A total eclipse of the Sun is the perfect opportunity to counter common misconceptions of how the Moon, Sun, and Earth move and interact in their orbits. Here’s how.

Most astronomy fans already know about (and are probably well prepared for) the great North American eclipse coming up on August 21, 2017. So you might be surprised to learn that, despite all of the frenetic activity and sustained advertisement many of us have engaged in during our preparations, the vast majority of the general public has little to no idea that an eclipse will soon be upon us.

What is perhaps more surprising is that in the days and moments leading up to the eclipse, astronomy enthusiasts will encounter many people who have almost no understanding of why eclipses occur. These people will look to you — the recognized and dedicated sky watcher — to help them learn.

One might think that eclipse phenomena are easy to understand. Indeed, what could possibly be so difficult about the fact that our orbiting Moon momentarily gets between us and the Sun, causing the sky to go dark for a few minutes? As it turns out, for people who only casually look skyward, astronomy contains many challenging, oft-misunderstood concepts. To help readers prepare explanations for family, friends, and passersby, we’ve outlined the three most common misconceptions that deceive viewers.

Total solar eclipse diagram
We see a solar eclipse when the Moon's shadow falls on Earth. But how does that happen? (Note: This diagram is not to scale.)

Why Does the Moon Have Phases?

The first conceptual challenge is that many people fundamentally misunderstand the phases of the Moon. This is important because solar eclipses can only occur during the new Moon phase, and even then only on the rare occasion that the Sun, Moon, and Earth perfectly align. Experienced astronomy teachers know that many people mistakenly believe that the Moon changes appearance every month because Earth’s shadow gets in the way. People naturally assume the Moon should always appear round and full so, if the Moon appears otherwise, it must be because something’s getting in the way — clouds, pollution, or Earth’s shadow.

Phases of the Moon
We see the Moon go through phases due to its orbit around Earth and the changing geometry of our view.
S&T: Ana Aceves

What actually happens is that one half of the Moon’s surface is always illuminated by the Sun and the other half is in shadow — the phases occur because Earthbound observers see more or less of the Moon’s illuminated half depending on where the Moon is in its monthly orbit around Earth. During a solar eclipse, the illuminated half of the Moon is facing toward the Sun, so observers on Earth can’t see the Moon’s illuminated half at all. They can be quite astonished when the seemingly invisible Moon reveals itself during a solar eclipse.

Some people also mistakenly believe that the Moon goes through its entirety of phases over the duration of a single night. And some people believe that the Moon is only visible at night; they might be quite surprised that the Moon is visible during the daytime for about two weeks of every lunar month. Keep in mind: Our ancestors spent considerably more time outside than people do today, so most people have never thoughtfully observed the Moon throughout the night or over the course of a month.

Why Are Eclipses Rare?

If people mistakenly imagine that the Moon is quite close to Earth, they’ll naturally infer that two eclipses should occur each and every month — a solar eclipse at every new Moon and a lunar eclipse at every full Moon. Indeed, if the Moon were very close to Earth, then it would fall into Earth’s shadow at every full Moon, but it’s actually quite far away. The Moon orbits Earth at a distance of nearly 30 Earth-diameters, so their shadows rarely intersect.

Moreover, the angle of the Moon’s orbit around Earth is not often precisely aligned along the line between Earth and the Sun. As a result, the Moon rarely passes through Earth’s shadow during its monthly trip. Likewise, Earth rarely passes through the Moon’s shadow either. As a result, solar and lunar eclipses each only occur twice a year on average.

What Determines When and Where an Eclipse Occurs?

The third conceptual challenge is related to why the eclipse is only visible at certain geographic locations on Earth. As shown in recordings taken from space, the Moon’s tiny shadow is quite small compared to Earth’s girth. The deepest shadow only covers a small area about 150 miles wide. As a result, you must be in just the right place at just the right time to observe a total solar eclipse. Eclipse chasers travel the globe, often at considerable expense, in the hopes of watching a total solar eclipse.

For similar reasons, total solar eclipses are extremely short in duration — usually just a few minutes. The darkest part of the Moon’s shadow moves quite quickly across the surface of our spinning Earth at about 1,700 mph. Even an airplane can’t fly fast enough to wring out more than a few additional minutes of totality. The result is that the time over which the Moon appears to completely obscure the Sun is quite brief. The August 21, 2017, eclipse will last about 2 minutes and 40 seconds at the location of “greatest duration,” and somewhat shorter elsewhere along the path of totality. These brief minutes are when an observer is within the Moon’s quickly moving shadow (and it’s also the only time when it’s safe to look at the Sun without serious eye protection).

People seem to have a natural curiosity about things in the sky—especially rare events. Eclipses provide one of those unique opportunities — teachable moments — to explain the underlying physical mechanisms that rule our natural universe. If you know ahead of time that these mistaken ideas persist in the minds of people you are teaching about the eclipse, you can be ready and anticipate their questions!

Center for Astronomy & Physics Education Research Educators
Center for Astronomy & Physics Education Research (CAPER) educators, from left to right: Tim Slater, Stephanie Slater, Tom Field, and Sharon Schleigh.

Tom Field has been an amateur astronomer for 20 years and is the author of the RSpec spectroscopy software as founder of Field Tested Systems. Tom loves to surprise amateur astronomers by showing them the easy and exciting science they can do with their current observing equipment.

Dr. Tim Slater is the University of Wyoming Excellence in Higher Education Endowed Professor of Science Education. An internationally recognized scholar in understanding how students learn astronomy, he serves as Editor-in-Chief of the Journal of Astronomy & Earth Sciences Education and as a Senior Fellow at the CAPER Center for Astronomy & Physics Education Research.


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March 30, 2017 at 3:25 pm

"The Moon orbits Earth at a distance of nearly 30 Moon-diameters" I think what you meant to say is that the Moon orbits the Earth at a distance of 30 Earth-diameters; that is, 8,000x30=240,000 miles. One moon diameter is ~2,160 miles (2,160x30=64,800).

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Monica Young

March 30, 2017 at 3:49 pm

Thanks for the catch! You're right, and I've made the change to the text.

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March 31, 2017 at 8:53 am

You're welcome. I learned that factoid, if you will, from reading astronomy from the incomparable Isaac Asimov.

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March 30, 2017 at 9:01 pm

The ancient Greeks understood eclipses and why they happened. From what I've read they also were able to deduce that Earth was round based on the shadow it cast on the Moon's surface.

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March 30, 2017 at 10:44 pm

"As a result, the Moon rarely passes through Earth’s shadow on its monthly trip around the Sun."
Don't you mean the Moon's monthly trip around the Earth?

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Monica Young

March 31, 2017 at 9:21 am

Yes indeed, and I've fixed this in the text.

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