Today at lunch Tony posed a teaser question:

Today the phase of the moon is waning gibbous (meaning it was full just a few days ago, so it’s nearly full now but getting smaller each day). What’s the phase of the Earth as seen from the moon?

We debated it for a few minutes and basically decided it would be opposite. In other words, the Earth would currently be waxing crescent (small and getting bigger).

Tony joked that I’d have to look up the answer tonight to make sure, so indeed I did. Here’s what I found:

The phases of the Earth and moon are always the reverse of each other. When we see the moon as nearly full, any moon people would see a slim crescent Earth. When we see a completely full moon, the moon, Earth and sun are in a line– with Earth in the middle. Then people on the moon wouldn’t see Earth at all because it would be hidden in the sun’s glare.

Props to Tony for a puzzler.