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Wednesday, May 31

Earth isn't the only planet that experiences seasons. If a planet's poles are inclined to the plane of the solar system (the ecliptic), then they have seasons. This includes all planets except Mercury. Its polar inclination is 0 degrees. (Well, maybe Jupiter too, because its pole is inclined a mere 3.1 degrees.)

Today, is the first day of spring in Mars's northern hemisphere. You could say that air temperatures on Mars should begin to moderate now and over the next several months as the martian spring turns to summer. But the word "moderate" has a different definition on Mars. Even during high summer, the north polar region rarely gets above the freezing point for water. In the planet's southern hemisphere, meanwhile, it is now autumn and the temperatures are dropping fast. By midwinter, the temperature at Mars' south pole bottoms out at a brisk minus 194 degrees Fahrenheit (minus 126 degrees Celsius).

Of course, this is far more tolerable than the seasonal conditions on, say, Uranus, whose rotational axis is tilted 98 degrees into the ecliptic plane. Because of this tilt and prolonged orbit around the sun, each pole alternately receives 42 Earth years of sunlight and 42 Earth years of darkness.

Getting back to Mars, the red speck of a planet is currently awash in the sun's glare and can only be seen with great difficulty, if at all. I've indicated its position on tonight's map. (You can, however, easily see Mercury floating high above Mars' location. More on that planet in tomorrow's Skywatch dispatch.)

The second map shows how sunrise might appear over the Ares Vallis region of Mars, the site the rover scuttled across a couple of years ago during the Mars Pathfinder mission. Earth's position is marked and it should come as no surprise to see that it, too, is located in the sun's glare, just as Mars is from our vantage point.

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Jeff Kanipe is the author of A Skywatcher's Year, an astronomy guide just published by Cambridge University Press. He is a former editor at Astronomy and StarDate magazines and a writer for the Earth & Sky radio series.

The images in Skywatch are produced by Starry Night software.