Lunar diet with yoyo-effect: the waxing and waning moon

According to an old astronomer’s joke, it’s hard to tell the exact mass of the moon, since Earth’s satellite keeps waxing and waning. In fact, the moon weighs a total of 73,000,000,000,000,000,000,000 kilograms – that’s one 80th of the mass of the Earth. Whether the moon waxes or wanes depends on its angle in relation to the sun and its position in the lunar rotation cycle around the earth. The moon does not shine by itself; we can only see it because it is illuminated by the sun.

If the moon is positioned right across the sun, we can see a full moon from Earth, because the side facing us is fully illuminated. But if the moon is positioned in the direction of the sun, between Earth and sun, the side of the moon facing us is completely dark. No sun rays can reach it: it’s a new moon. That’s when the back side of the moon is fully illuminated, which can’t be seen from our perspective. So, no matter what phase the moon is in, it is in fact always half-light.

And have you ever asked yourself, looking at the crescent of the moon, whether it’s actually waxing or waning? About a century ago, the German poet Christian Morgenstern came up with a good answer in a funny poem.

The moon: a German celestial body?

"When God created the moon, he gave him the following task, when he was to wax, and wane, like 'ab' and 'zu', what every German reader knew, / forming an A, also a Z – it couldn’t be easier for him to do/ obeying this was his delight, a perfect German satellite."

In fact, the crescent of the moon actually forms the bow of A or Z in the old German Suetterlin-handwriting. If the crescent bows to the right, a Z fits in, representing German word "zunehmen" – to wax. If the moon's sickle bows to the left, you could fit a handwritten "A" for "abnehmen" – or to wane, in German. But Morgenstern’s poem has become a bit tricky in more recent times: firstly, today not many young Germans know how to read the old German handwriting, let alone it’s A’s and Z’s. And of course – the rule is reversed on the southern hemisphere, where the moon is upside down.

What a relief that there’s a much better rule of thumb that applies anywhere: When you can seen the moon in the evening sky, it’s always waxing. And if it’s up in the morning, it’s always waning, no matter which hemisphere you’re in. So put away your text books, there is no need to start learning old German handwriting to understand the moon.

Author: Dirk H.Lorenzen (ara)

Editor: Neil King