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How big is the Distance from Earth to the Moon?

380,000 kilometres!

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Distance from Earth to the Moon

The Moon orbits Earth at an average distance of 384,400 kilometers (238,900 miles), measured from the center of Earth to the center of the Moon. The distance changes all the time, because the Moon's orbit is not a perfect circle. Light crosses it in about 1.3 seconds.

Distance

The Moon travels around Earth on an ellipse, a slightly stretched circle, once every 27.3 days. On an average orbit it comes within about 363,300 kilometers (225,700 miles) of Earth at its closest point, called perigee. It is about 405,500 kilometers (252,000 miles) away at its farthest point, called apogee.

The pull of the Sun stretches and squeezes the orbit a little from month to month. As a result, over a year the distance ranges from about 357,000 kilometers (222,000 miles) to about 407,000 kilometers (253,000 miles). The figure of 384,400 kilometers is the semimajor axis of the orbit, half its longest width. Averaged over time, the distance is slightly larger, about 385,000 kilometers.

Size comparisons

  • About 30 Earths lined up side by side would span the average distance, and about 110 Moons.
  • If Earth were shrunk to the size of a basketball, the Moon would be about the size of a tennis ball, about 7 meters (24 feet) away.
  • Kapteyn's Star, a red dwarf star and the next larger object in the explorer, is a little wider than this distance. TrES-4b, a giant planet and the next smaller object, is about two-thirds as wide.
  • The distance from Earth to the Sun is about 390 times as far.
  • A car driving nonstop at 100 kilometers per hour (62 miles per hour) would take about 160 days to cover it.

Measuring it with lasers

Since 1969, astronomers have measured the distance to the Moon with lasers. Astronauts on Apollo 11, 14 and 15 left arrays of reflectors on the surface, and two Soviet rovers, Lunokhod 1 and 2, carried more. A telescope on Earth fires short pulses of laser light at one reflector and times how long the light takes to come back, about 2.6 seconds.

Hitting a reflector is difficult. Even a narrow laser beam spreads to about 2 kilometers wide by the time it reaches the Moon, so only about one in 25 million photons, or particles of light, hits the Apollo 11 reflector. Even so, the measurements have become far more precise, from a few tenths of a meter at first to a few millimeters now.

The Moon is drifting away

These measurements show that the Moon is slowly moving away from Earth, by about 3.8 centimeters (1.5 inches) a year. The cause is the tides that the Moon raises on Earth, which gradually shift the Moon into a wider orbit. Since the first reflector was placed in 1969, the Moon has moved about 2 meters (7 feet) farther away.

Sources

Scale of the Universe > Distance from Earth to the Moon

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