The Messenger spacecraft captured a sliver of the southern
hemisphere of Mercury in this recent image. Although Mercury is
relatively close to Earth, it stays so near the Sun in our sky
that it is difficult to study with ground-based telescopes,
which reveal almost no surface features. In fact, astronomers
couldn't even measure the length of Mercury's day until they
began using radio telescopes in the 1960s. Even today, the only
way to get a clear view of Mercury is from a spacecraft like
Messenger, which is the first probe ever to orbit the Sun's
closest planet. [NASA/JHUAPL/CIW] As recently as the 1960s,
astronomy textbooks reported that the length of a day and a year
on the planet Mercury were the same. That’s because astronomers
thought the same side of the planet always faced the Sun, just
as the same side of the Moon always faces Earth. That would
create a world where one hemisphere broiled in constant
sunlight, while the other froze in perpetual night. It turns out
that that’s not the case, though. Mer