Monday, 29 March 2010

Waxing or waning?

I was asked an interesting question today. One I admit I've never really thought about too much. The question was this: if you haven't seen the moon for a few months and then suddenly you had a clear night and saw a quarter moon in the sky, could you tell if it was waxing or waning?

(Waxing means going from a new moon to a full moon, waning means the opposite).

The answer is actually very simple if you like those rule of thumb things but he doesn't want to know the answer right yet because he wants to see what other people say. So I won't give the answer here, or at least not in this post. If you feel like commenting with your answer though, feel free!

Although the answer is simple it depends whether you're in the northern or southern hemisphere and you're a little stuffed if you live on or near the equator! So I think I have a general answer as well which should work no matter where you are. I'm sure this is in Google somewhere but I promise I haven't looked (yet!). You need to know west from east, though, or at least where the sun sets or where it rises with respect to your location.

As for the above shot (with some nice earthshine), it was taken just after sunset from home looking roughly to the west. Is it a waxing or waning moon? You have enough information to answer the question!


Zuzana said...

It is too early for my brain cells to work just yet, but that is one incredible shot of the moon. Here it is full moon tonight.;)
Btw, *The halo around the moon* post keeps getting at least 20 hits every day.;)

Keera Ann Fox said...

I know this, in part because I have a rule of thumb in Norwegian: "Ser den ut som et komma, så kommer'n", which sounds corny(er) in English: "If it looks like a comma, it's a-comin'." So this is a waxing (growing) moon.

But tell me, Tom, what's the deal with quarter moons at equatorial latitudes? A Singaporan tourist looked up at our quarter moon in Bergen, visible in the sky while the sun was still up and told me that one couldn't see that sight closer to the equator. Why not?

Andrew Cooper said...

Evening sky? Waxing.

It is amazing how many folks have lost touch with our planet's natural cycles.

Tom said...

Zuzana - it's an old recycled picture I took last year, but it was quite a nice one I think!

Keera and Andrew, yes, you're both spot on. Keera - as to your question, to be honest I have no idea. We're not that far from the equator here and you can see a quarter moon in the sky during daylight. The only thing I can think of is that the moon may be more obvious at your latitude because the sun is lower and the sky isn't so bright, but I don't think your tourist friend is correct.


Keera Ann Fox said...

I couldn't figure out why a 90 degree angle wouldn't be visible from all over. My tourist friend's comment was made in May, BTW, as we were sitting outside looking up. I can't imagine that time of year would influence geometry, either. :-)

Maren aka hilobeads aka Palms, Etc. said...

Just had the same thing happening, not seen the moon in weeks, and it's almost horizontal here. The time of night when it was taken settles it. Just after sunset? Waxing.