Mostly-Full or Slightly-Empty

Last night we were driving back from San Francisco (details forthcoming) when we spotted the moon a few degrees above the horizon. This was around 7:30, so it was still before sunset, and the moon was rising. We noticed that the moon was not quite full, with a little bite taken out of it at the bottom.

We wondered whether the moon was nearly full, or just past full.

I said, “There has to be a way to figure this out logically.”

My Dad said that this is an empirical problem, so he was doubtful we could reason our way out of it.

I said, “Well, we know that on average there’s more than one full moon per month, and so we ought to be able to figure out from that whether the moon rises a little earlier each day, or a little later. And if we know that then we should be able to figure out whether it’s nearly full or just past full.” I decided that since there’s more than one full moon per month, that meant that the moon was rising a little earlier each day, and that meant that that moon was not quite full.

About 20 minutes later I said, “The moon looks a little more full to me now, so I think I’m right.” Much laughter ensued.

I think my reasoning was a little off, mainly because what I really need to know is whether the moon rises more than once per day, and using “full moons per month” as a proxy for that is not right, because they’re not the same thing. Indeed, since our months are somewhat based on the lunar cycle, “full moons per month” is a circular argument. Well, sort of.

But it turns out I was right anyway, since the full moon is tomorrow.

Which goes to show once again that it’s better to be lucky than good.