How Far Away is the Moon?
Written by Fraser Cain
Chandrayaan-1's first picture of the Moon. Credit: ISRO
How far away is the Moon? The Moon is an average distance of 384,399 km, or 238,854 miles from the Earth.
That's an average, though. The Moon travels in an elliptical orbit around the Earth. Sometimes it's closer and sometimes it's more distant. From here on Earth, the size of the Moon can vary by more than 15% in size from when it's at its closest to when it's at the most distant point. When the Moon is at the closest, called perihelion, it's only 363,104 km from Earth. And then at its most distant point, called aphelion, the Moon is 405,696 km from Earth.
The brightest full Moons occur when the Moon is at the closest. A large, bright full Moon can be 30% brighter than when it's at its the most far away.
Astronomers know the exact distance to the Moon because there are small retroreflectors sitting on the Moon which bounce laser light. They were put there by the Apollo astronauts during their missions for just this purpose. Astronomers fire a laser pulse at the Moon and time how long it takes to make a return trip from Earth to the Moon and back. They can then calculate how far away the Moon is.
We've written many articles about the Moon for Universe Today. Here's an article about how LCROSS discovered buckets of water on the Moon, and here's an article about how long it takes to get to the Moon.
If you'd like more info on the Moon, check out NASA's World Book on the Moon, and here's a link to the Lunar and Planetary Science page on the Moon.
We have recorded several episodes of Astronomy Cast about the Moon. Here's a good one, Episode 113: The Moon, Part 1.
Filed under: Astronomy
Tags: distance from earth, Earth, Moon, planet earth, Solar System, the moon
