How big is the moon really and how did we measure it?

The struggle to measure the moon is a longstanding one.
John Loeffler

Earth's moon is the largest and brightest object in the night sky by far, so it has always been a subject of human fascination and even worship, but how big is the moon really?

As a natural satellite of Earth, the moon is clearly the smaller of the two, but how much smaller is it? How much of the Earth would the moon cover if it sat on Earth's surface? And how does our moon compare to the moons of other planets in the solar system?

How big is the moon compared to Earth?

The moon, like the Earth, isn't a perfect sphere, so we measure its size by its mean radius, which is 1,079.6 miles (about 1,737.4 kilometers), with a mean diameter of 2,159.2 miles (about 3,475 kilometers), and an equatorial circumference of 6,783.33 miles (about 10,917 kilometers).

The mean radius of the Earth is 3,958.75 miles (about 6,371), with Earth's diameter measuring about 7,917.5 miles (about 12,742 kilometers), giving an equatorial circumference of 24,901.45 miles (about 40,075 kilometers).

This puts the size of the moon at about 27% of the size of the Earth, which is the largest ratio for a moon to its planet in terms of size in the entire solar system.

The moon is also the fifth largest satellite in the solar system, but its mass is relatively tiny compared to Earth's.

The moon has a mass of 7.342×1022 kilograms, compared to the Earth's mass of 5.972 × 1024 kilogram, giving the moon about 1.25% the mass of the Earth.

In terms of volume, the moon measures 2.1958 × 1010 kilometer3, compared to the volume of the Earth, 1.08321×1012 kilometers3, which puts the moon's volume around 2% that of Earth's.

How did we measure the size of the moon?

The general size of the moon has been known since antiquity, thanks to the Greek astronomer Aristarchus of Samos (310 BCE — 230 BCE).

Working off the calculation of the Earth's diameter calculated by the Greek mathematician Eratosthenes (276 BCE — 194 BCE), Aristarchus measured the ratio of the angular sizes of the shadow cast by the Earth on the moon during a lunar eclipse and recognized that the moon almost perfectly blocks out the sun during a solar eclipse (only the corona is visible).

So, using some trigonometry, Aristarchus worked out that the diameter of the moon was about 0.32 to 0.40 times the diameter of the Earth, which overshoots it a bit (the actual figure is 0.27), but all in all, not bad for an ancient using nothing but shadows and math.

The same technique was more or less employed ever since and was only replaced once we got a more proper measuring device with the invention of LIDAR in the 1950s. LIDAR gives us the current measurements that we use today.

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Does the Moon have more land than Earth?

The moon has far less land than Earth, even though Earth is mostly covered by oceans and the moon is all landmass.

The moon has a total surface area of 37,930,000 kilometers2 (about 14.6 million square miles) which sounds like a lot, but the total surface area of Earth (including ocean surface) is 510,072,000 kilometers2 (196,900,000 square miles). Now. if we just considered the actual landmass of Earth for an apples-to-apples comparison, Earth's surface area not covered by oceans is about 148,940,000 kilometers2.

This gives the moon just 25% of the surface area of the parts of Earth covered by land, or about 7.5% of the total surface area of Earth.  

Is the US bigger than the moon?

The width of the continental United States is about 2,800 miles wide (about 4,506 kilometers, when measured horizontally from the eastern seaboard to the west coast) compared to the moon's diameter of 2,159.2 miles (about 3,475 kilometers), so in terms of width versus diameter, the US is bigger than the moon.

But if you measured the US from north to south, you would get a different result. The US is about 1,582 miles (2,545 kilometers) north to south, so the moon's diameter is wider than the US is 'tall'.

How many states can fit on the moon?

If you strictly overlayed a map of the United States on an image of the moon, you would be able to fit all of the states from the Rocky Mountains in the west to the Appalachian Mountains to the east.

If you are talking about surface area, then the entire US could fit on the moon, along with China, Europe, Brazil, and many other smaller nations.

What country is closest to the size of the moon?

No one country comes close to matching the moon in terms of surface area.

But considering that Russia is the largest country on Earth by width and surface area, the Russian Federation would come the closest to matching the size of the moon.

For context, you would still be able to fit all of North America onto the remaining surface area of the moon after factoring in Russia.

The moon might be smaller than Earth, but that doesn't make it small. 

What happens if the moon crashed into Earth?

Thanks to the most recent Roland Emmerich disaster flick Moonfall, a lot of people are wondering what would happen if the moon ever did fall to Earth.

First, the moon is actually moving away from us by about four centimeters a year thanks to the tidal forces the moon exerts on Earth (which is actually slowing Earth's rotation down ever so slightly), so the moon is going in the wrong direction, so we aren't in any danger of getting hit by the moon any time soon.

Still, let's assume that the moon was going to smash into the planet, what then?

First, the tides produced by the moon would be significantly stronger, producing major flooding events in the lead up to a collision. These tidal forces would also pull on and disrupt Earth's interior, producing volcanoes and earthquakes of violence.

Obviously, the real devastation would occur with an impact event, one that would effectively kill the entire planet, but it's likely that nothing would be alive to see it since the approaching orbit of the moon would produce miles-tall tides that would sweep over the planet several times a day, similar to what happened around Gargantua in the film Interstellar.  

Assuming that somehow, something survived the extreme tidal forces wreaking havoc on Earth, the moon would be torn the moment it passed the Roche limit, which is the minimum distance from Earth where the tidal forces Earth exerts on the moon would be more powerful than the force of gravity holding the moon together.

It would break apart into very large chunks and rain devastation on the planet similar to the early bombardment period in Earth's early history. There might very well be pieces of the moon as large as some of the very largest asteroids, and an impact from one of those is enough to break open the Earth's crust, expose the mantle, and blanket the Earth in a superheated cloud of gas of more than a thousand degrees Fahrenheit for more than a year.

This is enough to boil off the planet's oceans and sterilize the surface of the planet, cleansing it of all traces of life that hadn't been wiped out yet.

Earth would also likely have a short-lived set of rings made from what's left of the moon, but that would soon fall to Earth and disappear. Eventually, the superheated gas would dissipate, and the supersaturated clouds containing all of the planet's water would inundate Earth.

Eventually, the scar from the moon's impact would cool and heal, and over time, the water cycle would return, temperatures would lower, and Earth would be reset to a time just before the emergence of the earliest single-celled organisms.

Needless to say, the moon falling into the Earth would be bad. Fortunately, despite how big the moon looks, there's no chance of that happening any time soon so we can continue to enjoy supermoons, blood moons, and all the other lunar phenomena we've come to celebrate throughout human history.

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