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Moon and Regulus

February 25, 2021

A bright star follows the bright Moon across the sky tonight: Regulus, the heart of the lion. It’s below the Moon as night falls, and closer to the left of the Moon at first light.

If the weather conditions are just right, you might see another bright companion for the Moon: a ring of light. If it’s there, it’ll be huge — it’ll extend well beyond Regulus.

Such a ring is known as a Moon ring or a winter halo. It’s caused by moonlight passing through cirrus clouds. Such clouds are thin, so you can see right through them. And they’re high — at least four miles up. They can produce halos around the Sun as well.

Cirrus clouds are made of ice crystals. The crystals are flat hexagons. They act as tiny prisms, bending the moonlight. It’s bent at a 22-degree angle, so the light forms a halo that spans 44 degrees — more than four times the width of your fist held at arm’s length.

But the crystals bend different colors of light at different angles, just as water droplets bend light to create a rainbow. So a halo sometimes shows a faint palette of colors — red on the inside of the halo, and blue on the outside.

Cirrus clouds often precede cold fronts, so a Moon ring has long been considered a sign of bad weather. There’s even a saying about it:

A ring around the Sun or Moon
Means rain or snow is coming soon.

Even if you don’t see a Moon ring, enjoy the Moon and the star Regulus as they arc across the winter night.


Script by Damond Benningfield

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