A reader writes:
The Oatmeal recommends sites get more traffic and Facebook likes by writing an epic love story involving cage-fighting nuns and tanks, or if that is not possible, explaining why the sun and the moon appear to be the same size in the sky.
Both of these seem right up your alley, but frankly I for some reason find the second one more interesting. Why ARE the sun and moon the same size? Is it just a bizarre coincidence, or is there some astronomical orbit reason for it?
I'm afraid it is indeed just a fluke. Which, furthermore, starts to look less amazing when you discover that the sun and moon don't actually have a particularly spooky similarity in size.
I remember reading some flaky book when I was a kid, possibly some von Däniken claptrap or other, that made much of the extraordinarily precise apparent-size match between the 0.55-Earth-diameter moon and the 109-Earth-diameters sun. Surely this cannot be mere coincidence, hence ancient astronauts and Nazi moon bases and the various Stargate series are all documentaries et cetera.
Unfortunately for these otherwise-very-plausible speculations, the sun and moon are not actually the same size in the sky. They can be, but they usually aren't.
The earth's orbit around the sun is not perfectly circular, but it's close. On average it's one astronomical unit (oddly enough), but we're closest, 0.983 AU, in early January, and furthest, 1.017 AU, in early July. The actual sun stays the same size, so from our point of view it ranges from 31.6 to 32.7 minutes of arc.
For visual learners, that's about this much of a range:
(I made this from this NASA picture depicting a gigantic magnetic filament erupting from the surface of the sun. The same filament would not, of course, be there in both January and July.)
The moon's orbit around us is more eccentric than the earth's orbit around the sun, so the moon changes in apparent size much more dramatically than the sun does. It ranges from 29.3 to 34.1 arc-minutes or, to the same scale as the above sun picture...
(I took that moon picture myself. Residents of the northern hemisphere are invited to stand on their heads to make it look more familiar.)
(UPDATE: I forgot to mention the moon illusion when I first put this post up. Yes, the moon, and the sun too for that matter, seems bigger when it's near the horizon. No, it actually isn't. If anything, it's smaller!)
Here's the two ranges compared.
The only time when ordinary people really compare the size of the sun and moon is, of course, when there's a total solar eclipse. Then it really does look as if the moon neatly covers the entire sun, helpfully giving us a nice view of the corona, which is normally washed out by the much greater brightness of the body of the sun. (You can actually view the corona from the surface of the earth at other times, but you need special equipment to block out sky-glare.)
At this point, you may be wondering whether the roughly-month-long lunar size cycle and the year-long solar size cycle can coincide with an eclipse in such a way as to put a minimum-size moon in front of a maximum-size sun (well, any size of sun, really, there's not that much difference), so that the moon fails to completely obscure the sun.
Yes, it can; it's called an annular eclipse, and there's one coming shortly, though I won't be able to see it from here in Australia.
Here is a lot more information about all of this.
And then commenters will, I hope, correct at least the most obvious flaws in my answer.