Solar Eclipses

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Podcast Transcript

Every few years, somewhere on Earth is witness to one of the planet’s greatest sights: a total solar eclipse.

A total solar eclipse is rare, but it can be calculated centuries in advance. 

However, that wasn’t always the case. For thousands of years, solar eclipses were rare events that were considered to be bad omens. 

Learn more about solar eclipses, how they work, and how people have dealt with them throughout history on this episode of Everything Everywhere Daily. 

I’m sure most of you know what a solar eclipse is, but we should probably start from the basics anyhow. 

When one astronomical object passes in front of another one, from the perspective of an observer, it is considered to be one of two things: either an occultation or a transit. 

A transit is when an object that appears smaller passes in front of an object that appears larger. For example, when Venus or Mercury passes in front of the sun, that would be a transit. Venus and Mercury do not block out the sun when they pass by. 

An occultation is when an object that is perceived to be larger passes in front of an object that is perceived to be the same size or smaller. This is also known as an eclipse. 

A solar eclipse is something that only happens on Earth in our solar system. It is when the moon passes in front of the sun. If conditions are right, you can have a total social eclipse where the moon totally blocks out the orb of the sun. 

The fact that we have total solar eclipses is ultimately just a cosmic coincidence. It just so happens that the moon appears to be the same size as the sun from our position on the surface of the planet Earth. There are no other planets and moons in the solar system where this occurs. There will be times when a moon of a planet passes in front of the sun, but it just appears as a transit. 

The coincidence of the perceived size of the moon and the sun isn’t just a coincidence of astronomy. It is also a coincidence of time. The moon is slowly getting further away from the Earth over time. Millions of years from now, the moon will appear smaller than it does now and will be unable to totally eclipse the sun. 

When the moon passes in front of the sun, it casts a shadow that is much smaller than that of the moon. The result is that only a narrow path on the Earth can actually see the sun get totally eclipsed. Other parts of the Earth, because they are viewing it from a different angle, will only see the moon eclipse part of the sun. 

There are several different types of eclipses that can occur. The first is the total social eclipse that I’ve mentioned, where the moon totally blocks the sun. 

The second is a partial eclipse. This happens when the moon and the sun aren’t perfectly aligned, and only part of the moon passes in front of the sun. 

There, there is an annular eclipse. An annular eclipse is when the moon and the sun are aligned, but the moon is farther away and isn’t big enough to block out the sun. 

This happens because the moon’s orbit around the Earth isn’t perfectly circular. It is slightly elliptic. That means at certain parts of the moon’s orbit it will appear slightly larger or smaller. 

The difference in the perceived size of the moon because of its distance from the Earth is enough to determine whether a total solar eclipse will occur. 

A solar eclipse can obviously only occur during a new moon. A question that many people have that is very reasonable is why there isn’t a solar eclipse every month. Shouldn’t every new moon come with a solar eclipse?

The reason why that doesn’t happen has to do with the moon’s orbit. 

The orbit of the Earth around the sun is known as the ecliptic plane. For the purpose of this discussion, you can consider it a flat plane.

The moon doesn’t orbit the Earth along the ecliptic plane. Its orbit is slightly tilted at 5 degrees. That means at some point during each month, the moon will be five degrees above the ecliptic plane and five degrees below the ecliptic plane. 

That is enough to ensure that the moon is usually above or below the sun when it passes in front of it each month.

If the moon orbited the Earth in the ecliptic plane, then there would be an eclipse every month.

However, because it isn’t in the ecliptic plane, the only time you can have a total eclipse is when the moon crosses the ecliptic plane at the same time it passes in front of the sun. 

That is why eclipses are so rare. 

One of the interesting phenomena that can be observed during an annular or total eclipse is what is known as Baily’s Beads. 

Baily’s beads were named after the English astronomer Francis Baily, who explained the effects in 1836. The moon is not a perfectly smooth sphere. It has mountains and craters, which create irregularities on its surface.

When the moon passes in front of the sun, sometimes you can see the outline of the topography of the moon. Sunlight will come through the irregular perimeter of the moon, creating an effect that looks like beads of light. 

When a total solar eclipse happens the shadow that is cast on the Earth is rather narrow. This is known as the path of totality. Everywhere on the path of totality will experience the total eclipse; however, it will be brief, and those in the middle of the path will experience it longer. 

The width of the path of totality will vary, but it is usually about 100 to 120 miles or 160 to 193 kilometers wide.

Because they happen so infrequently and because the path is so narrow, most people will never experience a total solar eclipse during their lives unless they go out of their way to see it.

As solar eclipses have been occurring pretty much since the Earth was formed, humans have been experiencing them since there were humans. 

When early humans encountered an eclipse, they had no idea what was happening. The sun disappearing and the world going dark in the middle of the day must have been terrifying and mystifying to ancient people. 

In many cultures, an eclipse was a bad omen. It could be interpreted in any number of ways that something bad was going to happen, or perhaps the gods didn’t favor a particular ruler. 

The first record of a solar eclipse was written down on clay tablets found in Ugarit, Syria. The eclipse is believed to have taken place on March 5, 1223 BC.

Recording eclipses is very important for establishing historical timelines and calendars. Because the moon orbits the Earth in a highly regular way, it is possible to calculate the exact dates and times of historical eclipses. 

So, if we have evidence of an eclipse that occurred on a particular date on an ancient calendar, we can then synchronize it to our calendar. 

There were records of eclipses from ancient China, Babylon, and Greece. 

While eclipses didn’t occur frequently, eventually, enough evidence began to pile up that some of the top minds in the ancient world realized that they happened on a regular basis.

The Babylonians were the first people to record planetary movements and eclipses. They actually tracked lunar eclipses because they were more frequent and easier to track. With this data, they began to see regular patterns in eclipses.

One of the landmarks in the study of eclipses took place 2600 years ago. A Greek philosopher named Thales of Miletus collected data from past eclipses and predicted when a future eclipse would occur. 

This wasn’t just a landmark moment in eclipses. This was arguably one of the first moments in the history of science. Thales collected data, created a hypothesis, and made a prediction. 

His prediction wasn’t what we would call precise. He simply predicted that an eclipse would take place during a given year. 

The eclipse he predicted was believed to have taken place in the year 585 BC. In fact,  it supposedly took place right in the middle of a battle between the Medes and the Lydians.

Supposedly, when the eclipse took place, both sides stopped fighting and called an immediate truce. 

Attempts at predicting eclipses continued for centuries, but it wasn’t until the early 18th century that a true prediction, down to a few minutes, was accurately made. 

The British Astronomer Royale, Edmund Halley, predicted the May 3, 1715 eclipse to within 4 minutes. Not only was he able to predict the eclipse, he was able to draw a map of the path of totality across England. 

Once it was possible to predict eclipses, it became easier to observe them. Given the short length of time they take place, you need to be in the right place and at the right time to get an observation. That was hard to do when they took you by surprise.

The corona of the sun, the sun’s atmosphere, was first identified during an eclipse in 1842. The first photograph of an eclipse was taken in 1851.

In 1868, a spectrograph analysis of the sun was taken during an eclipse, which determined its composition. 

Perhaps the most important scientific observation to have taken place during an eclipse was taken during the May 29, 1919 total eclipse. It was used to make an observation which confirmed Einstein’s theory of General Relativity.

Einstein’s theory predicted that gravity could bend light. To measure this, it was necessary to check the position of stars near the sun and compare them to their positions when they aren’t near the sun at a different time of year.

However, you can’t observe stars when near the sun because the sun is too bright. You can only do it during an eclipse. 

The amount of deflection measured in the starts was approximately 1.75 arcseconds at the limb of the Sun, aligning closely with Einstein’s calculations. 

With the increased precision in astronomical measurements, eclipse prediction has become increasingly more accurate and we now know when and where eclipse will take place years, and indeed, centuries, in advance. 

Many people have become eclipse followers, traveling around the world to witness solar eclipses. 

Even if they don’t travel around the world, many people will drive for hours just to see one that is within their area. 

In 2017, I made a trip to Nebraska to witness the eclipse, and we managed to view it right in the middle of the path of totality. It was the first time I had ever witnessed a total solar eclipse, and it was one of the most incredible natural phenomena I have ever seen. There is an enormous difference between totality and even 99% eclipsed. 

When totality hit, it became quite dark, and all of the street lights turned on. 

If you ever have the opportunity to see a total solar eclipse, I highly recommend you do it, even if you have to go out of your way to do so. The eclipse itself might only last for a few minutes, but it is something that you will remember for the rest of your life. 

It should go without saying that you shouldn’t look directly at the sun. This can do permanent damage to your eyes. If an eclipse is taking place, there will probably be plenty of cheap cardboard eclipse viewing glasses available that will block the vast majority of light and let you see what is happening. 

Total solar eclipses are relatively rare events that only happen on Earth, and millions of years from now, they will no longer happen at all. They have mystified and fascinated humans for thousands of years and will continue to do so for thousands more.