What Causes Solar Eclipses?

Since time began, the sun has been the bringer of light and warmth to the world, it has given us our days and our summers, and it 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 can find the same clues. Could you figure out on your own what causes solar eclipses?

If you pay attention to the world and the sky around you, perhaps you can notice the clues that you need. If there is a solar eclipse coming up, pay attention to the moon. For example, 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, 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 you can't see it anymore. Where is it now? If you continue the motion in your imagination, perhaps you can imagine 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, for example, occurred two weeks after the full moon of September 29th. Every solar eclipse occurs after a two-week process of crossing from the far side of the sky to the same side of the sky as the sun, waning from a full moon down to an invisible new moon.

So what causes solar eclipses? Here are your clues:

• We never see a solar eclipse in one part of the sky and the moon in another part of the sky at the same time.
• Solar eclipses only happen at the time of a new moon, when the invisible moon is probably passing by the sun in the sky anyway.
• The piece missing from the sun is always circular. It is the same shape and size in the sky as the moon.
• This circle moves across the sun from west to east, in the same direction and with the same speed as the moon.

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 an 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 an eclipse, 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.

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 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. It leaves a thin ring of the sun exposed around the edge, 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,” the moon is big enough to cover the sun completely, and we will have a total eclipse. If it coincides with a “minimoon,” then the moon is smaller and can't cover the sun completely, and we will see an annular eclipse.

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 was therefore annular. The full moons of spring 2024 will be “mini”, and the new moons will be “super”, and the eclipse of April 8 will therefore be total.)

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.