Full Moon

Let’s test your visual memory, eh?

Get a good mental picture of the full moon. You’ve seen it a couple thousand times in your life. In real life, like a few days ago if you happened to be looking up outside at night. In the movies, like on that poster of E.T. On the covers of countless books.

Now, a question.

If you were to take a whole bunch of full moons and stack them one atop another, how many moons would it take to reach the horizon to the zenith (the point in the sky directly overhead)?

A) 16

B) 24

C) 48

D) 120

E) over 120


Well, what do you think? The answer’s the last choice. Surprised? Most people are, myself included when I first encountered the question many years ago. Based on the images we see in the various media, we’re accustomed to an over-enlarged version of the moon. The full moon in actuality is so tiny, relative to the whole night sky, that it would take 180 of them stacked on atop another, to reach from horizon to zenith.

Now how about another question?

How many full moons would it take to cover the entire night sky?

This one stumped me. Initially I guessed about 60 full moons from horizon to zenith and thought I had to do some sort of trigonometric equation to find the surface area of a half-sphere. 4 π r-squared / 2. I plugged in 60 as r as a guess and came up with (4 * 3.14159 * 60 * 60) / 2 = about 22,620.

What do you think?

The answer is a bit more than that. Turn’s out it would take 105,050 full moons to completely cover the celestial dome. That intrigued me, it absolutely amazed me. No, not the number, not even the formula needed to figure it out (which obviously is a far cry from my weak guess).

What floored me is this: Stop a second, and simply imagine what the sky would look like with 105,050 full moons! You’d go blind instantly, I’d think (there’s a biophysics problem right there - calculate the luminosity of the full moon, multiply by 105,050 and compare result to the single sun, and decide whether the human optic nerve could handle it dispersed over the entire region of the sky). How could such a world even plausibly exist? What type of creatures would evolve on such a world? What would be their culture? Their belief system? Obviously they would worship the moon, or the moon-multitude, but what shape would that worship take?

There’s a science fiction short story in there somewhere if you have the patience and the time …