Why is the Moon the same size as the Sun?

August 4, 2013

The best time of day at our villa in Kas, Turkey, is early evening: it can't be a coincidence that the builders named it Sundown Villa. As a very broad rule of thumb, and whilst never staring directly at the Sun, it and the moon are typically a similar size: or, at least, the Sun is not massive compared with the Moon. Why?

I have recently got very interested in some aspects of the geometry of the Earth- blog warning: I will be doing some science blogs when I feel sufficiently confident that I am along the right lines. As a sneak preview, on a recent journey coming back from our cottage near Keswick, Lake District, to our home in Stockport, Jane remarked that there was a noticeable difference in time of sundown between the two towns, despite them not being much more than 150km apart: at spare moments, I have been giving some thought as to why this should be. That will be blogged about later when I am ready.

For now, though, I wondered recently why the moon is broadly a similar size to the sun, despite the fact that we know the Sun is huge and the moon small compared with the size of the Earth. Clearly, it is down to distance, but what are the maths?

When I started blogging in December 2012, I didn't know if I would enjoy it (I do, very therapeutic) and keep up with it (so far, so good) so didn't spend much time on the design, including in particular the heading: things that interest or amuse me. On reflection, I could have added 'and know something about, but not that much'. Part of the interest to me in blogging is to learn new things, find things out, think about things. On astronomy and the Earth, I do not even have 101 level knowledge: this is all new to me. I might regret some of these science blogs.

Why are the Sun and Moon broadly similar sizes?

Is it a fluke? I am sure it is, but the Sun is 389 times as far away as the Moon, and at the same time is 400 times larger. So basic math/perspective shows that on average, they are around the same size. To me, it is quite cool (warning- I have three teenage children so that word is used a lot) that the ratios are so close.

Astronomical data

I have started a note (h/t to Evernote, a great app, and to its sister app, Skitch) with some data; to be added or amended as I learn more.