The Science of Solar Eclipses

By JoEllen McBride, PhD 

As the sky darkens on August 21st, we will stand in awe of the first total solar eclipse to cross over the contiguous U.S. in almost 40 years. This is also a chance for scientists to do what they do best– science!


Total Eclipse of the Sun

Every month, the Moon passes between the Earth and Sun during its New Moon phase. We can’t see the New Moon because the side that faces us isn’t illuminated by the Sun but it’s up there. Solar eclipses happen only when the Moon is in the New Moon phase and crosses the plane created by the Earth-Sun orbit. All other New Moons are either too high or too low in the orbit, to cover Sun.


A total solar eclipse is even more special. The cosmos has gifted us with a spectacular coincidence. The distance between the Moon and Earth is 400 times less than the distance between the Sun and Earth. This wouldn’t be interesting except for the fact the Moon is also 400 times smaller than the Sun. Once the Moon hits that sweet spot in its orbit around Earth, it completely covers the Sun.


That also means that sometimes a solar eclipse occurs and the Moon doesn’t completely cover the Sun. These are partial or annular eclipses and it just means that the Moon was too far from Earth to hide the Sun completely.


A solar eclipse occurs approximately every year and a half (give or take a few months). What makes them seem so rare is our planet is mostly ocean, so the chances of the solar eclipse passing over land with people on it is reduced. That’s why Monday’s total solar eclipse passing over the entire mainland U.S. is such a big deal! Don’t let Neil deGrasse Tyson put a damper on it!


Predicting Eclipses

It is true that for centuries solar eclipses were thought of as omens and bringers of terrible things by many human societies. But once we figured out that they were predictable, we quickly used them to learn about the universe. The first predicted eclipse was done by Thales of ancient Greece around 610 or 585 BCE. Thales made the prediction using the idea of deductive geometry borrowed from the Egyptians. Euclid, much later, formalized this into what is now known as Euclidean Geometry. The historical record shows that Thales’s prediction only worked one time though because there are no other accounts of anyone successfully predicting an eclipse until Ptolemy used Euclidean geometry in 150 CE.


So how can scientists use this periodic alignment of celestial bodies to their advantage? The Sun is a pretty reliable part of our day, so having it gone for a few moments allows us to study the reaction of animals to an abrupt change in their environment. You’ll hear birds stop singing and frogs and crickets will begin chirping as the sky darkens. Mammals will begin their bedtime rituals also. But we can learn the most about the Sun itself from a solar eclipse.


Image of the corona created by placing a disc over the Sun to mimic a solar eclipse. These instruments, called coronagraphs, still allow a little sunlight to get through which can mess up measurements of the corona. So scientists still rely on real deal total solar eclipses to study the corona in detail.
Image of the corona created by placing a disc over the Sun to mimic a solar eclipse. These instruments, called coronagraphs, still allow a little sunlight to get through which can mess up measurements of the corona. So scientists still rely on real deal total solar eclipses to study the corona in detail.

Grab a Corona

The Sun has an outer atmosphere extending millions of miles above its surface called the corona. At temperatures reaching a few million degrees Fahrenheit, the corona significantly hotter than the Sun’s surface. The corona was first observed in 968 CE during a solar eclipse and for many centuries, scientists debated whether this bright wispy envelope was part of the Sun or the Moon. It wasn’t recognized as being part of the Sun until the eclipse in 1724 and then verified over a century later in 1842. Then, during 1932 and 1940 solar eclipses, scientists determined that the corona is significantly hotter than the surface of the Sun. Iron atoms in the corona are stripped of their electrons, which can only happen if the atoms are heated to millions of degrees. This discovery still summons solar physicists to all parts of the planet to observe solar eclipses. This solar eclipse is no different. They’re still not sure why the corona is so hot.


Get You Some Flare

Solar eclipses also allow scientists to study another extremity of the Sun, solar flares. Solar flares or prominences are as spectacular as they are dangerous– especially today. They can disrupt satellites and other communications devices as well as short out electrical grids. So it is crucial that we understand as much as we can about them. The first solar prominence was observed, with the naked eye, during a partial solar eclipse in 334 CE. Knowing this probably would have helped Birger Wassenius during the total solar eclipse in 1733. He noticed solar flares but suspected they were coming from the Moon. It wasn’t until a solar eclipse in 1842 that scientists verified the ejections were coming from the Sun.


The Sun goes through cycles of solar flare activity about every 11 years. This year, the Sun is approaching a low point in its activity, so scientists will use this total eclipse to study how flares differ from when the Sun is more active.


Other Notable Discoveries Thanks to Solar Eclipses

In 1868 the element Helium was discovered in the Sun’s light during the 1868 and 1869 solar eclipses and named after the Sun (Helios = Sun in Greek). Helium wasn’t identified on Earth until 1895. Another big win for physics came during the 1919 solar eclipse. Scientists used the darkened sky to verify that the Sun is massive enough to bend the light of faraway stars before it reaches us. Stars that should have been behind the Sun– and therefore not visible during the eclipse– were clearly seen. This proved part of Einstein’s theory of relativity that massive objects bend space around them.


Solar eclipses are awe inspiring and also useful to science. So make sure you grab your eclipse glasses or pinhole cameras or fists and get out there!