Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing "Hoax" - Philip Plait (2002)
Part III. Skies at Night Are Big and Bright
Chapter 13. Shadows in the Sky: Eclipses and Sun-Watching
e humans have spent a long, hard time learning that the Earth is not a special place. It's not at the center of the universe, there are probably millions of planets like it in the Galaxy, and we may not even be the only place with life.
But there is one thing special about our blue home. It's a coincidence of place as well as time, and it is unique among all the moons and planets in the solar system. The Sun is much bigger than the Moon-about 400 times as big-but it's also 400 times farther away from us. These two effects cancel each other out, so, from our perspective down here on Earth, the Moon and the Sun appear to be the same size in the sky.
Normally, you'd hardly notice this. For one thing, the Sun is so bright it's hard to look at, making its size difficult to judge. For another, when the Moon and Sun are near each other in the sky, the Moon is a thin crescent and difficult to see. (Check out chapter 6, "Phase the Nation," for more about the Moon's position and shape relative to the Sun.)
But there is one time when it's pretty obvious that they're the same size, and that's when the Moon passes directly in front of the Sun. When that happens, the Moon blocks the Sun and we get what's called a solar eclipse. The eclipse starts small, when only a bit of the Sun gets blocked by the edge of the Moon. But as the Moon's orbital motion sweeps it around the Earth, more and more of the Sun disappears behind the Moon's limb. We see the Moon in silhouette, a dark circle slowly covering the Sun. Eventually, the entire disk of the Sun is blocked. When this happens, the sky grows deep blue, almost purple, like at sunset. The temperature drops, birds stop singing, crickets will chirp, and it's like having a little night in the middle of the day.
This would be odd enough, but at the moment of totality, when the Sun's disk is completely covered by the Moon, the Sun's outer atmosphere, called the corona, leaps into view. Normally invisible because the Sun's surface is vastly brighter, the corona is wispy, ethereal, and surrounds the Sun like a halo or aura. When the corona becomes visible, viewers almost universally gasp in awe and delight, and some have been brought to tears by the sheer beauty of it.
Eclipses are magnificent, and they do not happen very often, but they are predictable. The Moon's path in the sky has been charted for millennia, and ancient astronomers could predict eclipses with perhaps surprising accuracy. It's not surprising then that historical records are full of tales about eclipses. Mark Twain even used one in his novel A Connecticut Yankee in King Arthur's Court. In it a young man from America is transported back in time to medieval England and, through a variety of circumstances, ends up being sentenced to be burned at the stake. However, he happens to know that a total solar eclipse is about to occur and tells his captors that if they don't release him, he'll take away the Sun. Of course, the eclipse happens right on schedule and he is set free.
That may sound silly, but it's based on an actual event, and none other than Christopher Columbus is in the leading role. In 1503, on his fourth voyage to America, Columbus was stranded in Jamaica, his ships too damaged to be seaworthy. He relied on the natives for food and shelter, but they soon became weary of feeding Columbus's men. When the natives told him this, Columbus remembered that a lunar eclipse-when the Earth's shadow falls on the Moon, turning it dark-would occur soon. Just as Twain retold the tale nearly 400 years later (with a solar instead of a lunar eclipse), the event terrified the natives, who then begged Columbus to bring back the Moon. He did, and he and his men were able to stay on the island until they were rescued.
Occasionally, events are antithetical to the Columbus storynot predicting an eclipse can get you in trouble. In ancient China it was the duty of all court astronomers to predict solar eclipses. The Chinese thought that a solar eclipse was a giant dragon eating the Sun, and if enough advance warning were given they could chase the dragon away by beating drums and shooting arrows in the sky. In 2134 B.c., as the story goes, two hapless (and perhaps apocryphal) astronomers by the names of Hsi and Ho didn't take their duties too seriously. They knew a solar eclipse was coming, but decided to hit the tavern first before telling the emperor. They drank too much and forgot to pass on the news. When the eclipse came, everyone was caught off guard. Luckily, the emperor was able to "scare off" the dragon, and the kingdom was saved. Hsi and Ho weren't so lucky. They were collected, thrust before the emperor, presumably chastised, and not-so-presumably had their heads cut off. Legend has it that the emperor threw their heads so high in the air that they became stars, which can be dimly seen between the constellations of Perseus and Cassiopaeia. (Today we know these two faint objects to be clusters of thousands of stars, and they're a pretty sight through a small telescope-prettier, no doubt, than this gruesome Chinese legend might have you think.) The lesson here is still relevant today: publish first, and then head off-so to speak-to the bars.
Even today, people are superstitiously terrified of eclipses. After a total solar eclipse in August 1999 that was seen all over Europe, I had an e-mail conversation with a young woman from Bosnia, which was then suffering from terrible fighting. She was shocked and saddened to see the streets deserted during the eclipse and the signs posted to warn of dangerous rays from the Sun that would kill people exposed to them. As if these people didn't have enough to worry about, they also had to hide in fear of something that might have actually given them a fair degree of much-needed joy.
Not all eclipse fears are so severe. There are many legends among ancient people about solar eclipses, and having the Sun eaten is a common thread. Others have seen it as a bad omen, so they pray during eclipses. Still others avert their eyes, lest they have a spell of bad luck cast over them .. .
... which brings us to a very interesting and somewhat controversial point about the Sun and eclipses. How many times have you heard that looking at an eclipse will make you go blind? Every time a solar eclipse rolls around, the news is full of warnings and admonitions. The problem is, they never say exactly why you can go blind, or what degree of eye damage you might suffer. Worse, they sometimes give incorrect advice on viewing an eclipse, increasing the danger.
I'll cut to the chase: viewing an eclipse can indeed be dangerous. Obviously, looking at the Sun is very painful, and it is extremely difficult to do so without flinching, tearing up, or looking away. The Sun is just too bright to look at. Every astronomy textbook I have ever read has an admonition against looking directly at the Sun, and it is common knowledge that looking at the Sun, even briefly, can cause permanent damage to your eyes.
While researching information on solar eye damage for this chapter, I stumbled across an amazing irony: while it is dangerous to view an eclipse with the unaided eye, it is actually far less dangerous to look at the Sun when it is not eclipsed! This may sound contradictory, but it actually is due to the mechanisms inside the eye that prevent overexposure from light.
There is copious evidence that little or no long-term damage results from observing the uneclipsed Sun. I was shocked to find this information; I have been steeped in a culture that says looking at the Sun with the unaided eye will result in permanent and total blindness. However, this is almost certainly not the case.
Andrew Young, an adjunct faculty member of the San Diego State University's Department of Astronomy, has collected an astonishing amount of misinformation concerning solar blindness (see http://mintaka.sdsu.edu/GF/vision/Galileo.html). His research flies in the face of almost all common knowledge about solar blindness. He is quite strong in his statement: under normal circumstances, glancing at the Sun will not permanently damage your eyes.
"The eyes are just barely good enough at rejecting [damaging] light," Young told me, because the pupil in the eye constricts dramatically when exposed to bright light, cutting off the vast majority of light entering the eye. Most people's retinae don't get overexposed when they glance at the Sun. Young quotes from a paper, "Chorio- retinal Temperature Increases from Solar Observation," published in the Bulletin of Mathematical Biophysics (vol. 33 : 1-17), in which the authors claim that under normal circumstances, the constriction of the eye's pupil prevents too much light from the Sun from actually damaging the retina. There may be a slight (4 degrees Celsius) temperature rise in the tissue, but this is most likely not enough to cause permanent damage.
However, natural variations in the amount of pupil constriction between different people means that some might still be prone to retinal damage this way. These people make up the majority of solar retinopathy patients-people who suffer eye damage from looking at the Sun.
According to physicians at the Moorsfields Eye Hospital in London, England, observing the Sun can cause damage to the eye but not total blindness. On their web site [http://www.moorfields.org uk/ ef-solret.htmll, they report that half their patients with eye injury recover completely, only 10 percent suffer permanent vision loss, and, most interestingly, never has anyone had a total loss of vision from solar retinopathy.
So there is damage, and sometimes it can be severe, but most people recover, and no one has ever become totally blind by looking at the Sun. However, due to the natural variation in pupil constriction from person to person, I think I still need to stress that while it very well may be safe (or at least not very dangerous) to glance at the Sun, staring at it may still cause damage. The damage is most likely minimal, but why take chances? Try not to stare at the Sun, and try to minimize any glances at it. You might be part of the group that will suffer some injury from it.
So, if the full Sun is not likely to be dangerous, why should viewing a solar eclipse cause eye injury? During an eclipse most or all of the Sun is blocked by the Moon. However, think about what happens inside your eye when you view an eclipse. During a total eclipse the Sun's surface is completely covered by the Moon, and the sky grows dark. When this happens, your pupil dilates; that is, it opens up wide. It does this to let in more light so you can see better in the dark.
The Moon completely blocks the Sun's disk for a few minutes at most. Suddenly, when this phase of the eclipse ends, a small sliver of the Sun is revealed. Even though the total light from the Sun is less than when it is not being eclipsed, each part of the Sun is still producing just as much light. In other words, even if you block 99 percent of the Sun's surface, that remaining 1 percent is still pretty bright-it's 4,000 times brighter than the full Moon. An eclipse is not like a filter, blocking the light from hitting your eyes. Any piece of the Sun exposed will still focus this harmful light onto your retina, causing damage.
So when the Sun becomes visible again, with your pupil dilated wide, all that light gets in and hits your retina-and it's then that sunlight can really and truly hurt your eye. The bluer light can cause a photochemical change in your retina, damaging it, although most likely not permanently. This effect, according to Young, is worse in children because as we age the lenses in our eyes turn yellowish. This blocks blue light, better protecting older retinae from the damage. However, children's lenses are still clear, letting through the bad light. So while it's dangerous to look at an eclipse, it's even more so for children.
I'll note here that another common misconception about an eclipse is that the x-rays emitted from the Sun's corona can damage your eyes. The corona is extremely hot, but so tenuous that the normally bright Sun completely overwhelms it, making it extraordinarily difficult to observe during the daytime.
The corona is so hot that it gives off x-rays. Most folks know that x-rays are dangerous; after all, you have to wear a lead shield when getting an x-ray at the dentist. So many people put these two facts together and assume that it's the corona that can damage your eyes during an eclipse.
This is wrong. X-rays from any source in the sky cannot penetrate the Earth's atmosphere, which, for all intents and purposes, absorbs every x-ray photon coming from space. It acts like a shield, protecting us. Even if the atmosphere were not here, the corona is also simply too faint to hurt our eyes. And remember: whether the Sun is eclipsed or not, the corona is still there; it's just too faint to see. So if it could hurt your eyes during an eclipse, it could do so at any random time. In reality, the corona can't hurt you.
There are several ways to enjoy an eclipse without risking your eyes. You can use a telescope or binoculars to project the image of the Sun onto a piece of paper or a wall. You can wear very dark goggles, like welders wear; make sure they are rated as #14 so that they are dark enough to be comfortable.
You can also use a solar filter on a telescope or binoculars, but only the kind that mounts in front of the main lens or mirror. This stops most of the light from entering the optics in the first place. Some companies sell filters that go on the eyepieces, which block the amount of light leaving the optics. However, the optics focus all that sunlight right onto that filter, which can heat up a lot. Filters like this have been known to melt or crack. I heard one story of a solar filter that actually exploded! That's bad enough, but then your eyes are flooded with all that sunlight concentrated by the optics. The lesson: stay clear of such devices.
Also, despite some advice I have seen, do not use unexposed film to block the light. Even as lofty a source as the CNN web site once claimed that it was safe to view an eclipse this way. This is actually a very dangerous way to do it; it lets through less visible light, so your pupils widen. However, it does not block the dangerous wavelengths of light, so that even more damaging light floods your eye. I and several hundred other people flooded CNN with e-mail, and the web site was hastily fixed.
Solar eclipses do not happen very often, and they usually occur over scattered parts of the planet. I have never seen a total solar eclipse, though I've seen a dozen or so partial ones. Someday I hope to see a total eclipse myself, but when I do, I'll be careful.
And I'd better hurry. As is discussed in chapter 7, "The Gravity of the Situation," the Moon is slowly receding from the Earth. It's only moving away about 4 centimeters (2 inches) a year, but over time that adds up. As it gets farther away, it appears smaller in the sky. That means eventually it will be too small to completely cover the Sun during a solar eclipse. Instead, we'll get an annular eclipse, a solar eclipse where the Moon's size is somewhat smaller than the Sun, and you can see a ring of Sun around the dark disk of the Moon. We get these eclipses now because the Moon's orbit is elliptical, and if the eclipse occurs when the Moon is at the highest point in its orbit, the eclipse is annular. But, eventually, these will happen all the time. The corona will forever be hidden by the glare of the Sun and solar eclipses will be interesting, but lack the impact they have now. That's why, in the beginning of this chapter, I said that total solar eclipses are a coincidence of space and time. Given enough time, they won't happen anymore.
I can't leave this chapter without busting up one more misconception. It is an extremely common story that Galileo went blind because he observed the Sun through his telescope. I have said this myself, even once on my web site. Andy Young e-mailed me about it and set me straight.
Galileo did indeed go blind. However, it was not due to observing the Sun. Galileo realized rather quickly that looking through his small telescope at the Sun was a quite painful experience. Early on he only observed the Sun just before sunset, when it is much dimmer and safer to see. However, he later used a projection method to view the Sun and observe sunspots. He simply aimed his telescope at the Sun and projected the image onto a piece of paper or a wall, casting a much larger image of the Sun. This method is far easier and produces a large image that is easier to study as well.
Certainly, using a telescope to observe the Sun can indeed cause damage to the eye, since a telescope gathers the sunlight and concentrates it in your eye (much the same way that you can burn a leaf with a magnifying glass). However, this sort of damage occurs very rapidly after solar observation, and Galileo did not go blind until he was in his 70s, decades after his solar observations. There is copious documentation that during the intervening years his eyesight was quite good. Galileo suffered from cataracts and glaucoma later in life, but this was clearly not from his telescopic observations.
Galileo's observations of sunspots on the Sun caused quite a stir; the Catholic church had considered the Sun to be unblemished and perfect. Together with his observations of Jupiter, Venus, the Moon, and the Milky Way itself, he revolutionized our way of seeing and thinking about science, ourselves, and our universe. Yet we still can't get simple stories about him correct. Maybe we are the ones who are sometimes blind.