Ring Around the Sun

by Fraser Cain on September 24, 2008

Solar halo - a ring around the Sun. Image credit: Matt Saal

Solar halo - a ring around the Sun. Image credit: Matt Saal


Have you ever looked up and noticed that there’s a big ring around the Sun? These rings are caused by ice crystals within thin cirrus clouds, and there several different kinds of sun rings you can see depending on the weather conditions.

One of the most common ones is called a 22° halo. They get this name because the ring is located 22 degrees away from the Sun itself. Both the Sun and the Moon block a 1/2 degree region of the sky at a time, so the ring around the Sun is about 44 times larger than the Sun itself.

Why do you get a ring at exactly 22°? The ring is formed because of the ice crystals suspended in the cirrus clouds. If you could look at the crystals under the microscope, you would see that they’re hexagonal in shape, and act as prisms for the Sun’s light. As light passes through the two sides of the prism, it’s deviated by exactly 22°. Since the ice crystals are jumbled up randomly in the sky, most of the light is deflected away. But from every position you’re always able to see the deflected light from some of the crystals in the sky. And this is why you see the bright ring around the Sun.

When you’re looking for halos, or rings around the Sun, make sure you always shield both eyes from the Sun. Even looking at the Sun for an instant can cause permanent eye damage.

Here’s an article from Universe Today that includes instructions for looking for Sun halos.

Here’s a great article from Atmosphere Optics that helps to explain the process.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

Comments on this entry are closed.

Previous post:

Next post: