Signs in the Sky

The Wonder of Our Solar Eclipses

Have you noticed something odd about the appearance of the Sun and full Moon at dusk —apart from the uninteresting fact that they are opposite each other and visible simultaneously? If so, you may have stumbled upon a fascinating celestial coincidence: their apparent sizes are strikingly similar. While this synchronicity might go unnoticed by the casual sky gazer, it didn’t escape the keen eyes of pre-telescopic astronomers, and it offers more than just a curious bit of science trivia. This geometric confluence sets the stage for the awe-inspiring phenomenon of solar eclipses.

The Babylonians were the most careful and prolific recorders of celestial events in ancient times. Motivated by their belief that a solar eclipse was a bad omen for a ruler, they discovered the lunar, solar, and planetary celestial cycles and used them to predict eclipses. Fragmentary astronomical tablets survive from about 700 to 50 BC. Over this period, they recorded only one total solar eclipse, on April 15, 136 BC. The records note the timing of each stage of the eclipse; mention Mercury, Venus, Mars, and Jupiter; and also note that bright stars became visible. This was the most careful observation of a total solar eclipse in pre-telescopic times. Greek astronomers built on the Babylonian discoveries and continued to advance the science of astronomy.

The Great Celestial Coincidence

These cosmic events come in various flavors, depending on the relative positions of the Sun, the Moon, and the observer on Earth. You will see a partial eclipse, offering an incomplete veiling of the Sun, when you’re positioned just outside the center of the Moon’s shadow. And if the center of that shadow never lands on Earth during the event? Then, a partial eclipse is the only show in town.

When the shadow’s core does fall on the Earth, however, those fortunate enough to stand along its direct path are in for a treat: either an annular or a total solar eclipse. With an annular eclipse —named for the Latin word annulus, meaning ring —the Moon finds itself just a tad too distant from Earth in its elliptical journey to fully cloak the Sun. During an annular eclipse, the sky watcher would witness a bright ring of light encircling the Moon’s darkened face. To the untrained eye, the subtle shift in light might go entirely unnoticed, as human vision swiftly adapts to moderate changes in brightness.

Yet when the Moon swells large enough in the sky to match the Sun’s apparent size, a total solar eclipse unfolds. This subtle shift in celestial geometry catalyzes a profound transformation in our experience of a solar eclipse. At mid-eclipse, the Moon covers the Sun’s bright disk entirely, casting an otherworldly twilight below that unveils normally obscured stars and planets. Even Earth’s wildlife may be fooled into believing that nightfall has arrived, the darkness triggering their routines for dusk.

The Sun’s faint corona, the outermost part of its atmosphere, becomes visible only during a total eclipse. Eclipse chaser Serge Brunier describes his emotional responses to experiencing total solar eclipses:

Each time ... the feeling has grown that eclipses are not just astronomical events, that they are more than that, and that the emotion, the real internal upheaval, that they produce … far exceeds the purely aesthetic shock to one’s system.¹

And describing his first total solar eclipse experience in Hawaii:

The sight is so staggering, so ethereal, and so enchanting that tears come to everyone’s eyes. A soft twilight bathes the Mauna Kea volcano. . . . The solar corona, which spreads its diaphanous silken veil around the dark pit that is the Moon, glows with an other-worldly light. It is a perfect moment.²

Today, astronomers fully understand the mechanics of eclipses and can predict their timing to within one second, decades in advance, anywhere on Earth. Yet, this modern understanding of eclipses has not removed our sense of wonder at the experience and at the great celestial coincidence that permits them. Atheist astronomer Neil deGrasse Tyson notes:

Earth’s Moon is about 1/400th the diameter of the Sun, but it is also 1/400th as far from us, making the Sun and the Moon the same size on the sky —a coincidence not shared by any other planet–moon combination in the solar system, allowing for uniquely photogenic total solar eclipses.³

Similarly, astronomer John Gribbin writes,

At the present moment of cosmic time, during an eclipse, the disc of the Moon almost exactly covers the disk of the Sun. In the past the Moon would have looked much bigger and would have completely obscured the Sun during eclipses; in the future, the Moon will look much smaller from Earth and a ring of sunlight will be visible even during an eclipse. Nobody has been able to think of a reason why intelligent beings capable of noticing this oddity should have evolved on Earth just at the time that the coincidence was there to be noticed. It worries me, but most people seem to accept it as just one of those things.⁴

Convergences in Both Time & Space

Tyson’s and Gribbin’s observations highlight other aspects of the great celestial coincidence. First, there are moons around other planets in the Solar System, and they create solar eclipses as viewed from their host planets’ surfaces. Some moons, such as Phobos and Deimos around Mars, only produce partial solar eclipses. Others, such as the four Galilean moons around Jupiter, produce what I call “super-eclipses,” wherein moons appear much larger than the Sun.

Only one other moon appears the same size as the Sun from its host planet —Prometheus, which is small and resembles a potato. This Saturnian speedy spud produces only fleeting eclipses as it whips around its massive host at the edge of the ring system. The two Martian moons also race quickly around their host planet, but only because their orbits lie so close to its surface.

In contrast, our Moon lumbers along in its wide orbit. This gives us relatively long-duration eclipses. What’s more, Earth is the closest planet to the Sun with a moon. This gives us the largest apparent size for the eclipsed Sun. Finally, the large size of the Moon gives it a more nearly perfect spherical shape, because of the crushing effects of gravity on its interior. For these reasons, we get to enjoy the best eclipses in the Solar System.

Second, the Moon is receding from us at about 1.5 inches each year. As Gribbin noted, the Moon would have appeared larger in the past, blocking the chromosphere (the part of the atmosphere just above the bright photosphere) and part of the corona. In about 250 million years, the Moon will be too distant to totally block the Sun’s bright disk, and total solar eclipses will cease. Thus, the great celestial coincidence involves coincidences in both space and time. This raises a fascinating question: Why do we live in the best place in the Solar System to view solar eclipses, and at the best time?

Earth: A Place for Observers & for Observing

While much must go right on a planet for it to be habitable, one of the most important single ingredients is water. And not just water but liquid water, in abundance and at its surface. This requirement defines a star’s “Goldilocks zone,” the band in which the light energy reaching a planet’s surface is just right. Too close, and the sunlight would evaporate too much water, causing the atmosphere to overheat and the planet to lose its water to space. Too far, and the water at the surface would remain frozen.

Also, not just any star will do. Massive stars are effectively flashbulbs in cosmic history. They have short lifetimes and change rapidly. At the opposite extreme, red dwarf stars are cosmic methuselahs, but their feeble power output means that their Goldilocks zones are tight little annuli. And when a planet orbits close to its host star, it risks becoming tidally locked, like our Moon. Everyone can see that it’s tidally locked because it always shows us the same face. But a planet that always shows its host star the same face is not life friendly.

What does all this have to do with solar eclipses? Turn the problem around and ask yourself what indigenous observers on a planet orbiting a life-friendly star would see. Well, they would have to see that their host star subtends an angle of about a half degree in their sky. In other words, how big the Sun looks in our sky is set by the need that we be in the Goldilocks zone. Given the narrow range of star types that are compatible with complex life and the relatively narrow width of the Goldilocks zone, there is a tight relation between our existence on Earth and how big our Sun appears to us. The very fact that we can stand on Earth and view the Sun is evidence that our planet resides in the Goldilocks zone.

What about the Moon? The Moon also takes part in making our planet life friendly. To the casual observer, the direct effects of the Moon on Earth are subtle. Ocean tides are the most obvious. The Sun contributes some tidal effects, but not as much as the Moon. The tides help mix nutrients from the land to the sea. They also help power the great ocean currents, which, in turn, help to moderate the climate.

The Moon aids life in another important way. The tilt of Earth’s spin axis relative to its orbit is not constant over time but varies over a range of about three degrees, with a variation period of 41,000 years. This is one of the so-called Milankovitch cycles, which are thought to drive the ice ages. The tilt variation may be small, but its effects are big enough to be clearly visible in the polar-ice and ocean-bottom-sediment climate archives. Without the Moon, Earth’s axis tilt would vary chaotically over a larger range. We would not enjoy prolonged periods of climate stability with mild seasonal changes like we have experienced over the course of the Holocene (the current interglacial period).

Furthermore, the very way the Moon formed may have benefited Earth’s life, assuming the giant-impact theory is at least partially correct. This theory posits that the Moon formed following the impact of the proto-Earth with a Mars-size body early in the Solar System’s history. The enormous energy of the collision produced the kind of core needed to generate a strong and long-lasting magnetic planetary field, and Earth’s magnetic field has protected our atmosphere and life.

The Moon is almost too big to do its life-aiding work. Had it been just three percent larger, its stabilizing effect would have ceased by now. So, as with the Sun, how big the Moon appears in our skies is also tied to our very existence. These two links go a long way in “explaining” our eclipses, but only at a superficial level. There is a deeper explanation that we can uncover with a little more digging.

Doing Science with Eclipses

If this were the end of the story about our wonderful eclipses, it would already be quite amazing. Yet, as they say on the TV ads, “But wait, there’s more!” Not only are total solar eclipses beautiful and awe-inspiring spectacles, but they have also enabled us to discover profound truths about the universe.

If you’ve ever witnessed a total solar eclipse, one of the first things you notice during totality is the pearly white glow extending several degrees from the Sun’s disk. The corona is the outermost layer of the Sun’s atmosphere, and eclipses are the best way to observe it. Astronomers are still trying to understand how the corona can be heated to about one million degrees C.

During the eclipse of August 18, 1868, French astronomer Jules Janssen observed a bright yellow line in the spectrum emitted by the chromosphere, which didn’t correspond to any known element. British astronomer Norman Lockyer observed the spectral line shortly after Janssen and named the element responsible for it helium, after the Greek word for the Sun, helios. Helium turned out to be very important, second in abundance in the universe after hydrogen.

Two years later, during the December 22, 1870, eclipse, American astronomer and onetime missionary Charles A. Young conducted an important experiment. He monitored the transition from the partial phase to totality with a prism. Young described the Sun’s spectrum as transitioning from its usual bright background with dark lines (absorption spectrum) to one with bright lines on a dark background (emission spectrum) over just a few seconds. This “clue” about the Sun’s light helped astronomers understand how and where the absorption line spectra of distant stars are produced.

In 1915, Albert Einstein published his theory of general relativity. At the time, physicists thought Isaac Newton had figured out gravity. Einstein’s approach to gravity was completely different, which made his theory controversial. He proposed an observational test, the only one possible with the technology then available. If his theory was correct, then light from background stars near the Sun on the sky would be deflected on its way to Earth by an amount twice that predicted by Newton’s theory. The only way to observe the shift in the stars’ positions would be to photograph the sky around the Sun during a total eclipse.

British astronomers Arthur Eddington and Frank Dyson confirmed Einstein’s predictions with observations of the May 29, 1919, eclipse. This eclipse seemed tailor-made for the experiment. It was the longest solar eclipse in 503 years, and the eclipsed Sun was near the Hyades, a bright star cluster. Following publication of the analysis of the observations, many physicists came to accept general relativity. The experiment has been repeated during many eclipses since then, and all have confirmed the theory.

Finally, ancient solar-eclipse observations provide us with the best method to measure the changes in Earth’s rotation. Surviving records from ancient Babylon, China, and Europe indicate that the length of a day has been increasing at a semi-regular rate. Geophysicists use eclipse data to model changes in the shape of the Earth, and historians use it to place ancient calendars on the Gregorian calendar.

Here for a Purpose

Are our perfect eclipses just an accidental alignment of celestial bodies and nothing more? It would be one thing if our eclipses were like the ones on Mars, lacking in dramatic beauty and scientific value and having no relation to the life friendliness of our planet. We could dismiss them as inconsequential, of no more significance than boulders on a hillside.

Even so, our total eclipses attract observers from far and wide. Again, it would be one thing if they were merely awe-inspiring —enjoy the eclipse, post pictures on social media, then go back home and continue with your regularly scheduled life. But no, the mystery of objective beauty in nature should give us pause.

It stretches credulity to believe that this awe-inspiring and science-advancing phenomenon —one that is best viewed at this most opportune time and from the most habitable place in the Solar System —is a mere accident of cosmic history. Once I saw this connection, more than 25 years ago, I looked at nature with fresh eyes. I discovered multiple cases of this remarkable link between the conditions for life and the conditions for scientific discovery. I wrote up my findings with my co-author Jay Richards in our book, The Privileged Planet: How Our Place in the Cosmos Is Designed for Discovery.

Two other familiar examples help illustrate this new way of seeing the world. Consider the starry heavens on a clear, dark night. Countless sages, dreamers, and prophets, at least as far back as King David in the Psalms, have written about how the stars inspire us and seem to point beyond themselves. I need hardly point out how much we have learned about nature from their study, not least of which is that the universe had a beginning.

Think about what is necessary for us to see the stars. First, we need either no atmosphere or at least an atmosphere that’s not opaque. Several other planets in the Solar System have thick atmospheres that would block the view of the stars. We need to be on a planet that is not tidally locked. We need a sky that doesn’t have multiple suns, nearby planets, or big moons. Of course, places like the Moon offer better views of the stars, but they would severely restrict our ability to do science in other important ways. For example, we wouldn’t be able to build a fire (from lack of both fuel and oxygen), the starting point for modern technology. We would lose out in more subtle ways as well. Earth provides us with all the conditions required for both life and observing the universe.

And consider rainbows, one of the most beautiful and awe-inspiring sights in nature. They, too, have taught us much. Once we learned how to “unweave” the rainbow and reproduce one at will, we were well on our way to unlocking the mysteries of the universe. All it takes to make an artificial rainbow —a spectrum —is to pass light through a prism. First, chemists discovered that each chemical element, when heated sufficiently, displays a unique spectrum. Then, astronomers examined the spectra of stars and discovered that they could identify the patterns produced by the same elements that chemists studied in their labs. Eventually, they were able to learn all sorts of details about stars, including their surface temperatures (something that the originator of positivism, Auguste Comte, said could never be done). It’s as if Someone is saying to us, “Look here and learn something important!”

When scientists read nature rightly, nature discloses herself in new and surprising ways, like a rich and multifaceted text to the patient interpreter. A proper reading creates new lines of research and exploration. Herein lies a virtue in seeing the correlation between life friendliness and discovery as a sign of purpose rather than mere coincidence: we should expect to find this correlation elsewhere, and we should expect to keep making discoveries because of it.

To one who has discerned that the cosmos is designed, this evidence is much like the sublime beauty of the mathematical elegance of the natural world. It’s no longer a troublesome anomaly to be explained away, but something fitting and wonderful. To dismiss it as “just one of those things” is both theoretically and aesthetically sterile.

Notes
1. S. Brunier and J. P. Luminet, Glorious Eclipses: Their Past, Present, and Future (Cambridge University Press, 2000), 6.
2. Ibid, 17.
3. N. D. Tyson, Astrophysics for People in a Hurry (W. W. Norton & Company, 2017), 171.
4. J. Gribbin, Alone in the Universe: Why our Planet is Unique (Wiley, 2011), 1.