Aside from the Sun, the Moon is the brightest and most recognizable object in the sky. The Moon has no air or water so its surface has been nearly unchanged from the earliest days of the solar system some 4 billion years ago. Many skilled stargazers have examined the surface of the Moon in detail for decades and still never tire of its stark beauty, and professional and amateur astronomers continue to study the history of our solar system as written into its rocky face.
At about 1/4 the diameter of the Earth and some 250,000 miles away, the Moon spans just a tiny slice of sky, about half the width of your little finger held at arm’s length. Even with your unaided eye, you can see light and dark areas on the Moon. The light-colored areas are the lunar highlands, the oldest parts of the Moon’s surface. The highlands are peppered with craters made mostly by stray asteroids and comets left over from the solar system’s formation about 4 billion years ago.
The Moon cycles through phases once every 27 days or so. As it waxes from its “new” phase to full, it’s visible in the evening or night sky; as it wanes back to new, it’s mostly visible in the daytime.
The darker patches on the Moon are newer, about 3 billion years old. They are called the lunar maria (“MAH-ree-ah”) or seas, a misnomer since they are bone-dry and covered with fine dust. The maria were flooded with lava after the solar system had been mostly cleared out of stray material that smashed into the Moon’s surface. That’s why these younger regions are smoother and have far fewer impact craters.
The seas of the Moon come into clear view with a pair of binoculars. So do about a dozen large craters. When the Moon is nearly full, you can see near the south-central part of the Moon (“south” is “down” for observers in the northern hemisphere) the crater Tycho, which has a series of “rays” that shoot out in all directions, giving the Moon the appearance of a peeled orange, with Tycho as the “pip”. The rays are material ejected when Tycho was created by a large asteroid impact about 110 million years ago.
When the Moon is about a day or two past “first quarter” (when it appears half lit), another large crater comes into view near the equator of the Moon. This is the crater Copernicus, a large crater nearly 100 km across.
While binoculars show dozens of sights on the Moon, a small telescope reveals thousands, including craters of all shapes and sizes, arcing mountain ranges that tower thousands of feet above the lunar surface, and cracks and fault lines and strange domes that hint at past geological activity.
Beginners often believe the best time to observe the Moon is when it’s full. But that’s almost always the worst time. You’ll get the best views of the Moon along the terminator, the line that separates night from day. On the terminator, the craters and mountains stand out more clearly in the long shadows of lunar sunset or sunrise.
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