Dr. Terry L. Goforth
Associate Professor of Physics, SWOSU
May 2003

Thursday, May 15, 2003, skywatchers will be able to witness one of the nighttime sky's great treats: a total eclipse of the moon.

A total lunar eclipse occurs when the moon, more or less lined up with the Earth and the sun, passes through the darkest part of Earth's shadow. (See diagram.) Moon shown moving through Earth's shadow The moon is visible only because it reflects sunlight. Put it in a shadow, and it appears dark. Obviously, the moon can only enter Earth's shadow when it is on the side of the Earth opposite the sun. At this time, viewers from Earth see the full daylight side of the moon-a full moon. As the moon's orbital motion carries it into the Earth penumbra, or partial shadow, the moon will dim a bit-too little to be noticed by the unaided eye. (An astronaut on the moon would see part of the sun's disk blocked by a large, dark Earth during this time. But since some of the sun is still shining (as seen from the moon), the moon's surface is still bright.) This penumbral phase of the eclipse will last for about an hour before the observable eclipse begins, and again for about an hour after the observable eclipse has finished.

The first noticeable phase of the lunar eclipse is the partial phase. This begins when the moon first enters the Earth's umbra, or dark shadow. Here there is essentially no sunlight, and the moon's surface is now in the dark. (For an astronaut on the moon, the Earth would now completely block the sun's disk.) Earth's observers will see a small bite disappear from the moon's eastern edge. This bite will grow larger as more of the moon enters the umbra. When the moon is entirely inside the umbra, the total phase, or totality, has begun.

NASA diagram showing moon going through Earth's shadow

If you expect the moon to completely disappear from view during totality (a completely reasonable assumption), you are in for a pleasant surprise. The Earth's atmosphere acts as a lens, focusing a small amount of light into its own shadow. Red light is preferentially focused, giving the moon a reddish or copper-colored appearance. (This preferential scattering is also responsible for the red color of the rising and setting sun.) Dust, pollution, and other impurities in the atmosphere can enhance this effect, deepening the red color of a totally eclipsed moon. This red color is noticeable only during totality. It is not seen during the partial phase because the bright, fully sunlight portion of the moon that has not entered the umbra is simply too bright.

For the May 15-16 eclipse, totality will last about 53 minutes. Then the process will reverse-the eastern edge of the moon will emerge from the umbra, appearing bright and white. The lighted portion of the moon's disk will grow until the full moon is again visible. Of course, the moon is still in a penumbral eclipse, but we won't notice that.

The table shows the times for each phase of the eclipse and the moon's location in the sky as seen in Weatherford. Note that the moon has already entered the penumbra when it rises here. (That happens at 8:05 p.m., 16 minutes before moonrise.)

Moonrise 15 May 2003 8:21 p.m. ESE horizon
Start of partial phase (Moon enters umbra) 15 May 2003 9:02 p.m. Low the ESE
Start of totality 15 May 2003 10:13 p.m. Low in the SE
Middle of eclipse 15 May 2003 10:40 p.m. Low in the SE
End of totality 15 May 2003 11:06 p.m. Moderately up in the SE
End of partial phase (Moon leaves penumbra) 16 May 2003 12:17 a.m. Moderately high in the SSE
Moon leaves penumbra 16 May 2003 1:14 a.m. Moderately high in the S
Moonset 16 May 2003 6:49 a.m. WSW horizon
Data courtesy Astronomical Applications Department, U.S. Naval Observatory,

For more information, check out Fred Espenak's (NASA/Goddard Space Flight Center) web page at This site also includes links to many other sources of information including Sky and Telescope and Astronomy magazines.

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Last update:  May 14, 2003