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Mercury in the Morning

January 23, 2010

The little planet Mercury just peeks into the dawn sky over the next few days. It's in the east-southeast about 45 minutes before sunrise, and looks like a bright star. It's so low in the sky, though, that it's hard to pick out -- any clutter along the horizon will block it from view.

Sunrise on Mercury can be an odd affair. At certain times in Mercury's orbit, the Sun can actually climb well up into the sky, then stop, reverse direction, and drop back below the horizon. And the reverse happens at sunset -- the Sun sets, climbs back into view, then sets again.

Among other reasons, that's because Mercury's orbit around the Sun is stretched out. When the planet is farthest from the Sun, it's about half again as far as when it's closest. In fact, Mercury's orbit is more elongated than that of any other planet in the solar system.

In addition to the odd sunrises and sunsets, that creates a couple of other interesting effects. The Sun looks about half again as big when Mercury is at its closest, for example, and it's about twice as bright. Mercury not only gets more visible light, it gets stronger doses of ultraviolet and X-rays, too. Engineers have to keep that in mind when they design spacecraft to head to Mercury -- a craft needs extra shielding against the heat and radiation of the nearby Sun.

Again, look for Mercury quite low in the east-southeast during early twilight over the next few mornings.

Script by Damond Benningfield, Copyright 2009

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