Apparent variations

A 340
It always seems a marvelous coincidence that the moon happens to be the right size for blotting out the sun at solar eclipse time. But that fortuitous match of the 2,160-mile diameter moon with the 865,000-mile-wide sun only works in the case of a total solar eclipse.

There is another type of solar eclipse, called an "annular" eclipse, where the moon isn't quite able to keep the sun under cover. During an annular eclipse, a thin ring of sun encircles the darkened moon.

Like most celestial bodies, the moon's orbit is not a circle, but an ellipse. Thus, as the moon swings through its 27-day orbit, its distance from Earth varies. At perigee, the closest point in its orbit, its is 192,317 nautical miles from Earth; while at apogee, the distance increases to 219,460 miles. This variation in distance is enough to change its apparent size. Without some frame of reference, however, the unaided human eye can't perceive this fluctuation in apparent size.

When the moon is at apogee, however, and is between the sun and Earth (on the plane of the ecliptic), we do have some way of seeing the change in apparent size. The moon is not large enough to obliterate the sun from the sky; a portion of the sun's disk is seen ringing the moon (annular is from a Latin word for ring).

This year, on May 10, some of us in North America will have a good opportunity to see an annular eclipse. The path of totality (or perhaps we should say "semi-totality") will come in from the Pacific, cross Baja California, enter the U.S. at the New Mexico/Texas border, make a bee line for Lake Erie and Lake Ontario, cross northern New England, hit the Gulf of Maine, the Bay of Fundy, and Nova Scotia before heading out across the North Atlantic. The eclipse will cross the U.S. border at about 1610 GMT, and pass into the Atlantic at 1800 GMT.

While the path of totality will be roughly 120 miles wide, all of the continental U.S. will see some effects of the eclipse. And while the geometry of sun, moon, and Earth is favorable, there is one wildcard in seeing any eclipse: weather. Those of us will the best potential view can only hope for clear skies.

Eclipse lovers will need to have luck with the weather this year, since there will only be a total of three: a total solar eclipse, an annular solar eclipse, and one partial lunar eclipse. (In any one year, there can be as many as seven eclipses: five of the sun and two of the moon.)

To see the total solar eclipse due for November 3, one will need to be on the South Pacific, ashore in South America or South Africa, or in the South Atlantic. (Maps showing the viewing area of both solar eclipses are on pages six and seven of the 1994 Nautical Almanac. Lunar eclipses are visible everywhere on the darkened half of Earth - barring inclement weather, of course.)

In addition to the annular eclipse on May 10, a partial lunar eclipse will be visible on May 25 from Africa, most of Europe, Antarctica, and the Americas (except for the northwest and extreme north).

During a partial lunar eclipse, a section of the moon passes through Earth's shadow. This section is darkened, and sometimes shows a coppery red or orange color. A total lunar eclipse occurs when the moon dives straight through Earth's shadow, blocking all light from the sun.

Since the moon makes one circuit around Earth every 27 days, one might expect two eclipses every month (one solar eclipse when the moon came between the sun and Earth, and one lunar eclipse when the moon entered Earth's shadow). However, since its orbit is tilted 5 degrees to the plane of the ecliptic, the moon passes above or below the sun and passes either above or below Earth's shadow, cheating us of biweekly eclipses.

The moon's orbit is like a hoop that passes through the plane of the ecliptic at two points. These points are called nodes and in each orbit of the moon there is a "descending" node (when the moon is moving from that half of its orbit above the plane of the ecliptic to the half below it) and an "ascending" node (from below the ecliptic to above it). Due to gravitational forces acting on the moon, the hoop of its orbit rotates roughly 20 degrees a year. This rotation is called precession of the nodes and it takes 18 years and 11 1/3 days for the moon to complete one precessionary cycle - a period called a Saros.

It is due to this precession of the nodes that eclipses occur. When a node crosses the ecliptic on the sun side of Earth, and the moon comes between Earth and the sun, a solar eclipse occurs. When the intersection is in Earth's shadow, we see a lunar eclipse. During the period of a Saros, there are generally 29 lunar and 41 solar eclipses. This cycle is repeated every Saros, but the eclipse point moves 120 degrees to west with each cycle. Thus, every 54 years and 34 days, an eclipse will reoccur at the same point on Earth.

So, on June 13, 2048, in the continental U.S., we'll see the same type of solar eclipse along the same path as the one we're about to view in May.

Now, if only the weather cooperates . . . . Tim Queeney

By Ocean Navigator