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Moon and Mercury

August 14, 2012

If you like sunrises and sunsets, the planet Mercury would be an interesting world to visit. The planet has no atmosphere, so there’s none of the vivid color that comes with sunrise and sunset here on Earth. But Mercury has something that Earth doesn’t. It can have two sunrises and sunsets, or two sunsets and sunrises, all within a few hours of each other — and all at the same point along the horizon.

Mercury is visible for a few days through the glow of our own sunrise. It’s quite low in the east about 30 or 40 minutes before sunrise. It looks like a fairly bright star, but you need a clear horizon to see it. It’s easier to find tomorrow morning, because it’s close to the lower left of the Moon.

The odd mixture of sunrises and sunsets is a result of Mercury’s long day and its orbit around the Sun. Mercury spins slowly, so on average, it’s about six Earth months between one sunrise and the next. But Mercury’s orbit is stretched out, so its orbital speed varies.

As a result, the Sun moves across the sky at an uneven pace. It can stop and even reverse direction — creating the odd sequences of sunrises and sunsets.

Although there’s no air to create twilight, the sky would brighten a bit before the Sun climbed into view — the result of the Sun’s hot outer atmosphere, called the corona. The silvery corona would form a hazy dome of light above the horizon — one more reason to enjoy a sunrise on the little planet Mercury.

Script by Damond Benningfield, Copyright 2012


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