, experiences changes due to its "eccentric" orbit (very elliptical, or egg- shaped). Mercury has the most eccentric orbit of any of the eight major planets, and since it also lacks any appreciable atmosphere, Mercury lacks seasons as we know them. The greatest cause of changing temperatures on Mercury is literally the difference between day and night, and temperatures between Mercury's daytime and nighttime sides vary by more than 1,000 degrees Fahrenheit. Venus' orbit is nearly circular, like Earth's, but there is hardly any axial tilt, so the weather has no major variations during a Venusian year. Venus also has an atmospheric pressure 90 times greater than Earth's, and the clouds in Venus' thick carbon dioxide atmosphere trap heat like a greenhouse. The surface temperature of Venus is 900 degrees Fahrenheit - hotter than a broiler! Some characteristics of Mars and its weather are similar to Earth's. A day on Mars is only 40 minutes longer than our 24-hour day, and the polar (rotational) axis on Mars is tilted only about one degree more than ours. But the orbit of Mars is significantly more eccentric than Earth's, which causes its seasons to be of unequal length. In the northern hemisphere, where summer lasts 178 days, "Martians" enjoy a shorter, 154-day winter. Mars also has weather features visible in telescopes. The most prominent features are icy polar caps in both hemispheres that shrink and expand during the changing seasons. Clouds and hazes also form over the surface, and are most often seen around specific features and along the terminator dividing night and day. Martian winds can whip dust into obscuring dust storms that may last for months and envelop the entire planet. The four outer "gas giant" planets have atmospheres completely unlike Earth's. Seasons as we know them don't exist. Jupiter, Saturn, Uranus, and Neptune do not have solid surfaces like the terrestrial planets Mercury, Venus, Earth, and Mars. Their gaseous material grows denser with depth. It's worth mentioning that Saturn is now approaching the spring season in its northern hemisphere, and as it does so, its rings are becoming progressively "thinner" in our telescopes. In a few months they will appear exactly edge-on, so thin that no earth-based telescope can detect them. Currently Saturn is visible in the evening sky in the constellation Leo the Lion, after sunset. Its rings should still be easily visible, even in small telescopes. Watch Saturn over the next few weeks to observe its "vanishing" rings. Note that the shadow of the rings may still be visible on the planet. The last time the rings appeared edge on was in 1996.