An annular eclipse occurs when the moon blocks out the center of the sun, leaving a glowing ring called an annulus around the moon’s dark silhouette. It’s a lovely sight and well worth seeing, although it doesn’t compare to the wonder of a total eclipse.
We experience both total and annular eclipses because the distance between the earth and the moon varies. When the moon is close to the earth, it appears as large as the sun and we see a total solar eclipse. When it’s further away, it appears smaller than the sun and we see an annular eclipse. This year, because the moon was at its furthest distance from the earth on May 19, the “ring of fire” was especially wide.
The moon casts a complex shadow during an eclipse. If the umbral shadow touches the earth, there’s a total solar eclipse. But sometimes, as shown in the diagram, the umbral shadow doesn’t quite reach us. Instead, an antumbral shadow falls on the earth, creating an annular eclipse. Areas within the much larger penumbral shadow experience a partial eclipse.
The Path of the May 20 Eclipse
Fortunately, the path of annularity—the area on earth from which the complete ring of the sun was visible—included many populated areas, so lots of people had the opportunity to experience this lovely celestial sight. Many more people were able to see a partial eclipse.
The antumbral shadow first touched southern China (on May 21 local time). Early risers in Guangzhou found the eclipse in progress at dawn. After leaving China, the shadow traversed the southern coast of Japan, and the Tokyo area was treated to a full five minutes of annularity. The shadow then raced across the Pacific, reaching the southern Oregon—northern California coast in the late afternoon. Parts of the western United States experienced well over four minutes of annularity, including Reno, Nevada; Albuquerque, New Mexico; and Lubbock, Texas. But by the time the shadow reached Albuquerque, the sun was only 5 degrees above the horizon, and it was only 1 degree above the horizon in Lubbock. For both cities, the eclipse was still in progress as the sun set. The eclipse ended just past Lubbock.
The darkest gray area on the world map shows the path of annularity. The medium gray indicates areas that saw a partial eclipse, and the lightest gray shows areas that saw only a part of the eclipse because the sun was rising or setting.
Eclipse-Watching in San Francisco
In San Francisco, the maximum eclipse occured at 6:32 p.m. when 84.22 percent of the sun was obscured as shown in the illustration. A clear sky provided a perfect eclipse-watching experience. About 1,500 people joined the Exploratorium and the San Francisco Amateur Astronomers to see this exciting event.
• The HM Nautical Almanac Office’s Annular Eclipse of the Sun: 2012 May 20-21 site has local circumstances and animations for 259 locations.
• The NASA Eclipse Web Site has an abundance of information about solar eclipses, lunar eclipses, and transits.