The Moon and Mercury line up low in tomorrow’s dawn twilight. Mercury looks like a moderately bright star. It’s to the left of the Moon by more than the width of your fist held at arm’s length. The little planet will climb slightly higher into the sky through the end of the month. Even so, it’ll remain low and tough to find.

That highlights a problem astronomers have had from the beginning. Mercury is the closest planet to the Sun, so it never stands far from the Sun in our sky. This month’s appearance is one of the best of the year, yet Mercury is in view only during twilight, which veils its surface. Astronomers also are seeing it through a thick layer of air, so the view is blurry. The combination makes it almost impossible to see any surface features.

As a result, scientists didn’t even know how fast Mercury turns on its axis until the 1960s. They saw the same features every time Mercury was closest to Earth, so they thought the same hemisphere always faced the Sun. That would make the day and the year the same length — about 88 Earth days. In the ’60s, though, they bounced radar beams off Mercury’s surface. That revealed that a day lasts about 59 Earth days.

We learned most of what we know about Mercury from the only two spacecraft to study it. But we should learn much more from a European mission that will orbit Mercury later in this decade — teaching us about a hard-to-see little planet.

Script by Damond Benningfield

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