It's a full moon tonight. You all know what that means - the moon will look like a big pale circle in the sky. Did you also know it means the moon will rise at around the same time as sunset, and the moon will set around the same size as sunrise? This is because the moon is full when it is directly opposite the sky from the sun, so they can't both be in the sky at the same time.
So if the moon is full tonight, what will it look like in one week's time?
a. It will be totally dark
b. It will be totally full
c. It will be a crescent like the letter "C"
d. It will be a half circle like the letter "C"
e. It will be a crescent like the letter "D"
f. It will be a half circle like the letter "D"
The answer is d. if you live in the northern hemisphere. If you live in the southern hemisphere the answer is f.
The moon completes a full cycle (new moon, waxing moon, full moon, waning moon, new moon again) in about 29 1/2 days, which is close enough to four weeks as a rule of thumb we can say it completes a quarter of its cycle in one week. This month encompasses a whole cycle quite handily, so the calendar I have reproduced above shows how the moon looks as it waxes and wanes. The thing I want you to remember is whether a "C" shape means the moon is waxing (getting bigger) or waning (getting smaller).
If you live in the northern hemisphere, the moon goes from dark, to a "D", to an "O", to a "C", then dark again. So the way to remember it is "Doctor Moon" - "D O C". Got that?
If you live in the southern hemisphere, the moon goes from dark, to a "C", to an "O", to a "D", then dark again. The way to remember it is "the moon if a fish" - "C O D". OK?
I find it oddly depressing that very few people seem to know this, but the majority of people can name Jennifer Aniston's husband.