The Earth's Moon is the easiest thing to spot in the night sky. Unlike a planet that orbits around the Sun, a moon is an object that orbits around a planet. We call the moon that orbits around the Earth "the Moon," but many more moons exist in out solar system.
Scientists believe the Moon has accompanied Earth since the beginning. Earth had barely formed when a Mars-sized object struck it obliquely. The blow sent a spray of vaporized rock into space and formed into the Moon. The Moon is 238,860 miles (384,400 km) miles from Earth. The Moon is 2160 miles (3475km) in diameter and is an airless, dry surface covered with plains and craters.
The moon circles the Earth every 29.3 days, moving through a cycle of phases. This cycle formed the basis for most early calendars and led to the 12-month year that is used today. The gravity of the Moon raises tides on Earth, while Earth's gravity locks the Moon's rotation to keep the same side always turned towards Earth. Lunar tides raise the ocean twice a day. One bulge of water, on the side nearest the Moon, marks where its gravity is pulling the water away from Earth. The other bulge, on the opposite side of Earth, marks where the Moon is pulling Earth away from the water.
Asteroids bombarded the Moon and so it has a heavily cratered surface. Some craters measure 55 miles (90 km) from rim to rim. Because the moon lacks air it can be very hot and very cold. The lunar high-noon temperature reaches 134 degrees Celsius. On the Moon's night side the temperature drops to about –170 degrees Celsius.
About 3 billion years ago, the Moon heated up and melted. The surface rock later hardened, and then molten rock below welled up through cracks in the surface, flooding huge areas. Today we see these areas of solid lava as the Moon's "seas," or "maria" from the Latin word sea.
The Moon now appears to be geologically dead. Six manned missions to the moon brought back hundreds of pounds of rocks that have done much to unfold the moon's history. The moon is our best known, as well as nearest neighbor in space.
Phases of the Moon
The relation of the Moon in relation to the Sun affects how we see the Moon from Earth. The Moon has no light of its own. It reflects sunlight. The Moon is always round. It looks like it changes shape because as it orbits the Earth, we see different parts of the lighted side. This is known as the Phases of the Moon. The Moon travels around the earth about every 28 days. As it travels its bright side (side facing the Sun) faces the Earth and looks like a big bright ball of light in the sky. As it moves around the Earth and only part of the bright side faces us, we see a
and then a
. The Moon continues to move and we see less and less of the lighted area. When we see no more of the lighted area and it looks like the Moon has disappeared, we call it a
. The Moon continues to move around the Earth, and as it does, we begin to see more and more of the lighted area. At last we see that big bright ball again and call it a
. There are eight phases: New Moon, Crescent, First Quarter, Gibbous, Full Moon, Gibbous, Last Quarter, Crescent.