Moon Facts Quiz
Orbit and phases
The Moon goes around Earth once every 27.3 days. From one full moon to the next takes a little longer, about 29.5 days, because Earth is moving around the Sun at the same time, and the Moon needs two more days to line up with the Sun and Earth again.
The Moon also spins, exactly once per orbit. This is called tidal locking, and it is why we always see the same face. The far side is not dark: it gets as much sunlight as the near side, we just never see it from Earth.
Phases happen because half of the Moon is always lit by the Sun, and as it travels around us we see different amounts of that lit half, from none at new moon to all of it at full moon. Earth's shadow has nothing to do with phases: it falls on the Moon only during a lunar eclipse.
Surface and gravity
On average the Moon is about 384,000 kilometers away. Light covers that in just over a second, but the Apollo astronauts needed about three days. The gap is so wide that all the other planets of the Solar System would just about fit into it side by side.
The Moon has far less mass than Earth, so its gravity is only about one-sixth as strong. Someone who weighs 90 kilograms on Earth would feel as light as 15 kilograms there, which is why Apollo crews could bounce along in their heavy suits.
The dark patches you can see with the naked eye are called maria, Latin for 'seas'. Early astronomers thought they were water; in fact they are wide plains of old, dark lava. Twelve people have walked on the Moon, on six Apollo missions between 1969 and 1972.
The Moon and Earth
The first people to walk on the Moon were Neil Armstrong and Buzz Aldrin of Apollo 11, on July 20, 1969, while Michael Collins circled overhead. They left a panel of mirrors behind, and scientists still bounce laser beams off it to measure the distance to the Moon to within millimeters.
Those measurements show that the Moon is slowly moving away from Earth, by about 3.8 centimeters a year, roughly as fast as fingernails grow. Over hundreds of millions of years, that adds up.
The Moon's gravity also pulls on the oceans, raising the water slightly on the side of Earth facing it and on the opposite side. As Earth turns, coastlines pass through these bulges, which gives most places two high tides a day. The Sun pulls on the oceans too, but less, because it is so much farther away.