The sun and the moon are not just the same shape, but the same apparent size in the sky. It’s this happy arrangement that produces total solar eclipses as seen from the earth’s surface.

Americans got a chance to view such an eclipse on Monday — an event we won’t see again from coast to coast until 2045. The moon’s 115-mile wide central shadow entered Texas from Mexico around 12:29 p.m. over Eagle Pass before grazing the edge of San Antonio, and then passing over Dallas-Ft. Worth. It continued on a northeasterly pass over 11 other states until it reached Maine. (You can find the precise path at NationalEclipse.com.)

In a total solar eclipse, just before “totality,” the last bright bit of the sun’s photosphere looks like a pink diamond in an engagement ring. When the moon covers the sun’s disk, the sky goes dark; the temperature drops; the stars appear. Bugs and animals get confused and go quiet or start squawking. And the dim outer atmosphere of the sun, the corona, reaches out from the black lunar disk like the gray iris of an eye with a black pupil in the middle. At that point you can take off your protective glasses and see it with your naked eyes.

Astronomer Guillermo Gonzalez (my co-author on The Privileged Planet) and I provided live commentary for an eclipse viewing in Waxahachie, Texas, south of Dallas — where totality lasted four minutes and 19 seconds. (You can find highlights at X on the @DiscoveryCSC feed.)

In order for a total solar eclipse to occur, the moon, sun, and Earth have to line up in a straight line. When the moon passes in front of the sun, you can see an eclipse if you’re in the path of the moon’s shadow.

Those fully in the shadow — the umbra — see the moon cover the sun. If you’re just outside that path, you see a partial eclipse — the penumbra. The difference between a partial and total eclipse is like the difference between day and night. It’s impossible to capture with mere words the experience of seeing a total eclipse. But words can help us ponder its meaning.

Finely Tuned

The sun is a giant ball of gas and plasma. The moon is a much smaller rock. And yet, during a total eclipse, they mark off the same space in our sky. They match. On Earth, we can see not just total eclipses, but what we might call perfect solar eclipses.

The moon is about 400 times smaller than the sun. But the moon is also about 400 times closer to the earth than is the sun. As a result, the size of the moon matches the size of the sun from our perspective. And since they appear as round disks, they match in both size and shape.

Physics doesn’t require this arrangement. There are 65 major moons in our solar system and many smaller ones. But only we enjoy perfect solar eclipses when a moon just barely covers the sun’s bright photosphere. If there were life forms on Mars or Jupiter, they wouldn’t see such eclipses.

So the best place to view total solar eclipses in our solar system is just where there are observers to see them. Let that sink in a minute.

A Habitable Planet

Most astronomers chalk this up to coincidence. And yet, without this precise arrangement of the earth, the moon, and the sun, we probably wouldn’t exist.

Let me explain. For lots of reasons, a planet almost surely needs liquid water on its surface to host complex life. Almost all places in the solar system and in the universe are either way too hot or way too cold for this. To be “habitable,” a planet needs to be in the “Goldilocks Zone” around its star: not too hot and not too cold. Think of this zone as a narrow, nearly circular ring of space around a star. (Netflix’s Three-Body Problem is fun science fiction, but any planet in a three-star system would almost surely be lifeless.)

The earth is, of course, safely inside the Goldilocks Zone. And as a result, the sun appears to be a certain size in our sky.

Our large, well-placed moon also plays a key role in making Earth habitable by stabilizing the tilt of its axis. That gives our planet a more stable climate. The moon also contributes to Earth’s ocean tides, which mix nutrients from the land into the oceans. The two tiny moons around Mars are much too small to serve in this role. As a result, Mars wobbles on its axis far more than the earth does. That’s bad news for Martians.

Now put these two facts together.

1. When a planet, like Earth, is in the cozy, life-friendly zone around a star, that star will appear to be a certain size in its sky.
2. A habitable planet like Earth also needs to have a moon of a certain size in its sky to create the right amount of gravitational pull to stabilize the planet.

Not just “certain” sizes, but nearly the same apparent sizes. So, two of the key ingredients for building a habitable planet also produce perfect eclipses for observers on that planet.

Our Eclipses Are a Gold Mine for Science 

That alone seems fishy. But there’s more: Our ability to see perfect solar eclipses has played a pivotal role in several major scientific discoveries. Those discoveries would have been hard to make on the planets that don’t enjoy such eclipses.

First, eclipses helped us unlock the mystery of stars.

Scientists since Isaac Newton (1666) have known that sunlight splits into all the colors of the rainbow when passed through a prism. But only in the 19th century did astronomers begin to observe solar eclipses with spectroscopes, which use prisms. This allowed them to discover how the sun produces its light.

The beginning and end of totality present the best chances to examine the thin middle layer of the sun’s atmosphere, called the chromosphere. It shines in the ruby-red light of hydrogen gas heated to more than 20,000° Celsius (36,000° Fahrenheit). Just beyond the moon’s silhouette during an eclipse, observers may also see solar prominences: brilliant red arcs, loops, and jets of hot gas propelled by the explosive release of the sun’s magnetic energy.

All of this gave astronomers a way to figure out the structure of the sun itself. Since the sun looks larger from the earth than from any other planet with a moon, we can discern finer details in its chromosphere and corona than we could from any other planet.

This knowledge, in turn, has allowed astronomers to make sense of the light from the distant stars. Perfect eclipses, then, have been a key that allowed us to unlock the physics of stars.

Testing Einstein’s Theory

Eclipses have done far more than help astronomers decipher starlight, however.

In the early 20th century, Albert Einstein predicted in his General Theory of Relativity that light passing near a massive object like the sun would be visibly bent. To test his theory, astronomers needed to measure the changes in the positions of starlight passing near the sun’s edge compared to their positions months later when the sun was in another part of the sky.

Have you ever tried to look at starlight right next to the edge of the sun? It’s a bad idea and wouldn’t work anyway. The test could only be done during a total solar eclipse. That’s why, during the 1919 eclipse, two teams of astronomers set out to confirm Einstein’s theory.

They succeeded, as did other astronomers during later eclipses. This led scientists to embrace Einstein’s theory, which is the basis of our current knowledge of the universe.

Conspiracy, Not Coincidence

There’s far more to this story. Indeed, the perfect eclipses we enjoy are just one of many examples of an eerie pattern Gonzalez and I discuss in detail in The Privileged Planet. That pattern points to a startling conclusion: Life-friendly places like Earth are also the best places, overall, for doing science. That is, the rare places where observers can exist are also the best overall places for observing. The universe seems to be designed not just for life but also for discovery.

Genesis 1 says that God created lights in the sky for “signs.” One of those signs has been hiding in plain sight all along.

Cross-posted from The Stream.