The October 2023 Solar Eclipse

On October 14th, 2023, you may be able to observe the second in a triplet of "Great American" eclipses.

Most residents of the United States Southwest are about to see something dark pass across the face of the sun. I'll use Denver, Colorado as an example: At approximately 9:15 in the morning on Saturday, October 14th, residents of Denver who look up will see the upper or western edge of the sun begin to disappear. (Actually, unless you have a way to reduce the glare and get a good look at the precise shape of the sun, you probably won't notice any difference from a normal day for another hour or so, even if you look directly at the sun.) More and more of the upper or western edge of the sun will disappear until, roughly an hour and a quarter later, about 70% of the face of the sun will be extinguished and the outdoors will be noticeably dimmer. If you have a way to get a good look at the shape and size of the sun, you may notice that it is still circular, but a portion of the face is missing — a circular portion, with a diameter roughly the same as the sun itself in the sky. After about 10:30 AM, this phantom circle will move away to the left or the east, revealing more and more of the sun until finally, at about noon, it will be gone and normal daylight will be fully restored.

In summary, residents of Denver will see a “lacuna” or gap in the face of the sun, this lacuna will have the shape of a circle, and it will appear to pass slowly across the face of the sun from the western side to the eastern side. It will be as if something huge and round and dark is passing between us and the sun in outer space, overlapping it in the sky, and depriving us of its light. Residents elsewhere in the southwestern United States will see something similar, but the mystery circle will appear to “aim” differently and “strike” the sun more or less off-center, as if we are viewing the same two objects in the sky from different angles. Depending on where you live, you may see the circle merely graze the edge of the sun, or you may see it “aim” perfectly, and pass directly across the center of the sun. (Residents of Santa Fe, New Mexico and others who live along the “centerline” of the eclipse path will see the phantom circle pass directly over the center of the sun, and will have a brief moment when they can see the circular “hole” perfectly centered in the face of the sun, leaving a thin ring or “annulus” of the sun exposed around the circumference.)

The exact shapes that you see, and the times when you see them, will depend on where you live. If you want to view a map of the eclipse path or to look up details for your city, I recommend the Time and Date website. You can enter your city, and it will calculate the schedule of events for your location.

Looking at the Sun

Isn't it Dangerous to Look Directly at the Sun?

Have you ever glanced at the sun, or any extremely intense light, and noticed that you see spots for a short time afterwards? These are normal afterimages, and they go away within a minute or so. But what happens if, instead of merely glancing, you stare at the sun for a long time? Most people wouldn't want to do this, because it hurts. The defensive mechanisms in your eyes are giving you a signal that maybe you should stop looking at the sun. But sometimes people stare at the sun anyway, perhaps for some ritual, or perhaps because something is blocking their pain signals and they don't realize that anything is wrong. These people see spots, too, but their spots take a long time to go away. These people have suffered from “solar retinopathy,” and they have a “scotoma,” or a hole in their vision, for several weeks or even several months. In extreme cases, their spots might never go away. (Ophthalmologists might disagree with this analogy, but I think of it like a cut or a scrape in your skin: Your retina can heal itself of small injuries, but in severe cases you might be left with a permanent scar. Your retina is just a patch of sensitive skin, after all, as you may have discovered if you have ever dissected an eyeball.)

So on normal days, most people have nothing to fear from the sun. As long as you allow your eyes' natural adjustment and defensive mechanisms to function normally, you have no need to worry about a quick glance at the sun. And by the way, this is especially true when the sun is close to the horizon. To see what I mean, let the sun shine on your face at midday today, and compare the light and warmth of the sunshine with those from the sunset tonight. When the sun is very low, its rays have to pass sideways through the atmosphere to reach you, and its power is reduced to much more comfortable and safe levels. So go ahead and enjoy the beauty of sunrises and sunsets all you want, and you won't harm your eyes.

The problem with solar eclipses is that you have a reason to want to stare at the sun. When the sun is high in the sky, there is so much overpowering light surrounding it that you can't get a good look at the exact shape. All you see is a puddle of glare. So if you want to see exactly what's going on up there, you are tempted to stare, or to find some way to get a better look. This is where a little more care becomes necessary. Another problem with solar eclipses is that they can “mislead” or interfere with your eyes' normal defensive mechanisms. With most of the sun covered, the sun doesn't seem nearly so bright and it doesn't seem as dangerous to stare, yet the portions of the sun that are still exposed are still emitting sunshine with the same intensity and can still harm your retinas. So with solar eclipses, extra precautions become necessary.

First of all, you obviously want to avoid staring. Do not stare at the sun when it is high in the sky, even if only a small portion of it is exposed. You also need to be extremely careful about looking at the sun through anything. You obviously don't want to look at the sun through binoculars or telescopes (at least not without special accessories). You may be tempted to look through binoculars or a telescope while wearing eclipse glasses on your face. Imagine holding a magnifying glass in the sunshine over your eclipse glasses, and you'll see why this is a bad idea. (You can see wonderful views of the sun with a telescope and a “solar filter,” but the filter has to go on the FRONT or over the OBJECTIVE LENS of the instrument, not over the eyepiece or over your eyes.) You should even be careful about looking at the sun through cheap sunglasses because, depending on the material, they can “trick” your eyes into thinking that it is relatively dark while still allowing all of the harmful UV rays into your eyes at full strength. You will also want to be wary about covering your eyes with “eclipse glasses” or otherwise allowing them to adapt to darkness, and then suddenly exposing them to direct sunshine without allowing them a moment or two to adapt to the light again.

A few websites tell you to never, ever look directly at the sun, because it can cause you to go blind. The sun is not as dangerous as this type of hysterical prohibition makes it out to be, and you should not feel as if you need to hide your eyes every time you are outdoors. Your eyes are perfectly capable of protecting themselves from harm in normal circumstances. A barely-eclipsed sun is no different from the sun on a normal day and is no more dangerous (other than making you want to stare at it). There are some risks to observing a deeper and darker solar eclipse, but as long as you act wisely and take some basic precautions, there is no reason you can't observe a solar eclipse safely. In a nutshell, don't stare even if it seems safe or comfortable to do so, and be very careful about looking at the sun through anything.

(Perhaps I should add that you might need to be even more careful during a total eclipse, because the near-total darkness causes your eyes to adjust to the dark and become more sensitive, and this makes the returning sunshine more dangerous. However, that advice doesn't apply in this case because the October 2023 eclipse will never become total. And even during a total eclipse, the events do not happen so rapidly that you can't simply look away from the sun or put your eclipse glasses on when you sense the light beginning to return.)

Getting a Better Look at the Sun

Whether or not it is harmful to look directly at the sun, it usually doesn't do much good. There is so much glare that it is very difficult to see the edges and the exact shape of things. If you want to get a better look at the exact shape of the sun, during an eclipse or on a normal day, you need some help.

One of the simplest and most effective ways is to buy a pair of “eclipse glasses.” These are specially made for looking at the sun, they are quite cheap, and they are similar to sunglasses but much, much darker. If you look through them at anything other than the sun, you will see nothing, because they are so dark. If you look through them at the sun on a normal day, you will see a beautiful, perfect glowing disk. If you look through them at a solar eclipse, you will see a beautiful glowing disk with a circular portion missing.

A Partially Eclipsed Sun (Seen Through a Solar Filter)

Another way to see the shape of the sun is to make a pinhole projector. I'll let you search for more detailed instructions elsewhere, but the basic idea is to place a tiny hole in the sunshine, and allow the sunlight coming through the hole to land on some kind of card or screen. The odd thing about pinholes is that, as long as the hole is small enough, the shape of the light spot that it makes has nothing to do with the shape of the hole and everything to do with the shape of the sun. Try making tiny holes with different shapes in a piece of black paper, and place them in the sunshine, and you'll see what I mean. We don't usually notice this effect in nature, because the sun is usually a circle, and circular light spots seem “normal.” But sometimes light spots are not circular. During a solar eclipse, try finding a place under a tree or next to a trellis where spots of dappled sunlight are landing in an otherwise shady place, and you'll see what I mean. If you can't find a suitable shady place, you can also try holding a colander in the sunshine.

Dappled Sunlight Underneath a Tree During a Solar Eclipse

Anyway, to return to the subject of an artificial pinhole projector, you will probably find the best results if you can place the “screen” several feet from the pinhole. The farther the screen is from the pinhole, the larger the image of the sun will be. Unfortunately, the farther the screen is from the pinhole, the dimmer the image will be, which leads me to my second point: Try to enclose the space between the pinhole and the screen to make it as dark as possible, while still allowing you to look inside and see the image on the screen. I once made a pretty good solar projector from a 10-gallon bucket and a long mailing tube, with a paper screen at the bottom of the bucket, and a pinhole in a piece of aluminum foil wrapped over the upper end of the mailing tube.

If you want to get an even better look at the sun, you can also — CAREFULLY! — use binoculars or a telescope. You can buy solar filters (which are basically “eclipse glasses” for your instrument) from various places online, and with solar filters on your binoculars or telescope, you can study a magnified sun as you would examine anything else in the sky. If you have a camera with a telephoto lens, you can place a solar filter over your camera lens and obtain pretty impressive photographs of the sun that way. (The auto-focus and auto-exposure features might not work properly, so you may have to adjust those manually.) You can also simply use your binoculars or telescope as a projector: Instead of looking through them directly, place a card of some kind where your eye would have been but a bit farther away, adjust the focus, and you should be able to project an image of the sun onto your screen. (Doing this successfully with a pair of binoculars may take a little practice and a little manual dexterity, but it can be a cheap way of creating some pretty impressive images of the sun. As with the pinhole projector, however, you will probably want to position your screen in a shady place to improve the contrast. If you have a mirror, perhaps you can reflect the image of the sun out of the sunshine and into a dark corner.)

What Causes Solar Eclipses?

Since time began, the sun has been the bringer of light and warmth to the world, has given us our days and our summers, and has been (almost) as regular and reliable as the stars. Three or four millennia ago, what must people have thought when this god of light and warmth suddenly faded from the sky? Two and a half millennia ago, the Ancient Greeks faced the world with analytical minds, they gathered the clues, they found the patterns, and they gave us the first non-mythological explanations in history, including the scientific cause of both lunar and solar eclipses. But how were they able to do this? The Ancient Greeks lived 2500 years before the invention of space travel, and 2000 years before the invention of the telescope. They had no authoritative professors or cartoonish scientific diagrams to explain things to them. They had to figure out reality for themselves, and the only clues they had were things that can be seen from the ground with the unaided eye. You have the same clues. Could you figure out on your own what is causing the upcoming solar eclipse?

If you are paying attention to the world and the sky around you, perhaps you have noticed the clues that you need. The September full moon (i.e. the “Harvest Moon”) of 2023 occurred on Friday the 29th. If you watched the sunset on that day, maybe you turned around and saw the full moon rising in the east just as the sun was setting in the west. If you got up early to watch the sunrise on the following day, maybe you noticed the full moon setting in the west just as the sun was rising in the east. Perhaps it occurred to you that the sun and the full moon both appear as circles in the sky and that both circles are nearly the same size. Perhaps you thought ahead to the next day and wondered if the moon would be in the same place at the same time.

If you observe the sunrise every morning for two weeks following a full moon, you will see the moon gradually work its way across the sky, from the western side of the sky towards the sun on the eastern side, “waning” or growing thinner and fainter as it moves nearer and nearer to the sun. It will start out full, setting in the west just as the sun rises in the east. A few days later, it will appear as a waning gibbous, high in the west at sunrise, and it will descend in the western sky throughout the late morning. About a week after a full moon (around Friday, October 6 this month), the moon appears as a quarter-moon, and it is usually quite easy to find in the morning skies, high in the south as the sun comes up, and it will descend through the western sky throughout the morning. At this point, it is halfway across the sky in its journey towards the sunrise. A few days after this, the moon will have thinned even further and approached even closer to the sun, and you will see it as a beautiful crescent moon decorating the sunrise, just over the rising sun. (This crescent moon will lead the sun across the sky throughout the day, preceding the sun on its western side. Knowing where to look, you may enjoy the challenge of trying to find the thin crescent moon in the daytime sky.) Finally, the moon will have “waned” so much that it will no longer be visible. Where is it now? If you extrapolate the motion in your imagination, perhaps you can visualize the invisible moon passing very near to the sun in the sky.

It is at this time that solar eclipses always happen. The solar eclipse of October 14th, 2023 will occur two weeks after the full moon of September 29th.

So what causes solar eclipses? Here are your clues:

You don't need anybody else to tell you what causes a solar eclipse. You can see it for yourself. By keeping track of the moon's motion through the sky (compared to the sun) and through the phases, you can watch it circle or “orbit” around us, from west to east, once every month. You can notice that every month at the time of the new moon it passes the sun in the sky. And you can figure out that eclipses happen when the new moon “aims” especially well and “hits” the sun in the sky, rather than passing invisibly alongside the sun.

By the way, if you want to follow up on the moon's progress, where should you look for it to reappear after the eclipse? If it continues its eastward motion past the sun, this will place it on the eastern or “sunset side” of the sun in another day or two, won't it? If you check the sunset a couple of days after the eclipse, say Monday the 16th, will you see a beautiful sliver of a returning crescent moon over the sunset?

Why Are There Different Kinds of Eclipses?

If you study a map of an “eclipse path”, which shows the parts of the world where a solar eclipse is visible, you will notice that there is a thin stripe of the world where people get the best view, and a somewhat broader path where people can see at least part of the eclipse. For everyone else outside that ribbon on the map, it is just a normal day, and none of the eclipse is visible. (You can see NASA's map of the “eclipse path” for the upcoming eclipse here. As much as I like the Time and Date website, their “detailed eclipse path map” wasn't working properly the last time I checked.)

Knowing that the Moon is passing between the Sun and the Earth, can you figure out what that path on the map represents? If people looking up see the moon covering the sun, they must be in the shadow of the moon, mustn't they? What would it look like if we could actually fly up into outer space during a solar eclipse, and turn around and look back down at the Earth? Would we see the moon's shadow falling on the Earth? Would it look something like this?

The Moon's Shadow Passing Across the Earth, Photographed from the ISS

The map of a solar eclipse shows the path that the moon's shadow takes as it passes across the Earth. By the way, the small size of the moon's shadow on the Earth helps to explain why solar eclipses are so rare. They actually occur somewhere on Earth a couple of times every year, but only a few people get to see each one. This means that for any given location on Earth, solar eclipses that are visible from that location only happen a few times in a lifetime. (To see a fuller list of past and upcoming eclipses, you may be interested in my tables of all eclipses from 2000-2050.)

Partial Eclipses

Have you ever noticed that the edges of a shadow are sharp when they fall close to the object making them, but they grow blurrier the farther they have to travel? The moon's shadow has to travel very far before it lands on the Earth. Do you suppose that the “true” shadow (or “umbra”) in the center is small, and that most of the moon's shadow is filled by the blurry edges (or “penumbra”)?

For every solar eclipse, most of the people who can see it will only see a “partial eclipse.” Looking at the situation from the ground, the moon has to “aim” very well to “hit” the sun dead center in the sky, meaning it will usually miss and pass off-center, and it will never cover 100% of the sun's face. Looking at the situation from space, the dark “umbra” of the moon's central shadow is quite small (and sometimes nonexistent), and most of the moon's shadow consists of the half-light of the penumbra, and it is this half-light portion of the shadow that passes over your city during a partial eclipse. On a map of the eclipse path, those observers inside the broad ribbon but outside the central path will be passed over by the “penumbra”, and will thus see only a “partial eclipse.”

Total vs. Annular Eclipses

The real excitement usually happens within the narrow “centerline” of the shadow's path. This is where the center of the moon's shadow passes across the Earth, and viewers inside this path will see the moon pass directly across the center of the sun in the sky. But even here, observers don't always see the same thing. Sometimes the moon covers the sun completely and cause a spectacular “total eclipse.” (During a total eclipse, the day turns to night for a minute or three, the stars come out, and the whole outdoors grows dark and a few degrees colder.) But during other eclipses, the sun seems to have grown a little, or the moon has shrunk, and the moon is no longer able to cover the sun completely. So even if it passes directly over the center of the sun, it can never block out the light completely, and the day never turns completely to night. (On the other hand, viewers along the centerline during these eclipses will have a few moments when they are able to see the sun as a spectacular “ring of fire” or “annulus” of sunlight.)

Why would this be? Why are solar eclipses sometimes total and sometimes annular? Is the sun coming closer and moving farther away? Is the moon? Neither the moon nor the sun can be growing farther or closer by large amounts, or we would notice them growing larger or smaller in the sky. However, if you have a way to measure the size of the moon in the sky, you might notice that it does shrink and grow slightly in size, as if sometimes it is slightly farther away, and sometimes it is slightly closer. (Every so often, the media becomes very excited about a “supermoon.” This is just the moon coming a little closer and appearing a little larger than usual.) You can probably see for yourself now why we sometimes have “total” solar eclipses and sometimes “annular.” If the eclipse happens to coincide with a “supermoon,” we will have a total eclipse, and if it coincides with a “minimoon” we will have an annular eclipse.

A "Minimoon" and a "Supermoon"
(photo credit: https://vanderbei.princeton.edu/images/NJP/SuperMoon.html)

Now, can you explain why the moon would be sometimes a little farther away, and sometimes closer? Is the circle of the moon's orbit growing and shrinking? Or perhaps the orbit is a little lopsided, and it has a “far side” and a “near side”, as if the Earth weren't quite at the center of the moon's orbital circle? Officially, the moon's orbit is a lopsided “ellipse”, and we call the “near side” the “perigee” and the “far side” the “apogee.” When the full moon is a “supermoon”, it is on the near side of its orbit, and slightly closer to Earth than usual. And a solar eclipse occurring two weeks later will always be an annular eclipse, because the moon will have circled half-way around its orbit from the perigee to the apogee. Conversely, if the full moon preceding a solar eclipse is a “minimoon”, the following eclipse will be total, because the moon will have circled half-way around its orbit from the apogee to the perigee. The full “Harvest Moon” of September 29 was a “supermoon”, occurring near the moon's perigee, and the eclipse of October 14 will therefore be annular.

Looking at the situation from space, what does this mean for the moon's shadow? How is the shadow during an annular eclipse different from the shadow during a total eclipse? Think of the moon's “umbra” or “true shadow” as a cone in space. Immediately behind the moon, the cone is as large as the moon itself, but the farther you fly away from the moon, the more the blurry “half-light” of the edges encroaches upon the center, and eventually the umbral cone tapers to a single point. If you are too far from the moon, where the moon does not appear large enough to cover the sun in your line of sight, you are past the point of the moon's umbral cone. During a total eclipse, when the moon is a little closer and appears a little larger, the tip of the moon's umbral cone must be striking the Earth. The cone reaches the Earth ... but just barely, and the umbral shadow landing on the Earth is quite small. During an annular eclipse, when the moon is a little farther from the Earth and appears a little smaller, the umbral cone doesn't quite reach the Earth. The Earth's surface lies just outside the umbral cone, and the entire shadow consists of the “half-light” of either a partial or an annular eclipse.