What I've Learned

I received a Christmas card that depicts carolers dressed merrily in coats, hats, and colorful scarves, singing with much gusto. There is snow on the ground. It's night. You can tell it's night because the artist has sprinkled some stars in the sky, plus a C-shaped sliver of a moon.

Nice card. But, Houston, we have a problem. Either the carolers are doing their business at an extremely rude hour or it has snowed in Australia.

A C-shaped moon is a waning moon. In the Northern Hemisphere, a waning crescent moon rises in the wee hours of the morning and is gone before evening.

I love Christmas and enjoy its hymns and songs, but if a bunch of carolers – no matter how well-intentioned – showed up at 3 a.m., I'd be tempted to pelt them with pieces of hard candy.

Sometimes it is tiresome being me. And so I rejoice when someone displays a like-minded attention to detail.

Peter Barthel, an astronomer in the Netherlands, recently wrote a paper, published in this December's issue of Communicating Astronomy to the Public, about inaccurate depictions of moon phases in Christmas cards and on wrapping paper. It's called Santa and the Moon.

Here are some quotes from his paper:

"We have established that illustrators and designers draw moons ad libitum, according to their taste, but often physically incorrect. The most common mistake is the early morning waning moon shown in an evening scene."

"The apparent lack of knowledge concerning the physics of the moon phases is most likely widespread and not just limited to the countries examined here."

"Naturally, the question arises: So what, who cares? The errors are innocent, somewhat comparable to incorrectly drawn rainbows, with the red color at the inside of the arc. Now, watching beautiful natural phenomena like rainbows and moon crescents is one thing, but understanding them makes them all a lot more interesting."

Amen.

So, here's the deal. A new moon – one that's hard to see because no sunlight is reflecting off it – rises as the sun rises and sets as it sets.

A full moon does the opposite. It rises as the sun sets and sets as the sun rises.

In between, the moon is either waxing (on its way from new to full) or waning (going from full back to new.)

A waxing moon is shaped like a backwards C that gets fatter and fatter with each appearance. A waning moon looks like a C that gets skinnier and skinnier. In the Northern Hemisphere, a waning moon is never seen in the evening. It sets before the sun does.

Thanks, Peter Barthel.

A PDF file of Bartel's interesting and informative paper can be read online for free. Go to arxiv.org and search for "Santa moon."