What is an Eclipse?

From a little pamphlet I found that was written for the 24 Jan 1925 eclipse:

How about that for imagery? I didn't know "to soul" was a verb. Ah, well....

Here we go. It's time for the geeks to get their slide rules out from the heavy-duty, industrial-strength pocket protectors they carry them in, and amaze you with their ability to simultaneously flaunt their superior knowledge by obliviously plowing their way through a description of what an eclipse is in great scientific detail (which no one cares about), and bore you to tears in the process. Aren't you ready to have that kind of fun? Won't that just turn you onto eclipses forever? Uh-huh. Sure.

Well, that's not how I play the game. Yes, I think you'd have to agree that it's important you know something about what an eclipse is before you go globe-trotting out to see one, but isn't everything that way, though? Aren't respect and admiration for new things always more easily built through familiarity? I know without a doubt that if I'm going to really enjoy something to its fullest, I'd better have digested every word I could find that's been written on the subject, and then have gone over it so many times in my head that I could probably explain it to someone in that kind of dry, scientific detail. My wife says that's weird, though, and part of the reason I married her was to inflict a certain amount of common sense on my otherwise blissfully abnormal thought processes. So, I'll grant that most of you don't want to be bored with the math. OK, fine:

My wife would love that description, but I hope you'll let me go just a little further with it. Just to make sure I don't lose you here, let me say that the purpose of this page is to be a kind of memoir of my experiences during the decade of the 1990's, and not to be an astronomy course covering every imaginable aspect of the subject of eclipses in gory detail. If you want that, there are plenty of books out there. In fact, in preparing for this section, I realized that there was nothing I could add to the body of knowledge out there that's just waiting to be devoured by a new eclipse devotee. I'm just going to give you the basics, because either you already know the science, and want to get through this part as fast as possible, or you just want to know enough to see if this whole eclipse thing is something that could interest you. You should be OK, regardless of which camp you fall into. I'll try not to ramble.

From our view here on earth, the sun and the moon both appear to move across the sky. In fact, for thousands of years, that was indeed the accepted fact among scientists, the church, and anyone else who gave it any thought. Of course, this whole earth-centered idea was thrown out once people started seeing all the holes in it. The development of a body of scientific thought and reasoning, combined with people's gradual acceptance of the fact that the church might not be as all-powerfully perfect as it claimed, allowed us to finally break free of our narcissistically geocentric view of the universe, and see the situation for what it really was.

As it turns out, though, there are some concepts involved in studying eclipses where it's actually preferable to consider the earth as stationary, with the sun and moon "moving" around it. If you've ever noticed, the sun and the moon seem to take almost the same path across the sky, moving in the same direction. Of course, the moon takes a month to go all the way around the sky, and the sun takes an entire year (or a day, depending on what we're talking about, I guess), but they still travel the same apparent path. This path is called the "ecliptic", and the sun and the moon aren't the only ones who use it. All the other planets (except Pluto) do, and the string of constellations we call the Zodiac happens to lie on it as well.

To be fair, the apparent path of the moon isn't the true ecliptic; that's actually the apparent path the sun takes in its annual voyage through the Zodiac. The moon moves in its own orbit that's tilted a little bit from the actual ecliptic. But the moon is always within a few degrees of it, and for practical purposes, we can say they take the same basic path through the sky. Any more precise than that, and I'd be throwing too much of the technical stuff at you.

With all these heavenly bodies traveling pretty much the same little strip of sky all the time, it makes sense that once in a while, at least as seen from earth, one or two of them should apparently "hit" each other, taking up exactly the same piece of sky at the same time. Well, that's exactly what happens. Sometimes, a couple of times per decade, the moon goes in front of a planet or star. This is called an occultation. Sometimes (very rarely), one planet covers up another planet. Ditto the name on this one, but it's such a rare event, I don't even know when the next one's going to be. (Not in my lifetime, for sure!)  Once in a while, Mercury or Venus will seem to travel right across the face of the sun. This is called a transit; it happens a handful of times per century for Mercury, and twice every other century for Venus. These are all relatively rare events, because let's face it: A planet is a pretty small thing, and it's a pretty big strip of sky we're talking about. The moon and sun are much bigger, though, so we'd expect them to hit each other a little more often.

And we get what we expect. No fewer than twice, and as many as seven times per year, the moon moves right across the face of the sun, and blocks out part or all of it for people who happen to be in the right place on the earth's surface. We call this a solar eclipse, and it can be of several different types, depending on the exact positions of the earth, moon, sun, and observer.

Why don't we have an eclipse every month, though? As the moon moves around the earth once a month, shouldn't it always appear to hit the sun at the right time? Well, remember, I said the moon's path around the earth isn't exactly the same as the sun's. Since the moon's path is tilted with respect to that of the sun, there are only a couple of times per month that the path of the moon's orbit intersects the path of the sun's orbit, as seen from earth. And, beyond that, we need to have that point of intersection of the two paths be at the spot where the sun happens to be at that time, if we want the moon to appear to hit the sun. Sounds pretty rare, huh? Well, it is, relatively, but it does happen, with great regularity, between two and seven times per year. Of course, when it does happen, you have to hope you're in just the right spot on earth if you want to see it. There's a lot of the earth's surface that's just plain inconvenient to get to, and if you refuse to leave home to see one, you'll wait a very long time before an eclipse decides to occur exactly in the spot where you are. How long? About two or three times per thousand years (for a total eclipse), on average. But that's only on average -- it may be you'll get two in your lifetime, and it may be that your great-great grandkids will still be waiting. Thank goodness for airplanes.

Now, we should shift gears for just a second, and look at the big picture of what makes an eclipse, from space. Before we do that, though, let's think about some things relating to the most major player in the story: The Shadow.  From a very young age, we see and understand the concept of a shadow from our experiences on earth. You've got some light coming from a source, and if it hits something in its path, then behind that something there'll be a dark spot that's roughly the same shape as the intruding object. Simple enough, right? Well, that's true, it is, but there are a couple more things we need to know about shadows before we can understand their importance in creating a solar eclipse. It's diagram time:

I've included this simple drawing to help show the important elements of the Shadow. Notice how I've drawn four lines from the sun, touching the moon in one place each, and then continuing on toward the earth? Those lines are called internal or external "tangent" lines to the two disks, meaning they only touch each circle in one spot. The shadings within their boundaries have significance also, and I'll try to explain what they mean (though it's really tough to do without being able to point at the picture. You'll get the idea).

First of all, the very darkly shaded, triangle-shaped area in the middle, coming off the moon, is called the umbra. (This region is actually a cone, because it's in three dimensions. The triangle is only a central cross-section.) In this dark area, all light from the sun is blocked, and it's totally dark. ("Umbra" is kind of like "umbrella", get it? All the light, all the rain, blocked?) Anyone standing or floating within this cone will have the sun completely blocked from them, and could rightfully say they were observing a total solar eclipse.

That doesn't really make much sense, though, because what about somebody standing on the surface of the night side of the moon? They're in this shadow, too, right? Why shouldn't they just call it "night" instead of an "eclipse"? And someone who was in space close to the moon would not really call what they were seeing an "eclipse", either. The moon's disk would be so large from their vantage point, it would not be natural to call the fact that the sun happened to be behind that huge moon anything more than, well, the fact that the sun was behind the moon. What gives? Why do we give this normal phenomenon a special name?

To find out, we note that the umbral cone has a tip that extends just barely to the surface of the earth. This is significant, because it means that the moon's and sun's diameters and distances are in such a special relationship that they appear almost exactly the same size, as viewed from Earth. If we had a moon that was huge in apparent size, then it would be a monthly event for the sun to get blocked out, and it'd be pretty much ho-hum. Our species would have adapted to those few hours of darkness every month a looooong time ago, each major religion would've incorporated something about them into its list of things to say something about, and we'd notice the event about as much as we notice the full moon nowadays.

If our moon were very small, compared to the sun, then we'd never have a total eclipse, and transits of the moon would garner about as much excitement from the general public as transits of Mercury and Venus do now, which is to say, none. (If you don't believe me, recall the excitement you remember from your area's last annular eclipse, when a very large moon "transited" the sun. 95% cover of the sun => a little darkening => almost no excitement whatsoever.)

But we have a moon that is almost exactly the same apparent size as the sun, so total eclipses are rare enough, and cool enough to look at, that they do generate some excitement. When the moon covers all of the sun, it covers up neither too much nor too little. It covers the sun, the whole sun, and not much else except the sun. That means we get to see the corona.

The corona? Yes, the corona. The brilliant, white, shimmering, radiant crown of intensely hot gas that constantly surrounds the sun, but which we don't get to see, ever, except during a total eclipse. Normally blocked from view by the overpowering brilliance of the sun itself, this magnificent halo blasts into view the moment before the last drop of sun is covered by the moon during an eclipse. It is wonderful to see, and no picture ever made can come close to reproducing it. (Though the techniques have gotten quite good with computer-aided enhancements. I will attempt my own go at this with pictures from the 1999 eclipse in Turkey.) You have to see it to believe it, and then you will know what I mean. The dynamics of what you see at a total eclipse are the narcotic that compels you to see them again and again. If the sun's or moon's diameter or distance were so much as 10% different one way or the other, we'd never get to feel this. (Then, of course, we wouldn't care, but that's a different story.)

Back to the diagram. See the lightly-shaded area on either side of the umbra? That's the penumbra ("almost-umbra", as in the word "penultimate" ["almost-last"]). In this weirdly-shaped region, part of the sun's light is blocked, and part is visible directly. The visible part is, of course, the sun's disk, but if you were to look at the disk from within this region, you'd see a "bite" taken out of it. The farther away from the umbra you are, the more sunlight is visible, so there's kind of a graduated effect from outside to inside. It gets dimmer and dimmer, until finally you're in the umbra, and you've got blackness.

Now, the sun's light is pretty bright. In fact, if only 5% of the sun's disk is visible, you almost can't tell without using a really good filter that the entire disk isn't there, blazing away! It's hard to believe, but it's true. Any amount of the sun's disk is incredibly bright, and can fry your retinas pretty quick. The point to that, though, is that the gradual dimming you'd experience by traveling from outside to inside within the penumbra is very small. Even with the scattering of the earth's atmosphere, we see a pretty stark boundary between the penumbra and the umbra during an eclipse. How do we know this? Who is it, that during an eclipse, takes the time to look at the moon's shadow to know how distinct that boundary is?

The answer is that we don't have to wait for an eclipse - we can go outside any sunny day, and look at our own umbra-penumbra boundary on the ground beneath our feet! We ourselves cast a shadow, and it acts exactly like the moon's shadow for such experiments! Whenever you look at any object's shadow, you'll notice a fuzzy gray area around the edge of the dark part of the main shadow. That fuzzy place is not due to atmospheric diffraction (well, OK, maybe a little of it is, but not enough to worry about), it's due to the fact that part of the sun's disk is directly visible from that spot on the ground. The moon's penumbra is much larger only because it's much farther away. If you go stand on a cliff, and attempt to view your penumbra from hundreds of feet away, then atmospheric diffraction will get you, and the penumbra will be so dim, you won't be able to see it. It's there, though, and from that distance, it would look pretty big!

Part of the effect of an eclipse is the sharpening of your own shadow on the ground as more and more of the sun's disk gets covered by the moon. Think about it - as the sun's apparent size shrinks (as it gets covered by the moon), your own shadow's penumbra shrinks. Point sources of light offer almost no penumbra, and so it is with your shadow as the eclipse progresses. We are so used to seeing that little fuzzy edge around our shadows, that when it's gone, it's a weird experience. People usually have a hard time putting their finger on just exactly what's causing the eerie feeling, too. They say the light looks "clearer", or that it seems like everything is coming into focus a little sharper. It is just like what happens to a near-sighted person when they first put on their very first pair of glasses. They never knew things could be any different than fuzzy, so it feels weird to see everything so clearly. It's one of the things you can't explain to someone who's never seen an eclipse, because the only way to get the effect is to be at an eclipse!

You can see what I'm talking about with this diagram. Note how, with an increasingly smaller source of light, a given object's penumbra becomes very small, while its umbra actually grows to fill more of the "shadow" space:

Of course, you only have a few minutes to enjoy it. First contact (the moment the moon's disk first touches the sun's disk) happens about an hour and a half before the main show, and it's easy to lose interest in the whole experience during the first 70 minutes. Once the sun's disk has a really large bite taken out of it, though, the show really begins. The next 20 minutes are what you've been waiting for. All the pre-eclipse phenomena, the shadow's approach, Baily's Beads, Second Contact (the beginning of totality), and totality itself, with the corona popping into view amidst the whoops and screams of your fellow eclipsers, are experiences not to be missed. I really can't overstate this: you will be amazed at the absolute impotence of your native language in its ability to provide you with suitable adjectives to describe the whole thing!

After third contact (the end of totality), it all happens in reverse. The sun's disk comes back into view over the next 90 minutes, and daylight returns. I've never noticed anything during that time, though, because I was too busy letting the adrenaline subside, sharing stories with others at my site, or stuck in traffic jams driving back to the hotel. My goal by fourth contact is not to even know when it happened!

If you were to watch the moon's shadow from some point out in space, you'd see it sweep across the face of the earth with amazing speed. How fast is it? Well, again, the math is a little complicated, but we'll throw out a ballpark figure of between 1500 and 2000 miles per hour (at points on the path closer to the middle than the ends)! That's about a half-mile per second!

What does that look like to an observer on the ground? Well, you have to see it to really understand, and even if you're there, if you're not quick enough, you'll miss it. But what you basically see is an area of darkness to the west of you, that looks kind of like a distant storm. As second contact approaches, of course your concentration is going to be centered mostly on the sun; it's about to go away, after all, and the excitement keeps your eyes religiously glued to it (through your filters, of course). But if you hazard an occasional glance, you'll notice the darkness getting, well, darker. If you happen to be standing on high ground, and can look out on lower elevations to the west, the effect will be heightened even more.

Just before second contact, and I mean seconds before, that darkness to the west will appear to rise up out of the earth itself and rush toward you like a gigantic, dark, transparent curtain being pulled over your head. The effect is magnificent, but is often missed, since it tends to coincide with the appearance of Baily's Beads and the Diamond Ring. If the ground to the west of you is lower than you are, you'll swear you can actually see the shadow race over the ground toward you in the last seconds, like some huge, insanely fast cloud that's set on devouring all mankind. It's really a primordial feeling, but such is the nature of eclipses. And, all this is happening as you stand there transfixed, watching that thin sliver of sun shrink in real time down to nothing. The sensory overload is amazing, and it's always fun to see first-timers completely forget everything they'd planned to do during the eclipse because of it! They prepared so much for the technical parts of the event, they didn't count on the emotional overload causing them to completely forget where they were and what they were supposed to be doing! (I say that completely sympathetically; it's happened to me!)

Of course, once the eclipse is total, the moon's shadow doesn't stop moving. It has a date with more eclipsoholics to the east, and so, on it goes, oblivious to your screams and cries and pleas for it to stay. But there is one way you could be in its shadow for a longer amount of time, and that is to be in an airplane. Not just any airplane, though; it would have to be fast enough to actually have a chance at being able to keep up with the shadow, and there aren't many planes like that.

I'm not exactly sure when this experiment was first tried, either, although I'm sure it was a long time ago. One recent time that I'm aware of was on 30 June 1973, when a Concorde was loaded up with instruments and scientists, and kept up a pretty steady pace over Africa to try and keep up with the shadow. They did a pretty good job, too, with the plane hitting its max speed of almost Mach 2! The sloooooooow advance of the moon's shadow must have been wonderful to watch (although, since the instruments got all the window seats, I doubt too many of the people on board saw much of anything!), and how about the length of their totality? I heard that they got an hour and 14 minutes!! What a great gig.

(Note: As I write this P.S., in July 2000, it is being bantered around to have the Concorde attempt another such flight for the 2001 eclipse over Africa. It will be an expensive trip, and there will not be as much totality to see, due to the repositioning intervals the plane will have to go through due to Angola's insistence on no supersonic flight in their airspace, but hey, what the heck.)

Update 2006: The 2001 flight of Concorde was canceled due to the tragic mishap that occurred in France (which grounded the entire fleet). In 2003, a plane flew through the shadow over Antarctica, and in August 2008, another big jet plied the skies over the Arctic to intercept the umbra. I was on that one!

© 1999-2008 Dan McGlaun