Open any astronomy text and check the index for “antisolar point.” Result: nothing. It’s ignored as thoroughly as a teenager’s curfew. Yet that strange and unique place hosts an exciting mixture of phenomena.
The antisolar point (duh) is the spot opposite the Sun. By day it’s marked by the shadow of your head. At night it’s the most happening place in the heavens.
Even ancient cultures picked up on the uniqueness of the antisolar point and kept track of where it lurked in the darkness of the starry sky. Whenever the Moon wandered into that spot, it was invariably full – just as it will be next week on Thanksgiving.
And whenever it was precisely opposite the Sun, as it will be two Full Moons later in late January, they always witnessed a total lunar eclipse. Since, as we’ve seen, that’s also the location of Earth’s shadow, they figured that a lunar eclipse must be due to our world’s shadow being cast upon the Moon. Check it out the night of January 20, when our region will see a good one.
This was the reasoning that convinced Aristotle that Earth could be nothing but a sphere. (Some of his C-student friends must have argued that a flat disc can also cast a round shadow, but he surely had the rebuttal handy: A flat disc would sometimes be oriented sideways to the Sun, or nearly so, and the shadow would then be a weird straight line: something never seen on the Moon.) Later, Aristarchus calculated the distance to the Moon using the dimensions of Earth’s shadow.
People like us, who live far from city lights, can sometimes see a faint oval glow in the night sky, marking the antisolar point. This is the Gegenschein, which means “counterglow.” It’s the reflection of sunlight from countless dust grains that float between the planets. Like a beaded movie screen or highway sign that always throws light straight back toward its source, only dust precisely opposite the Sun bounces light to us.
Anyway, that’s what they say. We live in an inky-dark area where the zodiacal light, auroras and other faint phenomena show up routinely, but I’ve never seen the Gegenschein. I trust it’s there. Why would anyone make it up?
By day, the antisolar point occupies the exact center of every rainbow. A normal rainbow is somewhat less than a full semicircle because its center lies below the horizon. But when you look down upon sunlit water droplets, like toward a lawn sprinkler’s spray, you can see more than half a rainbow, and the antisolar point at its center shows up nicely as the shadow of your head.
When walking on a dewy lawn or looking out an airplane window toward the ground, the antisolar point is marked by a bright glow called the Heiligenschein. (Somehow those German terms always sound like a prestigious government medal: “Stand at attention, Fritz; today we honor you with the Heiligenschein!”)
When your jetliner flies just above a cloud layer, the antisolar point (its position marked by the plane’s shadow) will always be surrounded by a vivid series of colorful rings. This is called the “glory.” It’s quite exciting to see glories, even though they’re totally predictable. In the current climate of air travel, it’s important to suppress the temptation to shout to the strangers seated all around you, “I see the glory!”
Unlike rainbows, glories come in a variety of sizes, depending on the size of the water droplets. They’re one more reason to choose an airplane seat with care, to be sure you get a window on the side opposite the Sun – meaning the left side when flying west-to-east (seat A).
Even twilight offers wonderful antisolar phenomena. The “twilight wedge” is that dark blue-grey band that hugs the eastern horizon every day for about 20 minutes, centered at sunset. This is nothing less than Earth’s shadow itself. It’s as reliable as dusk: On a clear day, you simply cannot have a sunset without the twilight wedge vividly materializing.
Less common – maybe five percent of the time – enormous converging rays radiate from the twilight wedge. These dark angled streaks or alternating blue-and-pink fans meet and precisely terminate like the point of a V at the antisolar point. Not surprisingly, they’re called antisolar rays.
Fifteen minutes after any November sunset, when the Sun is some three degrees below the southwestern horizon, antisolar rays will converge at a point that high above the northeastern horizon: Yes, Fritz, precisely opposite the sun. Give that man another Heiligenschein!
The Gegenschein. Twilight wedge. Antisolar rays. Heiligenschein. Lunar eclipse. The glory. Rainbow centers. What part of the sky offers a greater variety of phenomena? Or is more neglected?
Want to know more? To read Bob’s previous columns, visit our Almanac Weekly website at HudsonValleyOne.com. Check out Bob‘s new podcast, Astounding Universe, co-hosted by Pulse of the Planet’s Jim Metzner.
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