Source: Oregon State University

On Monday, August 21, the sun will be blocked from view for about three minutes across much of the United States. The last time a total eclipse was viewable across the contiguous United States was 1979, or 38 years ago. You’ve probably heard about the eclipse and may have even prepared to see it, but what are you preparing to see? What object blocks the sun during an eclipse? Why does is an eclipse viewable from some locations but not others? And why are eclipses so rare?

The object blocking the sun during an eclipse is the moon, not a planet, and it does so at rare intervals because the moon’s orbit is titled five degrees from the earth’s orbit. If the moon orbited the earth in the same plane the earth orbits the sun, there would be solar eclipses every month, but the difference in orbits means that the moon’s shadow typically misses the earth. Only twice per year does the moon’s path align with the earth’s path such that the moon casts a shadow on the earth, but the moon’s shadow hits the earth at different locations from one alignment to the next, making an eclipse a rarity at any particular location.

Eclipses are easy to imagine but difficult to explain. You know what to expect—a dark object moving across the sun, shrinking it from a circle to a crescent to a ring—but you may not know why. Many astronomical phenomena follow this pattern. We expect the tides to come in and out, the moon to wax and wane, and the seasons to cycle, but we don’t know why. When pressed for an explanation, we are typically wrong. The tides do not change because the moon pulls the ocean closer and further from the shore (they change because the moon pulls the earth’s oceans into two global bulges and the earth rotates through those bulges). The moon does not change because our view of it is blocked by the earth’s shadow (it changes because our view of the illuminated half of the moon changes). And the seasons do not change because the earth moves closer or farther from the sun (they change because the amount of sunlight hitting the earth’s northern and southern hemispheres changes across the earth’s orbit).

The reason that astronomical phenomena defy explanation is that they defy —they defy our earliest developing and most easily accessed interpretations of those phenomena. The earth is a rotating sphere, orbiting the sun, but we perceive it as flat, at rest, and orbited by the sun. Children in particular have trouble conceiving of the earth as a moving, spherical object. They hear that the earth is round, and they see it depicted as round in pictures or drawings, but they interpret “round” in non-spherical ways. Some think the earth is round like a disc. Some think the earth is round like a deflated basketball—mostly round but with flat parts for people to inhabit. Some think the earth is round like a snow globe such that the top part of the globe is the dome of the sky and people live on a flat surface inside the globe. And some think the earth is round like a ball but that people do not actually live on the earth; they live on the ground.

Children hold these misconceptions until they can come to terms with how a round earth can appear flat from its surface and how people can live on the “underside” of a round earth without falling off. At that point—around age ten—they must reconceptualize the earth’s place within the solar system. A child who understands that the earth is a sphere may still not understand that the earth is in motion. It takes children several more years to figure out that our experience of a day is caused by the earth’s rotation and our experience of a year is caused by the earth’s orbit. The causes of the tides, the seasons, and the phases of the moon are tackled later, if ever.

Learning about astronomical phenomena is slow and difficult because we are constrained by misleading perceptions and misleading language. The sun, for instance, appears to move across the sky each day, and we call the start of this trajectory a “sunrise” and the end a “sunset.” Even after we have mastered correct explanations for astronomical phenomena, we still have trouble shaking our earlier misconceptions. They reemerge if we are hurried or distracted. In my own research, we’ve asked people to decide whether statements about astronomical phenomena are true or false as quickly as possible. The statements are closely matched for length and complexity but differ in their intuitiveness, as in “The moon revolves around the earth” (which is scientifically true and intuitively true) vs. “The earth revolves around the sun” (which is scientifically true but intuitively false). When science and intuition conflict, people respond more slowly and less accurately. They may know that the earth revolves around the sun, but they are prone to respond “false,” and if they do respond “true,” it takes extra time to do so.

Many adults even harbor misconceptions about the shape of the earth, explicitly acknowledging that the earth is round but implicitly believing that the earth is flat. In one study, adults were asked to estimate the distances between six cities that span the globe: Berlin, Rio de Janeiro, Cape Town, Sydney, Tokyo, and Los Angeles Participants estimated the distances between all possible pairings, and those estimates were submitted to a computer algorithm that extracted the radius of the sphere that best fit each participant’s estimates. The earth’s actual radius is 3,960 miles, but participants were assigned a greater radius if they overestimated the intercity distances or a smaller radius if they underestimated those distances. In actuality, most participants were assigned a radius of infinite value, not because they vastly overestimated the intercity distances but because a sphere with an infinite radius is geometrically equivalent to a flat plane.

If we have trouble fully accepting that the earth is round, we must also have trouble understanding more complicated phenomena, like how the moon can eclipse the sun. Seeing is believing, but believing is not necessarily understanding. This coming Monday, the sight of one celestial body eclipsing another will inspire awe, but let it also inspire curiosity and humility—curiosity about the workings of the universe and humility towards all the scientific ideas we do not yet fully understand.