The Coast News Group
Coastal Cosmos

Solar system geometry on display with spectacular eclipse

The geometry of the solar system reveals itself in the coming weeks. On May 20, the Earth, sun and moon will align during the day, generating an annular solar eclipse. The straight alignment of three celestial bodies is called syzygy — pronounced “zizigee.”

On June 6, a different kind of syzygy will take place as Venus passes in front of the sun for a twice-in-a-lifetime event known as a Venus Transit (detailed discussion in the next “Coastal Cosmos”).

A solar eclipse happens when the moon passes between the Earth and the sun, partially or totally obscuring sunlight from reaching Earth. Because the moon’s orbit of the Earth is an ellipse and not a perfect circle, its distance from Earth changes. During an annular solar eclipse, the moon is farther from the Earth and hence, its apparent diameter is smaller than the sun’s. A ring or annulus appears as the sun’s edge shines around the moon’s shadow.


On May 20, the Earth, sun and moon will align during the day, generating an annular solar eclipse. The straight alignment of three celestial bodies is called syzygy — pronounced “zizigee.” Image courtesy of NASA

Because the geometry is so acute, the maximum eclipse is only visible from a very small strip of the Earth’s surface. The May 20 eclipse will best viewed on an imaginary line connecting Tokyo, Japan, through Northern California to Albuquerque, N.M. From San Diego, a partial solar eclipse will occur, giving the sun a crescent appearance as it sets in the west.

The next total solar eclipse for the United States will take place on Aug. 21, 2017. The longest duration of totality, two minutes and 40 seconds, will arise over Sinking Fork, KY. The last total solar eclipse in the U.S occurred on Feb. 26, 1979, over parts of Washington, Oregon, Idaho and Montana.

Within a few hundred million years, total solar eclipses will no longer be possible. The moon is moving away from the Earth at a rate of about 1.5 inches per year, as continuously measured by lasers fired from Earth to small reflectors placed on the moon’s surface by Apollo astronauts.

It is a fortunate coincidence that the current distance of the moon can exactly match the overwhelming size of the sun from our perspective. As the moon recedes from Earth, only annular eclipses will occur.

The predictability of eclipses derives from the precise geometry of celestial bodies orbiting each other in the solar system. Eclipse cycles are the intervals between recurring eclipses. One such cycle, the saros, has a period of 18 years and 11 days. Each sar, or half-saros, alternates between lunar and solar eclipses.

The May 20 eclipse is part of Saros Cycle 128. It follows the annular solar eclipse of May 10, 1994, which I remember watching during a sixth-grade school day in Ohio. Amazingly, the saros was developed by Babylonian astronomers 3,000 years ago and is still applicable today.

It is an absolute necessity to always view the sun with extreme caution. Never look directly at the sun without proper eye protection. Sunglasses will not safeguard your eyes during solar observations. Permanent damage is likely form improper protection. Options for safe viewing include: solar eclipse glasses (available for $2 online), constructing a simple pinhole projector, a hydrogen-alpha solar telescope, or projecting the sun’s image onto white paper through binoculars.

Put these celestial events on your calendars and hope for a couple days free of May Gray and June Gloom.