The Many Faces of an Eclipse

When we hear of a solar eclipse we conjure up images of the Moon passing in front of the Sun, covering its surface (photosphere) and leaving only the Sun’s corona visible behind the Moon. This is known as a total solar eclipse and it is only one of many types of eclipse that can be viewed from Earth. Solar eclipse can be total, partial, annular and even hybrid.

This weekend’s annular eclipse is one of the types of solar eclipses that we can enjoy, but it is very different from a total eclipse. To understand what makes an annular eclipse different we have to take a look at what makes a total eclipse happen.

The mechanics behind a total solar eclipse are relatively straightforward; the Moon is about 400 times smaller than the Sun, but the moon is 400 times closer to the Earth than the Sun is. This ratio makes it so that that moon can completely cover the sun in our sky.

The Moon of course does not always eclipse the Sun every month when we have a new moon. This is because the Moon’s orbit around the Earth is tilted about five degrees off of the Earth’s orbit around the Sun. When all the Earth, Sun and Moon align just so in their orbits we can see a total eclipse where the dark inside portion of the Moon’s shadow (umbra) hits the Earth in a narrow path about 257 km wide. Outside the umbral shadow, we fall into a region know as the penumbra, or the outer lighter shadow of the Moon, where we will see a partial eclipse.

A total solar eclipse on July 22, 2009 as seen from Varnasi, India (Source: Wikimedia Commons - Credit Manoj Dayyala)

However, the Moon’s orbit around the Earth is not a perfect circle, just like the Earth’s orbit around the Sun. Sometimes the Moon is farther from the Earth at the point in its orbit we call the apogee. If the Sun, Earth and Moon align for an eclipse near or at the Moon’s apogee, the 400 to 400 ratio that gives us a total eclipse no longer applies and the umbra does not fall on Earth, but on a point out in space just above the Earth.

When this happens we get an annular eclipse, the moon will only cover a part of the Sun’s surface leaving a ring, or annulus, of the Sun’s photosphere around the Moon. Because of this ring around the Moon, annular eclipses are also called a ‘ring of fire’ solar eclipse.

May 20, 2012 annular eclipse (Source: Wikimedia Commons - Credit Carl Drinkwater)

During a hybrid eclipse we experience both an annular and total solar eclipse. This type of eclipse generally starts out as an annular eclipse with the umbra falling in a point out in space just shy of the Earth’s surface. Because the Earth’s surface is a round uneven sphere, the Earth’s surface ‘reaches’ out to briefly touch the umbra before falling back down. This allows the eclipse to go from annular, to total and back to annular.

A very important note to keep in mind with all solar eclipses is that you must never look directly at the Sun with the naked eye as it can cause permanent damage to your eyes. Always use proper protective eye protection when looking at the sun like solar viewing glasses (not regular sunglasses) or a #14 arc welder’s filter. You can also use pinhole-projection. For more information about this weekend’s annual eclipse and how you can make your own pinhole-projection device, please read my earlier blog entry about the annular eclipse. You can also come join the Winnipeg Centre of the RASC at Assiniboine Park to view the eclipse.