During a total eclipse, when the full sun is covered by the moon, we only see light on the horizon that is scattered in our direction. (Photo credit: NASA)
All the attention of today’s eclipse has raised interest in the sky’s color.
To understand why the sky is blue, we need to understand a little about light. Light is a form of electromagnetic energy. This form of energy does not need matter to propagate.
We can characterize this energy by its wavelength — the distance along a wave from one crest to another. Our eyes are sensitive to light with wavelengths between approximately 0.4 to 0.7 microns (one micron is a millionth of a meter or one one-hundredth the diameter of a human hair). Blue colors have wavelengths between about .455 and .492 microns, while red colors have longer wavelengths between .622 and .780 microns.
When light beams interact with particles suspended in air, the energy can be scattered or absorbed. Energy that is scattered causes a change in direction of the light path. The amount of light that is being scattered is a function of the size of the particle relative to the wavelength of the light falling on the particle.
Particles that are tiny compared to the wavelength of the light scatter selectively according to wavelength. While all colors are scattered by air molecules, violet and blue are scattered most. The sky looks blue, not violet, because our eyes are more sensitive to blue light (the sun also emits more energy as blue light than as violet).
At sunset and sunrise, the sunlight passes through more atmosphere than during the day when the sun is higher in the sky. More atmosphere means more molecules to scatter the violet and blue light. If the path is long enough all of the blue and violet light gets redirected out of your line of sight, while much of the yellow, orange and red colors continue along the undeviated path between your eye and the sun. This is why sunsets often are composed of yellow, orange and red colors.
During a total eclipse, when the full sun is covered by the moon, we only see light that on the horizon that is scattered in our direction.
