With the sun at your back, imagine a line from the sun through your head. Extend it until it runs into something else. The "something else" will be located at the antisolar point. Interesting things happen there. Look at the shadow of your head on a clear day, especially when the sun is low and shadows are long. It will be at the antisolar point. When the moon is full, it is near the antisolar point. A lunar eclipse happens when it is exactly at the antisolar point.
All objects at the antisolar point are in front of their shadows. It is the brightest point around, because as you move away from the antisolar point more of the shadows become visible. Stated another way, as you move away from the antisolar point, a greater percentage of what you see will be in shadow.
Look out of the window of your airplane as it takes off. The shadow of the plane follows you, always at the antisolar point. Above 2000 feet or so, the shadow will look smaller and less distinct, but every tree, rock, blade of grass, sand grain, etc. will still be in front of its shadow, so if you watch carefully you will see a bright spot in the shadow of your plane. The bright spot will follow you at the antisolar point, regardless of your altitude, even when you are too high to see your shadow itself. Not only that, but the shadow of your plane will be centered on your seat. If you are seated in the tail section of your aircraft, the antisolar point will appear in the back rows of the plane's shadow.
When the sun shines on water droplets, a rainbow is created, with the antisolar point at its exact center. When the sun is above the horizon, the antisolar point is the same distance below the horizon as the sun's elevation is above it. Thus, you can never see a complete rainbow from the surface of the earth; the higher the sun is in the sky, the less of the rainbow will be visible. For this reason, the most striking rainbows occur in late afternoon or early morning, when the sun is low in the sky. But fly above the clouds, or stand on a mountain peak where your shadow falls on a cloud below you. It will be one of those rare times when you can see a complete rainbow. Early aviators saw the shadow of their wings and fuselage forming a crude cross, surrounded by a halo on the clouds below them. Some early pilot, possibly with religious but definitely with poetic tendencies, named them glories, and that name persists today.
Many years ago I owned a Piper Cub, and I enjoyed looking at antisolar phenomena when I flew. I would pick a "target," such as a tree or house, and try to hit it with my shadow. I once flew over a large parking lot. As my shadow passed over the parked cars, their tail lights appeared to turn on as the antisolar point moved over them, and turn off as I passed. It seemed that a giant hand lit each reflector in sequence, one by one. Tail lights are designed to reflect light back to its source. So are glass beaded reflectors. Watch your shadow pass over reflective glass-beaded signs, or even stripes on highways, and they will, like the tail lights, appear to turn on and off. Look for these things next time your airliner approaches the airport in a large city. If the angles happen to be just right, you will be delighted with what you see!
When solar system objects are near the antisolar point they are said to be in opposition. Naturally, only the moon and objects outside of the earth's orbit can be there - Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, and those tiny objects, such as asteroids, planetary satellites, comets, etc. Opposition is the best time to observe them, because they are at their brightest, and usually near their closest approach to the earth. Because Venus and Mercury are closer to the sun than we are, it is impossible for them to be at the antisolar point, so there can be no opposition for them, unless you count those oppositions when they (and the antisolar point) are on the opposite side of the sun from us. At those times they are so far away that they appear to us to be the smallest they will ever be. More important, they are bound to be invisible because the sun will be in front of them.
At certain times of the year, on exceptionally clear and dark nights, far away from city lights or other glare, a faint spot known as Gegenschein might be visible. This is where the sun lights up dust particles scattered throughout the solar system. The dust, and consequently the Gegenschein is always found at the antisolar point. Of course, there is dust in the rest of the solar system too, but it does not reflect enough light back to us to us to make it visible.
It was my privilege to be present at a moonlight hike to the top of Mount Baldy, in Sugarloaf Ridge State Park. On this particular night, the moon was to rise partially eclipsed. Members of the hiking group asked me where they should watch for the moon to rise. Although I had never been on Baldy before, I impressed a few hikers by pointing to a notch in the distant mountains. But there was no magic involved. I knew that during an eclipse the moon would be at the antisolar point, and that my shadow, growing longer in the sunset, was aimed exactly in that direction, so I merely pointed past the end of my shadow and tried to appear wiser than I really am.
Any evening at sunset, when the air is quite clear, look not to the west, where the sun is setting, but to the east. The eastern sky will have a pinkish color, but near the horizon there will be a distinctly darker band of blue. This is the shadow of the earth on its atmosphere, and like all shadows, will be at the antisolar point. As the sun slips below the horizon, the earth's shadow will rise an equal amount, until it becomes indistinct in the rapidly darkening sky. On the evening of the eclipse, the thin sliver of the uneclipsed portion of the moon appeared to be sticking out of the blue shadow. This was a sight that could only be seen by people on the earth's terminator (the line dividing day and night). Even though the eclipsed part of the moon is always in the earth's shadow (that is what causes an eclipse), nowhere else is the shadow of the earth and the uneclipsed moon visible at the same time, because it is the only place where the earth's shadow can be seen against a daytime sky. At night, the entire sky is deep in the shadow of the earth, so the edges of the shadow are not visible.
So - the next time you fly, see a rainbow, watch the full moon, or see a sunset, think about the antisolar point, and have a lot of fun doing it!
* Appeared in The Kenwood Press January 1, 1999
© December 21, 1998, by Nathan B. Miron, Ph.D.