The May 20 new Moon will produce a solar eclipse visible over the western half of the U.S. Those in a narrow band from the Texas panhandle to northern California will see a rather rare annular eclipse while the rest of us will see an impressive sunset partial eclipse.As the Moon orbits our planet each month, new Moon is the point at which it passes between the Sun and Earth. If the plane of the Moon’s orbit around Earth was exactly the same as the plane of the Earth’s orbit around the Sun, we would see a total solar eclipse every new Moon (and a total lunar eclipse every full Moon). But since the two orbital planes aren’t exactly the same, solar (and lunar) eclipses occur far less frequently than monthly. When the Moon does pass exactly between the Sun and Earth, it creates a total solar eclipse, temporarily blocking out the Sun and casting a moving shadow across part of Earth. If it passes partly, but not quite exactly, between and blocks out part of the Sun, in creates a partial solar eclipse.The Sun is 400 times larger than the Moon, and by coincidence, is also 400 times further away, thus they appear the same size. However, since the orbits of the Earth and Moon are elliptical rather than circular, the distances between the Earth and Sun, and between Moon and Earth, vary making their apparent sizes also vary.Since the Moon’s orbit around Earth is more eccentric (less circular) than Earth’s orbit around the Sun, the change in the Moon’s apparent size is more pronounced. When solar eclipses occur with the Moon further from Earth, the Moon isn’t large enough to cover the entire Sun, leaving a “ring of fire” around the Moon’s silhouette. What would otherwise be a total eclipse becomes an annular eclipse. This is what those within a narrow band less than 200 miles wide from Texas to California will see, weather permitting. At the extreme eastern end of this band in Texas, the eclipse will reach annularity as the sun is evening setting. The further west from which on views, the earlier before sunset the eclipse begins and the longer it will be visible.

The rest of the western U.S. not within the area of annularity will see a partial eclipse of the Sun where the Moon covers part of the Sun. The nearer one is to the area of annularity, the greater the percentage of the Sun will be eclipsed. In Central Texas the partial eclipse begins at 7:35 p.m. with the Sun less than a fist-width (held at arm’s length) from the western horizon. By sunset (8:21 p.m. in Waco), the Moon will be covering more than half of the Sun. Further east, the show will be shorter with less coverage; further west, longer and more coverage. At the extreme eastern end, in the Lubbock, TX, area, the partial eclipse reaches annularity just as the Sun is setting. Further west, like in Albuquerque, NM, the entire 4-minute period of annularity occurs just before the Sun sets. For points beyond the Nevada-Arizona-Utah border area, the entire eclipse – partial and annularity – will be visible before sunset. It is essential to view this event safely. Even when partly eclipsed and near the horizon, the Sun can produce serious and permanent eye damage if viewed with without proper protection. Make-shift devices like stacked sunglasses, soot-covered glass, and CDs are not adequate; while they block out visible light, they don’t protect against invisible but more dangerous ultraviolet and infrared radiation. For direct viewing use approved solar glasses or #14 welders glass. Even safer is indirect viewing of projected images of the Sun using “pin-hole” projectors or binocular images cast on a sheet of white paper. Never look a the Sun with binoculars (or a telescope) without using a proper solar filter.Here’s hoping for a cloudless western horizon May 20 before sunset. But even if you’re clouded out, the event should generate some exceptional photos of the eclipsed Sun near the horizon, so watch the media for postings the next day.