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Evening Mercury

April 26, 2015

The planet Mercury just peeks into view in the early evening the next couple of weeks. But it’s quite low in the sky, so you need good timing and a clear horizon to see it.

Because Mercury is the closest planet to the Sun, it’s almost always immersed in the twilight. It’s also the smallest of the Sun’s planets. The combination makes it tough to study. In fact, astronomers couldn’t even measure how fast it rotates until 50 years ago this month.

By trying to track surface features as Mercury turns, astronomers had concluded that the planet was locked so that the same hemisphere always faced the Sun. Since it takes 88 Earth days for Mercury to make one orbit around the Sun, that meant it would also take 88 Earth days to make one turn on its axis.

In the early 1960s, though, astronomers began bouncing radio waves off the planet’s surface. The way those waves “echoed” back to Earth revealed how fast the planet was spinning.

Observations made in April 1965 revealed that Mercury spins once every 59 days. That means it makes three turns on its axis for every two orbits around the Sun — a pattern unmatched by any other planet.

Look for Mercury quite low in the west-northwest beginning about 30 minutes after sunset. It looks like a bright star, far to the lower right of Venus, the “evening” star. Mercury will climb a bit farther from the Sun over the next few days, but it’ll also fade, dampening the view of this hard-to-study world.


Script by Damond Benningfield, Copyright 2015

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