Treebeard's Stumper Answer
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?
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.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!
"It is better to know some of the questions than all of the answers." - James Thurber
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:
- The moon and the earth are locked in synchronous orbit so we always see (almost) the same side of the moon from home. Therefore a future colonist on the moon will always see the earth in (almost) the same place. There are other examples of synchronous orbits in the solar system, some of which involve the earth. (What are they?)
- The earth rotates every 24 hours, much faster than the month it takes the moon to go around. So a lunar observer will see all the continents on the spinning earth pass by every 24 hours. What a great show!
- The moon orbits around the earth once a month catching the sun from all sides. So from the moon we'd see the sun rise and set in a monthly "lunar daily" cycle. We'll see the background stars behind the sun march through the zodiac just as we see from earth, with a bit of retrograde back and forth motion because of our orbit. I will see Orion/Taurus and Scorpio both appear behind the sun just once a year.
- As the moon orbits around the earth, I will see the background stars shift along the ecliptic behind the earth. Every 28 days, I will see Orion/Taurus and Scorpio both appear behind the earth. On earth, we only see this change once a year. After one full lunar trip around the earth, the sun will appear about one zodiac sign to the left of the background stars from where it was before, just like on the earth.
- The earth will go through phases from Full to New and back to Full again as viewed from the moon. This must be beautiful to behold as the earth turns all the time!
- There is no dark side of the moon, no matter what Pink Floyd sings. The moon rotates once every earth-orbit, so all sides get equal sun over a month-long lunar day. Anywhere on the Moon, you will get equal amounts of day and night, except for deep shadows, just like earth. (And my house on the shady north-facing slope of the Santa Ynez Mountains. Brrr!)
- We can always see stars in the black sky from the moon, even when the sun is high. There's no atmospheric scattering to create a blue "sky dome".
- A lunar eclipse (from the earth) is a solar eclipse from the moon, since the earth is between the moon and the sun. But the earth appears larger in the sky from the moon than the moon does from the earth. This changes the quality of eclipses as viewed from the moon, but not the number of eclipses. Every partial lunar eclipse on the earth is a total eclipse on the moon, somewhere at least. And every annular eclipse (from the earth) is a total eclipse on the Moon. During an eclipse, sunlight will pass through the earth's atmosphere creating a ring of fire around the dark New Earth. [NASA Link]
- A solar eclipse (from the earth) is an "Earth Eclipse" from the moon. We'd see a small shadow of the moon quickly pass across the bright Full Earth surface. [Link]
- I see the moon rise in the East and set in the West from home as the earth turns, but the moon moves just a little during that same period. Because of the changing perspective, I can see around the left-right edges of the moon between the first-quarter sliver and the last-quarter remnant. This creates some wobble, more correctly known as libration. [Link]
- The moon follows an elliptic orbit around the earth, so the moon is sometimes closer and farther to earth (and vice-versa) every month. This would effect the apparent size of the earth from a location on the lunar surface. That's more wobble.
- The moon's axis is tilted about 6 degrees from the plane of the ecliptic, less than the earth's 23 1/2 degree tilt, but it still makes a difference. Imagine yourself at the lunar north pole. There's a time every month when you are tilted towards the earth and a time when you're tilted away. This creates a wobble of about +/- 6 degrees. The disk of the earth as seen from the moon is only about 2 degree. Therefore at the poles, and along the east-west edges of the earth-facing side, there is more wobble. There are places around the lunar poles where you will see the earth rise and set. But they will do it in slow motion over a "lunar day" and not quickly over an "earth day"as we expect.
- Because the moon's axis is tilted about 6 degrees from the plane of the ecliptic, the earth will seem to move up and down in the sky wherever you are on the lunar surface. The earth's diameter is about two degrees as viewed from the moon. By comparison, the moon's apparent diameter from earth of about 1/2 degree. From the earth, we always see the full moon opposite the sun in the sky, so the moon rises about when the sun sets. In summer, the sun is high during daytime, so the opposite full moon (and the rest of the ecliptic) is low in the sky. In winter, it's just the reverse. The sun is low during the day, but the full moon and planets are high at night. That's part of the reason why the winter skies are so impressive. The moon also also moves above and below the ecliptic every lunar month/day. So from the moon, the earth will appear to rise higher and dip lower over the lunar month/day. These motions will someday effect lunar hotel prices because of the unusual changing view they provide! You can see the effects of lunar libration in animations here and here.
Here are some starting links for your own research:
- I thought of this stumper when I saw the NASA Astronomy Picture of the Day (APOD) for 2000 December 31, the last millennium. Here's the NASA APOD picture for the next millennium. The irony is that no one has seen this view of the earth since the Apollo 17 crew in 1972!
- I was there in 1968 (sitting on my butt watching TV), but it's just history for today's kids, The Apollo 8 mission is the one I remember, even more than the Apollo 11 mission that actually landed on the Moon. I was backpacking the High Sierra that summer, so I heard about the moon landing and the Chicago Convention riots at the same time. That was a mixed message!
- Want to put an object in synchronous orbit around the moon? Done! The moon orbits the earth in synchronous orbit, and vice-versa, so YOU and everything else around you are orbiting the moon right now! The Lunar Colony Homepage has good explanations and a classy certificate that you can print.
- This stumper is discussed elsewhere on the Web, e.g. Challenger Center, the Lunar Colony Homepage, and Dr. Sten Odenwald's Ask The Space Scientist. EarthRISE is a large database of photos of the earth from space. I have more stumpers (and links) about the Moon at Total Eclipse (14 Jan 2000) and More Lunacy (21 Jan 2000).
- NASA has tons of info on the Apollo Program. There's still a question who took the famous "earthrise" picture on the Apollo 8 mission. Robert Zimmerman has an answer. That was the night I still remember when the astronauts read from the book of Geneses on Christmas Eve. The history is available here and here. No one thought much about church and state at that moment.
Jim Lovell: "The vast loneliness is awe-inspiring and it makes you realize just what you have back there on Earth."
William Anders: "For all the people on Earth the crew of Apollo 8 has a message we would like to send you."
"In the beginning God created the heaven and the earth. And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters. And God said, Let there be light: and there was light. And God saw the light, that it was good: and God divided the light from the darkness."
Jim Lovell: "And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day. And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so. And God called the firmament Heaven. And the evening and the morning were the second day."
Frank Borman: "And God said, Let the waters under the heavens be gathered together unto one place, and let the dry land appear: and it was so. And God called the dry land Earth; and the gathering together of the waters called he Seas: and God saw that it was good."
"And from the crew of Apollo 8, we close with good night, good luck, a Merry Christmas, and God bless all of you - all of you on the good Earth."
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Copyright © 2001 by Marc Kummel / firstname.lastname@example.org