Why I Can’t Wait For the Eclipse

In 1991, I went to Teotihuacán, Mexico, to watch a total solar eclipse from the Pyramids of the Sun and the Moon. Alas, thick clouds moved in during the morning. At the time of totality, it just got really dark. I did not see a hole in the sky surrounded by a shimmering corona or any other eclipse phenomena: Baily’s beads, the diamond ring. I was crushed.

This year, I am planning to go to Plattsburgh, New York, near the U.S.-Canadian border, to watch the April 8 eclipse from a city beach on the western shore of Lake Champlain. If the clouds come in again … I do not even want to think about it.

Astronomy is unlike economics in that astronomers can tell us with great accuracy what will happen in the heavens not just weeks from now, but hundreds of years from now. Eclipse-watching, on the other hand, is very much like economics in that it is vulnerable to all kinds of uncontrollable effects. Like clouds. And traffic jams. And exorbitant rates or no vacancies in hotels and motels in the path of the eclipse, lending a new meaning to the term “blackout period.”

When the moon fully blocks the sun, the sky darkens, the temperature falls, birds roost for the night, and some stars pop out. “People laugh, cry, stare dumbfounded, jump up and down,” Peter Tyson, editor in chief of Sky & Telescope magazine, wrote in a special issue this year. “The hair on the back of your neck rises, and goose bumps cover your arms,” Kate Russo, who says she has witnessed 13 solar eclipses on six continents, wrote in the same issue.

I am hoping for some of that craziness in Plattsburgh. In the meantime I have been thinking about eclipses in a more mechanical way.

One counterintuitive aspect of an eclipse is that the shadow cast by the moon starts in the west and travels east, even though the moon itself crosses the sky in the opposite direction, from east to west. This eclipse, for example, will travel northeast from Mexico through the United States to Canada.

Why does an eclipse go against the one-way traffic of the heavens — not just contra the moon but also contra the sun, the stars and the other planets? If you could look down on Earth from far above the North Pole, you would see what looks like a ball turning counterclockwise. What appears to an earthbound observer like the sun, the moon, the stars and the planets moving clockwise across the sky is really the Earth turning counterclockwise while those other heavenly bodies are fixed (the sun, the stars) or moving slowly (the moon, the planets).

Another key eclipse fact is that the moon actually revolves around the Earth in the same direction that the Earth is spinning. We do not see it that way from Earth. It takes longer for the moon to go around the Earth than for the Earth to rotate on its axis, so the moon constantly falls behind from an Earth observer’s perspective. As a result it seems to be going clockwise, from east to west, like the sun and other bodies. That is the opposite of its true direction.

In an eclipse, the moon’s true direction is revealed. The shadow that the moon casts moves rapidly across the Earth’s surface from west to east as the moon passes in front of the sun.

The reason people get confused about the eclipse’s direction of travel is that they are mixing up two different concepts of velocity. One is angular velocity, or how much of the horizon, in degrees, an object transits in a given amount of time. The Earth is the clear winner in angular velocity. It rotates 360 degrees in a day, while the moon takes nearly a month to cover the same 360 degrees.

In terms of linear velocity,

Hoping the clouds stay away for an awe-inspiring event.

though, the moon is the clear winner. It has much farther to go in covering those 360 degrees. In its monthly circuit, the moon travels through space roughly twice as fast as a given spot on the Earth’s surface travels (though they are going the same direction).

Linear velocity, not angular velocity, is what matters in an eclipse. The moon’s advantage in linear velocity versus the Earth’s surface will cause its shadow to zip across North America on April 8. In those few hours, the moon will complete only a tiny fraction of its orbit; its angular movement will be small. But its linear movement will be sufficient to paint an eclipse across the continent.

I realize that this explanation is going to leave a lot of readers in the dark, so to speak. This is confusing stuff.

Just thinking about eclipses can be mind-bending, even without seeing one. On the other hand, thought experiments go only so far. Like millions of other people, I really want to feel those goose bumps when the eclipse comes.

Here is hoping for clear skies on April 8.