Tech Talk
Celestial Mechanics / by Michael E. Duffy / October, 2017

The Great American Eclipse of 2017 has come and gone, and I was fortunate enough to see it. Although total eclipses are fairly common (one takes place somewhere here on earth every 18 months or so), they’re the result of an amazing cosmic coincidence: viewed from earth’s surface, the discs of the sun and the moon are almost exactly the same size. This is due to the fact that the moon—while 400 times smaller than the sun—is also 400 times closer to earth, allowing the moon’s disc to completely obscure the blindingly-bright disc of the sun without hiding the sun’s outermost atmosphere, the corona. It’s the corona that gives a total eclipse its otherworldly quality and scientific value.

A larger lunar disc would blot out not just the sun, but its dramatic corona as well. Were the moon slightly smaller, we would see an annular eclipse, where the disc of the sun is visible as an extremely bright ring around the moon. In fact, in some millions of years, inhabitants of earth will see only annular eclipses, since the moon is gradually moving farther away from earth (an odd side-effect of the tidal interaction between Earth and the moon).

Totality itself is all too brief. Totality during The Great American Eclipse lasted at most two minutes and 40 seconds. Compare that to “maximum theoretical totality” at seven minutes and 32 seconds. The last eclipse lasting more than seven minutes was in 1973. Alas, the next eclipse is in 2150. During totality, the moon’s 60-mile-wide shadow is moving over the Earth at the speed that the moon orbits the Earth (2,290 mph). So, totality should last about 60 ÷ 2290 hours, or about a minute-and-a half. Right? Nope.

That might be true if the Earth was flat and stationary. But the Earth is rotating in the same direction as the shadow, reducing its speed over the surface and lengthening totality. And, the Earth is a sphere! As the shadow of the moon “slides” onto and off of the sphere, it travels more quickly over the surface. It moves most slowly at the point directly opposite the sun (for the Great American Eclipse, this was in Illinois). Since the sun is not directly behind the moon at every point along the path of totality further complicates things: the moon’s shadow is elongated, changing the amount of time it takes to pass over a given point. Finally, because the shadow is roughly circular, the farther from the center of the shadow’s path, the less shadow will pass over you, shortening the eclipse. I won’t even mention that lunar mountains and earthly terrain affect the shape of the shadow as well.

During totality, I wanted to be “in the moment” for the full minute and 52 seconds in our Oregon location, rather than fiddling with cameras. Getting to that moment, however, took a bit of time. We set up chairs and put on our eclipse glasses to watch the progress of the lunar disc across the face of the sun, which took 72 minutes. And in the last minute or two, so many things happen. The sky finally becomes noticeably darker (even with just 1 percent of the sun’s disc exposed, it’s still pretty much “daytime”). The temperature drops. Birds, crickets—everything—becomes still. And then, totality. Time to take off the glasses and look around. And “around” it is—it is sundown in all directions. Thirty miles away in every direction, it’s daylight under a partially-eclipsed sun. The brightest stars are visible in the darkened sky.

Most pictures of totality are close-ups of the lunar disc and sun’s corona, perhaps with tiny red-orange solar prominences visible. But for me, it was the larger vista that was compelling. In the deep twilight sky hung a small, jet-black dot, surrounded by a white nimbus of starlight, with longer, wispy streaks extending outward for some distance. When I say “a small, jet-black dot”, you must imagine something half the width of the nail on your pinkie, held at arm’s length (technically, about 30 seconds of arc). Binoculars (safe during totality) brought into view solar prominences at the edge of the moon’s disc and details of the corona. But that first sight of a black hole in the sky, surrounded by soft white light, is what I will remember best. Celestial mechanics at work, creating a transcendent, ephemeral beauty.

Having now seen a total eclipse as well as partial ones, nothing compares with totality. The good news is that another total eclipse will cross from Mazatlan, Mexico to Newfoundland in 2024, passing over Dallas, Niagara Falls, Vermont and places in between. Maximum totality falls over land in northern Mexico and will last four minutes and 32 seconds.

There are other total solar eclipses between now and 2024, if you’d like to visit South America or Antarctica. Or, perhaps you’d prefer to see total eclipse on July 2, 2019, on a two-week cruise of the South Pacific. But wherever it may be, get thee to totality! For more information, go to, an excellent resource for all kinds of eclipses.