Hey, sky fans! Well, my family and I are back from our vacation. It’s always great to get away. I have a couple of sov… souvie… sigh… souvenirs (got it!) from northern New England for you over the next few days. I’m not going to share this cold I picked up, though. You’ll have to get your own. Let’s see if I can muscle through, though.
There’s been a lot talk on the Internet over the last week or so about the total solar eclipse that’s going to visible in the US next August 21, about a year from now. This is the first total solar eclipse for the US since 1918, almost a hundred years, so it’s pretty big deal. There’s a lot written about it already, and I hope to be able to get somewhere to see it. I hope you can, too. I’ll write about it more over time, too.
Tomorrow, though, August 32, or if your one of those sorts of people, September 1, 2016, there’ll be another solar eclipse. It’s funny to think of something happening before something else as being “another,” but in this information-heavy world of ours, that’s the life we’ve picked. This one will be visible from much of Africa and the Arabian Peninsula. Here’s a link to the Wikipedia page about this eclipse, if you want some maps, times, and things like that. The Moon is small, so it casts a small shadow, which means eclipses can only be seen along a narrow path called its track. The track of this one makes it invisible in the US.
If you’ve been reading the news about it, you might have noticed tomorrow’s eclipse is being called “annular,” while next year’s is “total.” Maybe we should talk about this.
Before I go any further, do not look at solar eclipses, any solar eclipse, with your naked eye, and especially not with binoculars or a telescope, unless it’s a specialized solar telescope. You can seriously and permanently damage your eyes. In fact, it’s a bad idea to ever look directly at the Sun without very specialized equipment. Instead, use heavy filters or, eclipse glasses, or even welders glasses to look at it, or a pinhole to project an image into the ground. Here’s some more information on eye safety.
First, some background
A solar eclipse is simply when the Moon gets between the Sun and the Earth, and the Moon appears to block out all or part of the Sun as seen from the Earth. A lunar eclipse, which we’ll talk about again another time, is when the Earth gets between the Moon and Sun. For the rest of this post, it’s all solar eclipses.
Actually, an eclipse is any time something blocks something else out as seen by a viewer at a given location. Like this, here’s a film of my coffee cup eclipsing this green robot guy just a few minutes ago, as seen in the astrokitchen down at Sky Watch HQ.
As with lots of things in astronomy, and most things in life, perspective is everything. As we see it by way of the camera, the robot is being eclipsed by the mug. From the perspective of the robot, though, the camera is being eclipsed. From the mug’s view, there’s nothing strange happening. It’s just another day moving along the countertop by itself, as it does, while staring straight at the robot, at the camera, or off to the side. What’s more, if you’re looking from the perspective of a spider on the back of the robot’s head, all you’d see is the kitchen wall as it’s been all along, possibly without even knowing the mug exists at all.
To apply this to solar eclipses, imagine the mug is the Moon, the robot is the Sun, and you on Earth are the camera. Cookie Monster is the Man in the Moon. In a solar eclipse, you (the camera) would see the Moon (the mug) slide across the face of the Sun (the robot). The Earth drops into the Moon’s shadow. The Moon blocks the Sun out, and then it clears the way a little while later.
Partial, Annular, and Total Eclipses
If you remember back to earlier this summer, we talked about the dark side of the Moon and the difference between it and the far side. We figured the dark side is wherever it’s dark on the Moon; wherever it’s night, and the far side is the part of the Moon that always faces away from the Earth. In a solar eclipse, the Moon gets between the Sun and the Earth, which means its what we call the “new moon” phase. At this spot in the Moon’s orbit, the far side is getting all of the sunlight that hits the Moon, and the near side, which faces us, is fully darkened.
Since the Moon has to be between the Earth and Sun, and new Moon is the phase when the Moon is between the Earth and Sun, solar eclipses can only happen at new Moon. They don’t happen every time, though, because the Moon’s orbit isn’t oriented perfectly relative to the Earth and Sun. Most of the time they don’t line up just right and the new Moon passes above or below the Sun as seen from Earth, and everyone expect for the people with the exotic telescopes just go about their day. A couple times a year, the Moon’s orbit crosses the Sun, at least by a little bit, and we get at least a little bit of an eclipse. This little bit, when the Moon partly covers the Sun, is called a partial eclipse.
Also, the Moon’s orbit isn’t round. It’s elongated, like an egg. At some points on the orbit, the Moon is farther from the Earth than at others. When the Moon is its farthest point from the Earth (about 252,000 miles / 405,000 km), it’s said to be at apogee. At its nearest (about 226,000 miles / 363,000 km), it’s at perigee. Things that are closer look bigger than things that are farther away. When the Moon is near apogee, it appears slightly smaller in our sky than it does when it’s near perigee.
When an eclipse happens near perigee, the Moon’s closest point to Earth, the Moon is bigger as seen from Earth, and it’s able to block out more of the Sun than it can when the Moon is further away; near its apogee. When the entire Sun is eclipsed, it’s called a total eclipse. This is the famous sort of eclipse most of us think of us when we think of solar eclipses. In near-apogee eclipses, the Moon cruises across the Sun, and at the maximum eclipse, there’s still a ring of the Sun clearly visible behind the Moon. The word for this is annular, which means “ring-shaped.” I’ve also seen these called “Ring of Fire” eclipses. Johnny Cash would be proud. Annular comes from the same Latin root as annual, meaning yearly; the year is a ring.
Eclipses are always incredible things for many reasons, and I love watching them. It’s amazing, as I’ve said a bunch of times, to be able to see the solar system in action. We tend to be so focused on our lives, that we sometimes forget there’s all of these other things happening with or without us. To actually be able to see the Moon blot out the Sun for a time is something that has been stopping people in their tracks for ages. I’ve only ever seen a couple of solar eclipses, partial ones, many years ago.
If you happen to be tuning in from somewhere near the path of the Moon’s shadow tomorrow, thanks, and I hope you enjoy the show, and have clear skies!