Why You Should Observe the Eclipse from the Center Line

In all likelihood, the most important thing you’ll hear from someone speaking about the eclipse or that you’ll read on any website is that, on August 21, 2017, you must get to the path of totality. It’s true. As I like to say in my talks, the difference between viewing a partial eclipse and experiencing a total one is the difference between almost dying and dying.

Once you’ve decided to take this sage advice, I suggest that you consider taking one further step: Try your best to position yourself on the eclipse’s center line. Any quality map that shows the path of the 2017 total solar eclipse will have three curved lines on it. The two outer ones show the northern and southern limits of totality. It’s within these lines that the Moon’s umbra — its dark inner shadow — falls on Earth.

But it’s the line midway between the extremes that’s most important. Astronomers call this the center line, for obvious reasons. It’s along this path that the central part of the Moon’s shadow falls, and that’s where you should try to be on eclipse day.

Here’s why. Because the Moon is spherical in shape, its shadow is round. During total solar eclipses, the round shadow falls on Earth’s surface. It’s then your choice where you stand under the shadow.

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(If you’re on the umbra’s center line (red arrow), the Moon’s shadow will cover the Sun’s disk for a longer time than if you position yourself away from the center line (yellow arrow). // NASA/Holley Y. Bakich)

Consider the image of the Moon on this page. We’ve added two lines. If we imagine the shadow cast by our nearest satellite as having the same shape, you’ll enjoy a longer duration of totality if the shadow traces the pink line through your location than you will if it traces the yellow line, which, by the way, is exactly half as long. So, if the duration of totality on the center (pink) line is, say, 2 minutes, you’ll only experience 1 minute of totality along the yellow line.

If you take this example to the extreme, you could select a terrestrial position at the edge of the path of totality. At such a location, the duration of totality would be the briefest moment, much less than one second. And, in fact, some observers will be at the shadow’s edge to record the irregularities along the Moon’s limb (edge) as a tiny percentage of the solar disk becomes visible shining through valleys or between mountains. It’s important work, but it’s a job for scientists. You, as a first-time eclipse viewer, want to maximize your time under the umbra.

So, get to the center line!