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4 Questions

There are several things that people often ask us about. What makes the rainbow? Why should the sky be blue? Why is the Moon so much brighter when it’s full? And what makes the “shadow bands” seen at the time of the solar eclipse?
 

Well, the last one is easy. The shadow bands are simply the shadows of the atmospheric turbulence seen with the slit illumination of the crescent Sun just before and just after the totality. If, at the time of a total eclipse of the Sun, you happen to be standing in a creek bed with high maple-tree overcover, then, just before and just after totality, you’ll see three and four inch pinhole images of the crescent Sun, under the trees, flickering over the stones. It’s charming. Of course, you can see pinhole images of the sun on any sunny day, but, except at the time of solar eclipses, they are round and attract little attention. Likewise, the shadows of the turbulence may be seen on the ground almost any sunny day and attract little notice. But at the time of an eclipse they become conspicuous in the direction parallel to the solar crescent. Then, only, they appear as parallel waves moving with the breeze.

 

The sky is blue simply because the shorter wavelengths of the sunlight are more easily scattered by the molecules of the atmosphere (and the sky is brighter in ultraviolet than it is in blue). But it’s blue for the same reason that Crater Lake is blue. The shorter wavelengths are scattered back. And the distant mountains are blue because there is blue sky between them and you. Sometimes, when the Sun goes down, it appears to be orange or red because more of the longer waves get through. But sometimes, if you look to the east, you can see the blue light scattered back.

The rainbow is in the form of a circle around the shadow of your head. And the size of the circle is determined by the wavelength of the light. That is what separates the colors of the rainbow. In a prism, the refraction of the different colors comes at slightly different angles, and for the same reason, they come at slightly different angles from the raindrops. Sometimes you see a second rainbow, with the colors reversed, making a larger circle around your shadow. But when you see a circle around the Moon, or “sundogs” around the sun, they are in the opposite direction from the shadow of your head, and are caused by the ice crystals in the upper atmosphere.

 

But, why is the Moon so much brighter when its full? There are two reasons, and they are both related to seeing the Moon from the Sun’s position. (And the full Moon is much brighter than two quarter-moons would be.)

First, the Sun sees no shadow, because every rock, every grain of sand and every mountain (and on Earth, every leaf), hides its shadow from the Sun. When flying in a small plane, you may have noticed that when the plane gets high enough above the ground–so that the shadow of the plane is no longer seen–the shadow is replaced with an apparent bright spot that follows you wherever you go. It is simply the spot where you see no shadows because you’re looking from the Sun’s position. But we see the brightness of the full Moon a little to the side of straight down the shadow of the Earth. Otherwise, it would be darkened in eclipse. And, just before and after toatlity, it will be partially darkened by the partial shadow of the Earth (the penumbra).

The other reason that the full Moon is so very bright is because one-third of the surface material on the moon is glass. It is all over glass beads, and the surface beads, which have not yet been dulled by mircometeorite impacts, shine back the sunlight toward the Sun. That is what makes those brilliant rays from the crater Tycho. When the Moon gets hit by a large asteroid at some twenty or thirty miles per second, the asteroid goes several miles into the rocks where the kinetic energy of the impact is converted to a tremendous explosion of vaporized stone. In the absence of an atmosphere, this condenses to spherical glass beads. That’s why the Moon is all covered with glass. And that is why the brightness of the full Moon is so enhanced. The bright rays from the crater Tycho, which are so conspicuous when the Moon is full, are simply glass bead streamers, less than a million years old, that are long enough to reach from San Francisco to Denver.

 

Sometimes, people ask us why the moon looks so big on the horizon. That is because of your genetic expectations. Your genes have it hard-wired in that the things you see at or near the horizon are farther away than the things seen overhead. It’s true for the gulls, the zepplins, the blimps, the planes and the clouds. And your expectation tells you that it should also be the same for the Moon. But it’s not. When the Moon is seen on the horizon, your expectation tells you that it must have gotten bigger to look so big when it’s that much farther away.

 

 

The sky is blue simply because the shorter wavelengths of the sunlight are more easily scattered by the molecules of the atmosphere (and the sky is brighter in ultraviolet than it is in blue). But it’s blue for the same reason that Crater Lake is blue. The shorter wavelengths are scattered back. And the distant mountains are blue because there is blue sky between them and you. Sometimes, when the Sun goes down, it appears to be orange or red because more of the longer waves get through. But sometimes, if you look to the east, you can see the blue light scattered back.

The rainbow is in the form of a circle around the shadow of your head. And the size of the circle is determined by the wavelength of the light. That is what separates the colors of the rainbow. In a prism, the refraction of the different colors comes at slightly different angles, and for the same reason, they come at slightly different angles from the raindrops. Sometimes you see a second rainbow, with the colors reversed, making a larger circle around your shadow. But when you see a circle around the Moon, or “sundogs” around the sun, they are in the opposite direction from the shadow of your head, and are caused by the ice crystals in the upper atmosphere.

But, why is the Moon so much brighter when its full? There are two reasons, and they are both related to seeing the Moon from the Sun’s position. (And the full Moon is much brighter than two quarter-moons would be.)

First, the Sun sees no shadow, because every rock, every grain of sand and every mountain (and on Earth, every leaf), hides its shadow from the Sun. When flying in a small plane, you may have noticed that when the plane gets high enough above the ground–so that the shadow of the plane is no longer seen–the shadow is replaced with an apparent bright spot that follows you wherever you go. It is simply the spot where you see no shadows because you’re looking from the Sun’s position. But we see the brightness of the full Moon a little to the side of straight down the shadow of the Earth. Otherwise, it would be darkened in eclipse. And, just before and after toatlity, it will be partially darkened by the partial shadow of the Earth (the penumbra).

 

The other reason that the full Moon is so very bright is because one-third of the surface material on the moon is glass. It is all over glass beads, and the surface beads, which have not yet been dulled by mircometeorite impacts, shine back the sunlight toward the Sun. That is what makes those brilliant rays from the crater Tycho. When the Moon gets hit by a large asteroid at some twenty or thirty miles per second, the asteroid goes several miles into the rocks where the kinetic energy of the impact is converted to a tremendous explosion of vaporized stone. In the absence of an atmosphere, this condenses to spherical glass beads. That’s why the Moon is all covered with glass. And that is why the brightness of the full Moon is so enhanced. The bright rays from the crater Tycho, which are so conspicuous when the Moon is full, are simply glass bead streamers, less than a million years old, that are long enough to reach from San Francisco to Denver.

 

Sometimes, people ask us why the moon looks so big on the horizon. That is because of your genetic expectations. Your genes have it hard-wired in that the things you see at or near the horizon are farther away than the things seen overhead. It’s true for the gulls, the zepplins, the blimps, the planes and the clouds. And your expectation tells you that it should also be the same for the Moon. But it’s not. When the Moon is seen on the horizon, your expectation tells you that it must have gotten bigger to look so big when it’s that much farther away.