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Moon and Mercury

April 8, 2016

Astronomers have not only discovered lots of planets in other star systems, they’ve also learned quite a few details about many of them. The list includes a planet’s size and mass, the length of its year, and in some cases, even the length of its day.

Given these amazing discoveries, it’s even more amazing to think that we didn’t know the length of the day on one of the planets in our own solar system until the 1960s.

Mercury is both the smallest planet and the closest to the Sun — and that was part of the problem. It stays quite close to the Sun as seen from Earth, so it’s almost always immersed in the twilight, making it difficult to study. It’s tough to see any features on the surface through a telescope, so it’s hard to measure how fast the planet is rotating.

Even so, careful observations seemed to indicate that Mercury’s day and year were the same length — 88 Earth days.

In the 1960s, though, radio telescopes began scanning the planet. They acted as a radar, sending out a beam of radio waves and then capturing their “echo” for study. That revealed that Mercury spins on its axis once every 59 days, so it makes three full turns for every two orbits around the Sun.

And Mercury is in view right now. It looks like a fairly bright star, quite low in the west not long after sunset. It can be difficult to spot through the twilight, but this evening it lines up to the lower right of the Moon — helping point the way to this tricky little planet.


Script by Damond Benningfield

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