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Treebeard's Stumper Answer
26 January 2001


This famous photo is one of the most memorable images of our time. It shows the fragile-looking Earth above the lunar horizon as viewed from the Apollo 8 spacecraft during Christmas week in 1968. This was the first time that humans ever orbited another world. The photo is usually titled "Earthrise," and that's what it looks like. The stumper is to understand what this picture really shows. Suppose we were stationed on the Moon's surface with this great view. When would we see the Earth set? How would our view of the Sun and Earth and stars change during our long lunar day?

"Earthrise" photo taken December 24, 1968 by the Apollo 8 crew:
Frank Borman, James Lovell, and William Anders.

The Apollo 8 crew saw the Earth rise because they were orbiting the Moon, but it would look different from the lunar surface. The Moon is locked in its synchronous orbit so we always see the same face from Earth. That means we would always see the Earth in about the same place in the Moon's sky. (There's some wobble.) Over the month-long lunar day, we'd see the Earth turn in place every 24 hours. It would go through phases, and we'd see the background stars shift through the zodiac. The full Earth at lunar "midnight" and solar eclipses (by the Earth) must be especially beautiful.


This is my favorite kind of stumper question. It's a simple question about a picture we've all seen, but it forces us to re-think our assumptions. It makes the familiar strange and makes us to think deeper about who and where we are.

"It is better to know some of the questions than all of the answers." - James Thurber
But this advice goes against the grain when more testing is promoted as the answer for education. I'm opting for more questions rather than better answers!

The best way to understand this stumper is with a classic Trippensee Planetarium/Orrery. (Expensive but well-made. I found mine cheap at a garage sale. It's not as fancy as this classic.)

A grapefruit-sun, an apple-earth, and a tangerine-moon will also work. Either way, you have to mentally put yourself on the moon and do a few turns to appreciate the situation. Of course the scale is wrong, which makes a difference. We do not have eclipses every month, though the simple models don't show why. (See my Total Eclipse (14 Jan 2000) stumper for real picture.) I'm short of time this weekend, but I hope to add pictures to this answer when I catch up. Here's how I see the situation:

Here are some starting links for your own research:

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