Hey, sky fans! Thanks for dropping by. I hope you’re enjoying these last few days of summer. Here at the outdoor offices at Sky Watch HQ, there’s been a big grasshopper, one of those ones with the giant red eyes, staring me down from across the table for a while. I hope it’s not after my lunch.
On we go to the big show! Did you see that Moon last night? Wow! Where I was it seemed giant, positively huge, and really bright. Well, if you head outside tonight, you’ll see what looks like a big, swollen full Moon shining at you from overheard. Don’t be fooled, though. It’s not quite there, and while it might seem like we’re splitting hairs to say that it being only about 99% full by mid-evening tonight isn’t full, this month it matters.
September’s full Moon, which this year falls on September 16, is called the Harvest Moon. If you’re a fan of Neil Young, this Moon’s for you! Along with October’s Hunter’s Moon, it’s probably one of the two most famous full Moon nicknames of them all. Traditionally, the Harvest Moon is the one that happens closest to the autumnal equinox, which, here in the northern hemisphere is on September 22, about a week away; welcome to fall!
The Moon is said to be full when it is in the spot its orbit around the Earth where it’s directly at opposition from the Sun. If you remember back a couple of months ago when we got talking about Mars, and then Saturn, being at their oppositions, this is the same sort of thing. The Sun-Moon line will pass directly through the Earth. The three will be in a straight line with the Sun and Moon opposite each other in the Earth’s skies; we here on Earth will be stuck in the middle of the line. You’re right. The Moon is at opposition every month, every full Moon.
While the Moon will look full tonight, and, by all means, enjoy it, tomorrow is the real deal. If you’re up for a couple minutes of fun, I like to look really closely at the Moon to see if I can see the slightly shaved-off edge, the truncation, the foreshortened shadowed limb of the Moon when it’s just barely not full. Depending on the timing of it all, the Moon could appear to be close to full for a day or so on either side of the actual moment of full…itude.
The reason this is important this month goes back to the talk about the solar eclipse from a couple of weeks ago. Then, we talked about a solar eclipse that was visible over Africa and some of Asia. Well, solar and lunar eclipses usually come in pairs. After there’s one, the other can follow about two weeks later, when the Moon has gone half way around the Earth. Since solar eclipses only happen at new Moon, two weeks later, when the Moon has swung around to the other side of its orbit, is at opposition, and is full, it can dip into the Earth’s shadow and we have a lunar eclipse. Lunar eclipses only happen at full Moon.
During a solar eclipse, from Earth we see the Moon cross in front of the Sun and block it out. Part of the Earth drops into the Moon’s shadow. Things are reversed during a Lunar eclipse; it’s Earth doing the crossing. So, if you were standing on the near side of of the Moon, you’d see the Earth cross in front of the Sun, and block it out. The Moon drops into the Earth’s shadow. If you were on the far side of the Moon, you’d just keep saying “What’s an Earth?”
This isn’t going to be the best and easiest eclipse to see. It’s not visible in the US; only in parts of Africa, Asia, Australia and Europe—eastern hemisphere places. Also, this one is what’s called a penumbral eclipse, which is when the Moon only slides through the penumbra, the paler and shallower outer part of the Earth’s shadow. It won’t turn that deep red you might remember from last September’s total lunar eclipse. It’ll only be a dimmer brown color, which you’ll have to look for it to see, if you happen to be in the right place at all.
By the way, if you’re wondering if the Moon seems a bit bigger and brighter than usual, you’re right. This month’s full Moon will happen when the Moon is close to perigee: the closet point in its orbit around Earth. It’s not quite there, the next perigee is on September 18, but it’s enough to make the Moon look bigger. It’s not a giant difference from perigee to apogee, only about 25,000 miles or 40,000 kilometers, but that’s about 10% of the average Earth-Moon distance; it’s enough. Technicalities aside, the full Moon will definitely look bigger than usual in our skies over the next couple of nights, especially tomorrow night.
Until next time, enjoy the Moon, and clear skies, everyone!
(Edit: I mistakenly wrote originally that all eclipses come in pairs. This usually happens, but not always.)