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Moon and Mercury
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 at the same point along the horizon.
Mercury is visible through the glow of our own sunrise right now. It’s quite low in the east about 45 minutes before sunrise. It looks like a fairly bright star, but you need a clear horizon to see it. It’ll stand to the lower left of the Moon tomorrow, and quite close above the Moon on Thursday.
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, there’s a gap of 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 be a bit brighter before sunrise or after sunset — the glare 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.
Tomorrow: Mapping out the Milky Way.