It's been 47 years since we made our first footprints on the moon — here's what you might not know about Earth's dusty companion

It's one of the most defining moments in American history: On July 21, 1969, Neil Armstrong became the first person ever to walk on the moon, taking what he called "one small step for a man, one giant leap for mankind."

Forty-seven years later, we're still learning new things about Earth's dusty companion. And some of the things that we do know about it are pretty mind-blowing.

Here are a few facts about the moon that you might not know:

SEE ALSO: Today is the anniversary of the first moon landing — here's why we're not going back anytime soon

DON'T MISS: SURPRISE: Earth has 2 moons

An astronaut's footprint can last a million years on the surface of the moon.

It may have been decades since we last set foot on the moon, but its surface is still marked with the historic footprints of the 12 astronauts who stomped across it. That's because the moon has no atmosphere. It exists in the vacuum of space where there are no gentle breezes to sweep up the dust and erase the footprints.

There are such things as moonquakes.

During the Apollo missions, astronauts placed seismometers at their landing sights around the moon. They found that the moon actually experiences moonquakes, similar to earthquakes on Earth.

There are at least four different types of these moonquakes:

1. Deep quakes hundreds of miles below the surface, likely caused by tides.

2. Shallow quakes about a couple of dozen miles below the surface.

3. Vibrations that occur when meteorites hit.

4. Quakes caused when the sun heats up the moon's icy crust.

Some of these moonquakes can register up to 5.5 on the Richter scale, which on Earth would be powerful enough to crack plaster. Unlike earthquakes, which last up to only a few minutes, moonquakes can go on and on for as long as 10.

The moon is not shaped like a perfect sphere. In fact, it's more like an egg.

The moon is kind of shaped like an egg. That's because the moon's center of mass is actually a few miles closer to Earth than its actual geometric center.

In addition to being off-center, the moon also has a sort of lumpy gravitational field because of pockets of concentrated mass under some of its basins. One possible explanation for this is that asteroids hitting the moon billions of years ago caused regions of the moon to melt. As these regions dried, they became more dense and concentrated.