The first time I saw it, I found the video utterly fascinating, but I didn’t make a personal connection, at least not immediately. A few years later I thought about it again, and suddenly it struck me that I'd had a private-universe experience myself. When I was a child, I somehow got the notion that the Moon stored light from the Sun during the day and then released it back to us at night--not unlike a modern solar walkway light. I’m certain that at some point I was told the Moon reflected light from the Sun, but since I didn’t know (or understand) the relative motions and positions of the Sun-Earth-Moon system, I must have very creatively decided that “reflected” meant “stored it up and shone it back later.” Go figger.
Here’s the embarrassing part: I’m not sure precisely when I abandoned this notion for the astronomical truth, but it was well into my adulthood. And I can’t say for sure what triggered my epiphany, but I’m guessing it was seeing a diagram of the Sun-Earth-Moon system.
Since luna is the Latin word for moon (and the source of English words such as lunatic), I call my persistent erroneous thinking about the moon “my own private lunacy.” But I know I’m not alone. I see this at public astronomy events, when adults confidently tell me that the non-illuminated part of the crescent moon they’re looking at is the Earth’s shadow. They too are sorely in need of a diagram.
On this simple moon phase diagram,
you can see that the inner circle of moon icons depicts the moon’s orbit around the Earth, which takes about a month (29.5 days). The ABCD indicators show the direction of the Moon’s orbit. Only one hemisphere of the Moon is illuminated by the Sun at a given time, with the other hemisphere non-illuminated or dark (shine a flashlight at a baseball, and you‘ll get the idea). However, because of Earth’s position at the center of the lunar promenade, we only ever see part of the Moon’s Sun-illuminated hemisphere. The exceptions are New Moon and Full Moon.
The outer circle on the diagram depicts how each moon phase appears to us here on Earth. During New Moon, the Moon lines up between Earth and Sun and we can’t see any part of its illuminated hemisphere, so it vanishes from our view. During Full Moon, the Earth lines up between Moon and Sun, so we see all of the Moon’s illuminated hemisphere. Now, if the Earth and Moon were on the same orbital plane relative to the Sun, we would have a total solar eclipse and a total lunar eclipse once a month, respectively, when the Moon covered the Sun during New Moon and when the Sun cast the Earth‘s shadow onto the Moon during Full Moon. But that doesn’t happen, does it? This is because the Moon’s orbital plane around the Earth is tilted about five degrees from the Earth’s orbital plane around the Sun. These out-of-kilter orbits make those perfect line-ups needed for total eclipses a bit rare.
The Moon in Earth's shadow, during a total lunar eclipse
This week, go outside and look at the Moon naked eye. Then wait a week and do it again. Each time, notice what portion of the Moon’s face is illuminated and match it to the moon phase it most resembles on the diagram. Then think about where the Sun must be in relation to the Moon and you, to produce the phase you see. Know that you are looking at nearly-real-time reflection of sunlight from the Moon’s surface. I say nearly-real-time because it takes that reflected light about 1.25 seconds to traverse the quarter of a million miles separating Moon and Earth.
On Saturday, November 24, the Moon will be full. November’s Full Moon is called Beaver Moon by the Algonquin Indians, Moon of the Falling Leaves by the Lakota Sioux, and Freezing Moon by the Cheyenne. Full Moon is a great time to look at the Moon, not only because it is fully illuminated, but because it is in the sky all night long, from sunset to sunrise. If you can drag your bad self home before sunrise, you might just get to bask in a little of that reflected glory.