Question: how big is the moon in diameter?

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  1. The Moon is approximately 3500 km in diameter, that’s about 1/3 of the diameter of the Earth. Just like the Earth, it has a slightly bigger diameter at the equator, compared to the polar diameter – it is a bit squashed. But only a little. The diameter at the equator is only about 5 km more than the polar diameter. From the Earth, the Moon has an angular diameter of around about 0.5 degrees. That is if you put lots of moons around the sky, like on a string, you would need 720 of them to get all the way around and back to the start.


  2. Steven’s answer is very good here. I would listen to him about space stuff πŸ™‚



  1. Hi addzyy69,
    Steven has hit it on the head with size. But just to add something that I find interesting, the distance the moon is from earth, because it looks so big in the sky.

    If you imagine the earth was the size of a basketball, the moon would be about the size of a tennis ball. So go get yourself a basket ball and a tennis ball. Now take the tennis ball about 10m from the basket ball and that is how far the moon is from the earth (relatively)

    Here is a vid about it


  2. Thanks Steven and addyzz69 for teaching me some things about the moon. Dustin, that’s a nice way to visualise the relative sizes of the Earth and moon. πŸ™‚


    • Sorry, that should be addzyy69! πŸ™‚


    • Steven

      No problem. Just to take this thread a bit further. It turns out that the size of the Moon as seen from the Earth is very close to the size of the Sun as seen from the Earth. That means that the ratio of the Moon’s diameter to its distance from us is very close to the ratio of the Sun’s diameter to its distance from us. Further, the Moon, Earth and Sun all lie very close to the same plane (the disk of the solar system). This incredible coincidence means that every so often the Moon passes in front of the Sun and eclipses the Sun. When this happens, we are able to look directly at the solar corona, chromosphere and solar prominences when the photosphere (very bright disk of the Sun) is blocked. There will be a total solar eclipse observable from northern Queensland in November of this year. I’ll be going to see it and speak at a conference being held at the same time and place. I’ve never seen a total solar eclipse, so I’m very excited (hoping for good weather). The meeting is taking place at the Palm Cove resort near Cairns – it is a tough life being a scientist!


  3. how do you know that is the actual size of the moon?


    • Hi Horselover101

      One way to measure things is through a technique called trigonometry. If you haven’t done it in school yet you certainly will.

      So imagine a triangle with one point at your eye, and the two other points being on each side of the moon.

      This gives us a very tall triangle with two very long sides that are equal to the distance to the moon, and because one end is at your eye, or better still through a telescope, we can measure the angle at that point of the triangle.

      Once we have these numbers we can work out the third side of the triangle, which will be the diameter of the moon.

      Very good question

      Here is a link to more trigonometry stuff if