How Big is the Moon?

As our closest neighbor, the Moon has been a focal point of many societies throughout the ages. Although we can cover up the Moon with just our thumb, its exact size baffled humanity for centuries. Just how big is the Moon?

How Big Is The Moon

The Moon in Measurements

Our Moon was once a source of endless volcanic activity that would have constantly shaped and reshaped its surface. Over time, the frigid effects of space set in and cooled it to the lifeless rock we see today. This allows us to gather very detailed information about the satellite’s size.

Size of the moon

As it exists today, the Moon is on average 2,158 miles (3,474 kilometers) in diameter. Due to its axial spin, however, the Moon isn’t quite a perfect sphere. As it rotates, it stretches out ever so slightly at the equator. Since the Moon only rotates once every 28 days, the diameter at the poles versus the equator only differs by 2.5 miles (4 kilometers).

If you were to draw a line around the equator, the distance would be 6,783.5 miles (10,917 km) all the way around. If you set out to walk around the Moon on foot at an average speed without stopping, it would still take you three months to do so. That doesn’t even take into consideration stopping to sleep, eat, or any craters you may encounter along the way.

The Moon’s surface covers an area of 14.6 million square miles (38 million square kilometers). It weighs a whopping 16.2 x 1022 pounds (7.35 x 1022 kg) if it could be placed on a scale.

The Moon Compared to Earth

Compared to the Earth, our unique satellite is quite large. The Moon represents 27% the size of the planet we call home. This moon to planet ratio of one to four is more significant than any other planet/moon combination in the Solar System by a wide margin.

Earth-moon size comparison
The Earth and the moon with their size at the exact same scale. (Image credit: Lsmpascal on Wikimedia Commons CC BY-SA 3.0)

To put some of the above numbers into perspective, the Moon’s diameter fits entirely inside the United States, but not by much. From coast to coast, the U.S. averages 2,800 miles (4,500 kilometers) across. The same holds true for Australia at just under 2,485 miles (4,000 kilometers) from west to east.

If you could unravel the Moon, its surface area would easily fit into the confines of the continent of Asia at 17.2 million square miles (44.5 million square kilometers). For all its heft, the Moon only has 1.2% of the mass of Earth.

The Moon Compared to the Sun

It should come as no surprise that the Moon is tiny next to the Sun. Our star is only 400 times its diameter, and you’d have to place over 64 million Moons inside to fill the Sun up.

The Moon’s Size and Eclipses

These sizes lead to some interesting facts about the Sun, Moon, and eclipses. You see, the distance between the Earth and the Sun is almost exactly 400 times the distance between the Earth and the Moon.

During an eclipse, the Sun and Moon are nearly perfectly aligned on top of one another.
During an eclipse, the Sun and Moon are nearly perfectly aligned on top of one another.

In simple terms, this means the Sun and the Moon appear roughly the same size in our sky. Thus, when we have a solar or lunar eclipse, these two celestial objects fit nearly perfectly on top of one another. This would not be the case if the Moon were any different in size!

How Does the Moon Fare Against Others in the Solar System?

When looking at other moons, ours doesn’t do so bad. The Moon ranks number five out of the over 200 known satellites orbiting planets in the Solar System. Our satellite barely loses out to Jupiter’s moon Io, while other Galilean moons Ganymede and Callisto make up the one and three spots, respectively. Saturn’s largest moon Titan sits comfortably at number two.

Moons of the Solar System To Scale
Size comparison of moons based on mean radius. (Image credit: “Moons of the Solar System To Scale” by Kevin Gill on Flickr CC BY 2.0)

At 3,273 miles (5,268 kilometers), Ganymede is over 1.5 times the size of our only neighbor. Moons start to trail off in size significantly after Triton at number seven, and no one really knows how many other tiny moons are floating out there somewhere.

How Does the Moon’s Size Affect the Earth?

We’re actually quite fortunate to have a Moon of the size that we do. It helps keep many things in check down here that otherwise may compromise life as we know it.

For one, the Moon has a significant impact on our tides. Its pull on the Earth causes water levels to rise and fall at various points of the day and creates the currents that ships use to get around. This further impacts species that rely on the tides to travel and reproduce.

The Earth is known to wobble like a top, but it’s the Moon that keeps us from shaking out of control. Our global climate would feel this the most, throwing the Earth into chaotic and unpredictable weather patterns. Thanks to the Moon’s calming effect, we have at least a relatively stable climate.

What If the Moon Was Bigger?

A larger Moon up in the sky would undoubtedly make for brighter nights and a higher likelihood of solar eclipses in the sky. If the satellite doubled in size, there’s a chance we could become tidally locked, meaning the Moon would always be in the exact same part of the sky. Some people would have to travel halfway around the world to ever even see it.

This supermoon would also significantly slow the speed at which we rotate. We may be looking at 30 or 40 hour days. The Earth would see even less wobble and stabler climates, but the resulting effect on the tides would lead to massive waves or tsunami.

The Moon itself could look a lot different if it were double the size. Chances are it would have a molten core, and regular tectonic movement like the Earth does. This would, in turn, give the Moon a significant magnetic field and perhaps a shot at an atmosphere.

What if the Moon were 10x bigger?

Final Thoughts

The Moon is one of the largest satellites in the Solar System and certainly looks the part floating next to us. Without a doubt, life on Earth would be significantly different without our close friend up in the sky. The satellite is the perfect size to provide a stable climate and help maintain life while offering us a spectacular view.

About Noah Zelvis

Noah is a content writer who has had a love of all things astronomy for as long as he can remember.
When not reaching for the stars, you’ll likely find Noah traveling or running.