Hey, sky fans. The big astronomy headline this week is there’s a solar eclipse on the way! The sky will be going dark in some parts of the world in the middle of the morning this coming Wednesday the 9th… or is it Tuesday the 8th? Here’s the scoop (do people say “scoop” anymore? I’m nothing if not 20 years behind). The moon is going to pass directly between the earth and the sun for a few spectacular minutes that morning. The moon will cast a shadow, and people on the ground will see an ominous black disc cover the sun. Didn’t Monty Burns try something like this a few years ago?
This will be a partial eclipse for a huge part of the Pacific, Hawaii, Alaska, south and east Asia, and Australia. If you happen to be in Indonesia and Malaysia, and a few other places along a very narrow track that is directly under the very small moon’s very small shadow, it’ll be a total eclipse — the moon will completely block the sun. Everywhere else in that eclipse zone, the sun will be covered less, and the sky will stay bright. Most of the world, though, including all of North America except for parts of Alaska, will see nothing out of the ordinary at all.
Eclipses work because we happen to be alive at a terrific moment in the history of eclipsing. The sun’s diameter is 400 times bigger than the moon’s but, at the moment, it’s also 400 times further than the moon is. That’s why, as you may might have noticed, the full moon, when the moon looks its biggest, and the sun seem to be the same size in the sky. Thanks to that coincidence, the moon can pass right between the sun and earth from time to time and entirely block out the sun. As time goes on, though, the moon, which is slowly sneaking away from the earth, will get smaller and smaller as seen from Earth, and will eventually become too small to eclipse the sun. When our insect overlords read this millions of years from now, they’ll have missed the bus.
Also, the path of the moon’s shadow moves along the ground as the moon moves. Thanks to the International Date Line, this eclipse actually starts on Wednesday over southeast Asia, and, as the eclipse track moves eastward, ends on Tuesday out over the open Pacific.
If you’re having a hard time picturing how this works, imagine if some prankster moved a ball in front of your kitchen light so that the bulb gets completely blocked out from where you sit. Blammo. An eclipse localized entirely within your kitchen. At the same time, though, a friend sitting at the other end of the room, or even the other end of the table, will have a different line of sight on this bulb-ball-you situation. They might see some shadow, but they won’t see the light get completely blocked out. Then, the person with the ball moves, as the moon does in its orbit around the earth. Now your friend sees the bulb eclipsed, and everything is back to normal for you. Like so many things in astronomy — eclipses, lunar phases, constellations — it’s all about point of view.
Back to the real deal. Solar eclipses can only happen when the moon crosses directly between the sun and the earth. This can only happen during the moon’s new moon phase because that’s the only time that particular straight line is possible. At all other times, except full moon, the Earth-Sun-Moon line isn’t straight, and the shadow cast by the sun and moon misses the earth off to one side or the other, and goes off into empty space. This means there’s always a point somewhere where the sun-moon-[something] line is straight. So, something orbiting in just the right place might see a new moon, and possibly a solar eclipse, when the nighttime skies here on Earth are bright with moonlight. There are new moons every month, but we don’t have an eclipse every month. This is because the moon’s orbit isn’t perfectly in line with the ecliptic — the apparent path the sun takes across the sky; it’s tilted by about 5 degrees. Most of the time the new moon happens when the moon and sun are off to one side or the other of each other. Every now and again, things line up just right. A solar eclipse happens when the moon crosses the ecliptic at new moon. In fact, that’s how the ecliptic got its name!
All solar eclipses are followed by a lunar eclipses two weeks later, when the alignment swings to sun-earth-moon. So, we have a lunar eclipses on its way on March 23.
Most of us here in the US won’t be able to see this one. We’ll need to wait until next August, but, then, boy, we’re in for quite a show. Clear skies, everyone!