Solar eclipse on Sunday evening

On late afternoon of Sunday, May 20, the southwestern United States will be treated to a rare astronomical event — an eclipse of the sun. Weather permitting, this will be the first annular solar eclipse visible in the Southwest since May 9, 1994.

A solar eclipse occurs when our moon passes between the Earth and the sun, blocking our view of the sun. Depending on how the moon is aligned between Earth and sun, and depending on where on the Earth we are standing, the Moon will cover the Sun, creating a partial, annular or total solar eclipse.

If the Moon is aligned exactly between Earth and sun, it will cast its shadow for hours along a linear track about two hundred miles wide across the surface of Earth for thousands of miles. If we are standing inside the track of that shadow, and if the moon is close enough to Earth, it will cover the entire Sun, creating a total solar eclipse, one of the most spectacular sights we ever can witness.

But the moon’s orbit around Earth is elliptical, so at its closest to Earth, the moon is about 30,000 miles closer than it is at its farthest from Earth. Its average distance is about 239,000 miles.

The moon appears larger when it is closer to us and smaller when it is farther from us. At the May 20 solar eclipse, the moon will be farthest from Earth, so it will appear at its smallest. It will not be able to cover the sun entirely and will not create a total eclipse.

Instead, the moon will create an annular eclipse in which the outermost edge of the sun is visible as the moon passes in front of it. The sun then appears as a ring of light around the dark face of the moon. The word “annular” is derived from the Latin word “anulus,” meaning “ring”.

On May 20 in the Four Corners, the moon will start covering the sun at roughly 5:45 p.m. Mountain Daylight Time. At roughly 7 p.m., the entire face of the moon will cross the sun for as long as four minutes, thirty seconds. Then it will gradually uncover the sun until roughly 8:15 p.m.

Since these times are early evening, the sun will lie low in our northwest sky about eight degrees above horizon at eclipse midpoint. If clouds get in the way, our skies still can get noticeably darker close to eclipse midpoint. If skies are clear, watch for crescent images of the sun projected onto buildings or the ground as sunlight is filtered through leaf canopies of trees.

The center line of the eclipse shadow track will pass close to Zion National Park in Utah, Canyon De Chelly in Arizona, and Gallup and Albuquerque in New Mexico. Along this center line, the annular eclipse phase will last over four minutes.

Southwest Colorado towns will lie near the north edge of the eclipse shadow, so they may see the annular eclipse phase for only seconds, or only a partial eclipse. Whatever part of this eclipse is visible, caution must be taken while viewing.

Never view solar eclipses directly with the naked eye. Invisible ultraviolet rays of an eclipsed sun will damage your retinas or blind you permanently, even if you feel no discomfort. Always view solar eclipses indirectly as follows.

One way is to poke a small hole in a cardboard sheet, then hold the sheet to project sunlight through the hole onto another sheet or a wall. An image of the sun appears on the second surface.

Another way is to aim a telescope or binoculars at the sun and project a solar image through the instrument onto a wall or cardboard. Never view solar eclipses directly through telescopes or binoculars without a correct solar filter. Solar rays are extremely concentrated as they pass through eyepieces, like a magnifying glass. Such intense rays will damage your eyes severely.

Another way is to use an approved solar filter or a #14 welder’s glass to block ultraviolet and infrared radiation and excess visible light, which allows safe direct observation of the sun. Welding shops may sell appropriate welder’s glass for such use.

Using those precautions, enjoy the most awesome sight we ever can see in our sunny Southwest skies.

Copyright 2012 James F. Andrus;all rights reserved. Andrus earned a degree in meteorology from the University of Wisconsin, and an additional degree in computer science from Jan Juan College. He lives in Cortez, where he is a National Weather Service Cooperative Weather Observer. He has a lifelong interest in the physical sciences.

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