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An eclipse of the Moon can only take place at Full Moon, and only if the Moon passes through some portion of the Earth^s shadow. The shadow is actually composed of two cone-shaped components, one inside the other. The outer or penumbral shadow is a zone where a some portion of the Sun^s rays are blocked. In contrast, the inner or umbral shadow is a region devoid of all direct sunlight. Astronomers recognize three basic types of lunar eclipses: 1. Penumbral Lunar Eclipse * The Moon passes through the Earth^s penumbral shadow. * These events are of academic interest only since they are suble and quite difficult if not impossible to observe. 2. Partial Lunar Eclipse * A portion of the Moon passes through the Earth^s umbral shadow. * These events are easy to see, even with the unaided eye. 3. Total Lunar Eclipse * The entire Moon passes through the Earth^s umbral shadow. * These events are quite striking for the vibrant range of colors the Moon can take on during totality. As the Moon orbits the Earth every 29.5 days, it usually passes north or south of Earth^s shadows so no eclipse takes place. But every once in a while, the Moon passes through some portion of the Earth^s penumbral or umbral shadows and one of the above three types of eclipses occurs. During a total lunar eclipse, the Earth blocks all direct sunlight from the Moon. Astronauts on the Moon would then see the Earth eclipsing the Sun. While the Moon remains completely within Earth^s umbral shadow, indirect sunlight still manages to reach and illuminate it. However, this sunlight must first pass deep through the Earth^s atmosphere which filters out most of the blue colored light. The remaining light is a deep red or orange in color and is much dimmer than pure white sunlight. The Earth^s atmosphere also bends or refracts some of this light so that a small fraction of it can reach and illuminate the Moon. The total phase of a lunar eclipse is so interesting and beautiful precisely because of the filtering and refracting effect of Earth^s atmosphere. If the Earth had no atmosphere, then the Moon would be completely black during a total eclipse. Instead, the Moon can take on a range of colors from dark brown and red to bright orange and yellow. The exact appearance depends on how much dust and clouds are present in Earth^s atmosphere. Total eclipses tend to be very dark after major volcanic eruptions since these events dump large amounts of volcanic ash into Earth^s atmosphere. During the total lunar eclipse of December 1992, dust from Mount Pinatubo rendered the Moon nearly invisible. All total eclipses start with a penumbral followed by a partial eclipse, and end with a partial followed by a penumbral eclipse (the total eclipse is sandwiched in the middle). Since the penumbral phases of the eclipse are so difficult to see, we will ignore them. From start to finish, January^s lunar eclipse lasts nearly three and a half hours. The partial eclipse begins as the Moon^s eastern edge slowly moves into the Earth^s umbral shadow. During the partial phases, it takes just over an hour for the Moon^s orbital motion to carry it entirely within the Earth^s dark umbra. Since no major volcanic eruptions have taken place recently, the Moon will probably take on a vivid red or orange color during the 77 minute long total phase. After the total phase ends, it is once again followed by a partial eclipse as the Moon gradually leaves the umbral shadow. Eclipse Frequency and Future Eclipses During the five millennium period from 2000BC through 3000 AD, there are 7,718 eclipses1 of the Moon (including both partial and total). There are anywhere from 0 to 3 lunar eclipses (including partial and total) each year. The last time that three total lunar eclipses occurred in one calendar year was in 1982. Partial eclipses slightly outnumber total eclipses by 7 to 6. The last total lunar eclipse visible from the United States occured on Sept. 26, 1996. North Americans won^t have another opportunity to see a total lunar eclipse until May 16, 2003. However, on July 16, 2000, Hawaii, Australia and Asia will see the longest total lunar eclipse in 140 years (since 1859). It will last 1 hour and 47 minutes. Fred Espenak NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771 USA Ed. note: This news report was edited for brevity. Complete details, charts and references can be found at: http://sunearth.gsfc.nasa.gov/eclipse/
Uploaded: 2/21/2004
Uploaded: 2/21/2004