Ring Around the Moon or Sun? It's a Halo

Updated: Mar 3

Have you ever looked up at the moon and wondered why it looked as though it was surrounded by a perfect circle of light? Were your eyes playing tricks on you? Don't worry. Your eyes are fine. These rings of light sometimes appear around both the moon and the sun, and it's nothing to be concerned about.

A lunar halo. (Photo via Shutterstock)

These rings are called halos. Scientists call them 22-degree halos because they are about 22 degrees from the center of either the sun or the moon. When they form around the sun, they are called solar halos or sun halos. Rings around the moon are called lunar halos or moon halos. These halos are always a perfect circle, but you may not always see the full circle if clouds are in the way.


Solar and lunar halos are visible to us because light from the sun or moon is being refracted off tiny ice crystals in the high, thin cirrus clouds. The ice crystals act like a prism, allowing us to see the white ring.


The halos normally look like white light, but they can sometimes appear to have faint color as well. Usually, when color is visible, the inner edge of the halo will appear reddish. Less often, the outer edge looks violet in color. Color is more common in halos around the sun than the moon, but remember that you should never look directly at the sun because it can damage your eyes.

 

Words to know

Halo: A disk or circle of light.

Horizon: The line at which Earth’s surface and the sky appear to meet.

Prism: A transparent object with refracting surfaces at acute angles that separate white light into the spectrum of colors.

Radiate: To emit in the form or rays or waves.

Reflect: To throw back heat, light or sound without absorbing it.

Refract: To make light change direction with it enters at an angle.

 

There is an old saying, "Ring around the moon means rain soon," that is about moon halos. It turns out that these rings of light may predict rain. It may not always be accurate, but a moon halo may mean there will be rain soon. That's because the thin cirrus clouds that allow us to see these halos often move through before a storm.


Halos aren't the only optical phenomenon caused by the reflection and refraction of light on ice crystals in the clouds. If you've ever seen sunny, rainbow-like spots either to the left or right (or both) of the sun, you've spotted a sundog. These spots are caused by the refraction of light through ice crystals. They are always about 22 degrees to either the left or right of the sun, depending on where the ice crystals are.


Sundogs can look like rainbow spots on either side of the sun, although the colors are not as well defined. You'll see red light on the side closest to the sun, and the colors of the rainbow generate outward.


Another optical phenomenon involving the sun is called sun pillars. These are columns of light that radiate vertically above the sun. Sun pillars develop when light from the sun reflects off ice crystals slowly moving down through the atmosphere. They are most often seen at sunrise or sunset, and they are visible low on the horizon.

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