In our region of the Milky Way Galaxy, the stars are a long way apart. The Sun’s nearest neighbor is more than four light-years away — 25 trillion miles. In the core of a globular cluster, though, the stars are packed hundreds of times more densely. That means the stars are only a few […]

The geography of the sky owes a lot to Claudius Ptolemy. He was a Greek astronomer who lived and worked in Alexandria, the capital of Greek-controlled Egypt. Almost 1900 years ago, Ptolemy published one of the most important astronomical works in history. Known as the Almagest, it contained […]

Most of the “star pictures” in the night sky look nothing like their namesakes. But one beautiful exception lunges across the southwestern sky on May evenings: Leo, the lion. It’s high in the sky at nightfall. Leo consists of two patterns of stars that the brain puts together to make a […]

Over the centuries, no planet has been as frustrating to study as Mercury. The Sun’s closest planet never moves far from the Sun in our sky. So when astronomers pointed telescopes at Mercury, it was almost always screened by twilight and a thick layer of Earth’s atmosphere. So most of what we […]

The atmosphere of Mars is quite different from Earth’s. It’s colder, thinner, and it’s made mainly of carbon dioxide. In one way, though, the skies of the two planets are similar: both of them have clouds. And for the most part, the clouds themselves are alike as well. Because Mars is so […]