Question: What is the distance to the Moon?
Answer: Here’s a quick answer. The average distance from the centre of the Earth to the center of the Moon is 384,403 km (238,857 miles).
Wait!
Before you write that into your homework assignment, you’ve got to realize that the Moon travels around the Earth in an ellipse. That means it gets closer and further, depending on where it is in its orbit. At its closest point, the Moon gets to 363,104 km (225,622 miles), and at its furthest point, it’s 405,696 km (252,088 miles).
This variation can make the size of the Moon noticeably different from full Moon to full Moon. When the Moon is at its closest point, this is called perigee. Here’s a picture that illustrates the difference.
How have scientists been able to measure this distance so accurately? When the Apollo astronauts visited the Moon, they left behind some special mirrors that reflect light back towards the Earth. Scientists here on Earth can fire lasers at the Moon, and measure the time it takes to make the trip within the scale of picoseconds. Four of these lunar retro-reflectors are still functional.
In fact, their experiments are so accurate they can measure the distance to the Moon within millimeters. For more information on this project, check out the APOLLO experiment page.
Now that you know the distance to the Moon, find out how long it takes to get to the Moon.
Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay. He's the forum co-administrator of the Bad Astronomy/Universe Today forum with Bad Astronomer Phil Plait.
