More space stuff

I read somewhere that Copernicus, the huge rayed crater on the Moon (it’s the most obvious crater on the side of the Moon that always faces us because the Moon is tidally locked to rotate at its orbital rate) is so large that if you were standing on it you would not know that you were in a crater. The horizon on the moon is only two miles away and Copernicus is almost 300 miles in diameter.

I also read that the horizon on the Earth is seven miles away.

Now, this is something I figure I could use all that fancy algebra I learned back in high school to figure out something about the sizes of the Earth and the Moon.

But I don’t know what that something is and now I don’t remember a bit of the math!