By Gabrielle Galloway
The Moon Phases
The Moon circles Earth under the pull of the planet's gravity. The satellite passes across the sky once every 24 hours, exerting its own considerable gravitational pull on all of Earth's oceans and seas.
The Moon remained an object of legend until 1609. In that year, Galileo first focused his telescope on the details of its surface. He recognized mountains. He saw large, flat, dark areas, which he called "maria" (the Latin word for "seas"). Astronomers now know that there is no water on the Moon, although the term "maria" is still used.
Since then astronomers have built more-powerful telescopes that allowed them to map lunar details. Beginning in the 1960s, artificial satellites and crewed spacecraft were launched to pass near the Moon and eventually to land on it. Pictures from these missions revealed the surface in extraordinary detail. Rocks as small as 12 inches (30.5 centimeters) in diameter were visible. Moon probes have provided detailed photographs of the far side of the Moon . This dark side is always hidden from an Earthbound observer's view.