Shadow Play

“Uncle Sy! Uncle Sy! You’re back! Didja see the red moon?”

“Hi, Teena. Good to be home. No, I didn’t get to see the red moon. Where I was it didn’t even get red.”

“I saw it! I saw it! Mommie put me to bed early so I could wake up to see it earrrly in the morning. I saw the red part but the Moon looked smaller than it does coming up from behind the houses and they said it was going to be sooo big but it wasn’t. Anyway, I didn’t stay awake. Why was it red?”

“Was it really red red like your favorite crayon?”

“Mm-no, more like orange-y red.”

“Sunset color, right?”

“Uh-huh. Was it sunset on the Moon?”

“Sort of. The sunsets we see on Earth are red mostly because our air absorbs the Sun’s blue light when we’re looking across the atmosphere. Only the red light gets through to our eyes. Remember the solar eclipse we saw, when the Moon came exactly between us and the Sun? Moon eclipses are inside out from that. We come between the Moon and the Sun. The only light getting past us has gone across our atmosphere just like sunset light does so it’s orange‑y red like a sunset.”

“Oooh … does the Sun ever get between us and the Moon?”

“Don’t worry, Sweetie. We’re far, far from the Sun. Mr Newton’s Laws of Motion say that we and the Moon will be waltzing out here for a long, long time.”

“Whee, we’re dancing around the Sun! MOMmie, Uncle Sy’s here!”

“Hi, Sis. You saw the eclipse, then.”

“Mm-hm. I realized while I was watching it that lunar phase shadows work differently from eclipses.”

“Oh? How so?”

“The shadow shapes are different, for one. The edge of the lunar phase shadow always passes through both poles. In a solar eclipse the shadow only reaches the poles at totality, and in a lunar eclipse there’s this almost straight shadow arc that marches across the whole face.”

“Interesting. You said ‘for one,’ so what else?”

“Eclipse shadows move in the wrong direction. Starting from a full moon, the shadow comes in from the right until you get to new moon, then it falls away to the left until you get back to full moon. Agreed?”

“I always get confused. I’ll take your word for it.”

“I looked it up. In two places. Anyhow, in both kinds of eclipse the shadow creeps from left to right. Just backwards from the lunar phases. I wonder if that has anything to do with ancient societies thinking that an eclipse is somehow evil.”

“Mommie, you know you’re not supposed to use words I don’t know unless you’re keeping secrets. What’s lunar faces?”

“Sorry, Teena, not secret. Lunar means Moon. Sy, can you show her phases on Old Reliable?”

“Sure. Here’s a quick sketch, Teena. Pretend that the little ball is the Moon going around the Earth. The Sun is off to the right. You know the Moon goes around the Earth and it always keeps the same side towards us, right?”

“That’s the Man In The Moon except it’s really mountains and stuff pointing at us.”

“That’s what the little triangle shows, like it’s his nose. See how sometimes it’s in the light and sometimes it’s in shadow? The big ball is what we see when the Moon is in each position. When the Man is facing straight towards the Sun we call that the Full Moon phase. When he’s completely in shadow that’s the New Moon phase. There’s names for other special positions, and all of the special positions are phases, OK?”

“I suppose you have a logical explanation for the shadows?”

“Sure, Sis. It’s all about where the shadow’s being cast and how the shadow caster is moving at the time. This diagram tells the story. Nearly everything in the Solar System runs counterclockwise—”

“Widdershins.”

“… Right. Every orbit runs left‑to‑right half the time, right‑to‑left the other half. The two kinds of eclipse happen in opposite halves. The geometry works out that we see both eclipse shadows move left‑to‑right. See?”

“Cool.”

~~ Rich Olcott

  • Thanks to Alex for the question, and to Lori for the shadow observation, which I hadn’t seen discussed before.