Dr. Terry L. Goforth
Associate Professor of Physics, SWOSU
January 2000

On Thursday, January 20, 2000, Earth will witness its first total lunar eclipse since September, 1997. This heavenly event will be easily visible in the late evening skies of Oklahoma (weather permitting).

Moon shown moving through Earth's shadow, side view The moon does not produce its own light. Instead, what we call moonlight is really sunlight that is reflected from the surface of the moon back to Earth. A lunar eclipse occurs when the moon enters the Earth's shadow, where the Earth blocks the moon's source of light and leaves the lunar surface in the dark. The Earth's shadow consists of two regions. The outer region is an area of "partial shadow" called the penumbra. As the moon enters the penumbra, a person sitting on the surface of the moon would see the Earth blocking part of the sun's disk, with more and more of the sun's surface being hidden as the moon moves deeper into the shadow. The "dark shadow" or umbra occupies the center of the penumbra. When the moon enters the umbra, the Earth would completely block out the sun for our moon-based of observer.

At point A, the moon begins to enter the penumbra. This event is essentially unobservable. With good viewing conditions, a careful observer may notice a faint shadow on the moon's surface after the moon is about halfway into the penumbra. At point B, the moon begins to enter the umbra. It is here that the moon will begin to "disappear" from view. This is the beginning of the partial phase of the eclipse.

Moon shown moving through Earth's shadow, direct view

As the moon moves from point B to point C, more and more of the moon's face will darken. Although the part of the moon that is still in the penumbra will appear relatively bright, look for a faint, ruddy glow in the darker regions and other colors along the boundary between bright and dark. (It may help if you use your hand or some other object to "shade" your eyes from the brighter, uneclipsed region.)

Totality begins at point C. Here the sun is completely blocked by the Earth at all locations on the moon. However, the moon doesn't go completely dark and disappear from view. Instead, it is likely to glow with a dull red light! The culprit here is the Earth's atmosphere. It acts as a lens, focusing a very small fraction of the sun's light into the umbra. Blue light is more likely to scatter in random direction in the Earth's atmosphere and is therefore less likely to be focused. (This also results in blue skies for the Earth.) Red light is less affected by this random scattering, so it is primarily reddish light that is focused into the umbra and lights the surface of the moon during totality. (This reddening effect is also seen at sunrise and sunset, when the sun looks orange or even red.) The amount of light that enters the umbra depends heavily on atmospheric conditions; moisture, dust, pollution, and other "contaminants" all play a role. Look for variation in the darkness of the surface from one side of the moon to the other. The side of the moon that is deeper in the umbra is likely to appear darker, and the entire surface should darken as the moon approaches midtotality, then lighten after passing the midpoint of the eclipse.

At point D the moon begins to emerge from the umbra, totality is ended, and the second partial phase of the eclipse begins. As the moon moves out of the umbra, it will gradually "reappear" as a relatively bright object, and the darkened regions in the umbra will again be difficult to see. The moon completely leaves the umbra at point E. A partial shading of the surface might be noticeable for a while, but it is very difficult to pinpoint just when the moon emerges from the penumbra (point F) and the eclipse is complete.

The timetable for the eclipse is given below*.

Event Time (CST)
A Moon enters penumbra 8:03 p.m.
B Moon enters umbra (partial eclipse begins 9:01 p.m.
C Moon inside umbra (totality begins) 10:05 p.m.
  Middle of eclipse 10:44 p.m.
D Moon begins leaving umbra (totality ends) 11:22 p.m.
E Moon leaves umbra (partial eclipse ends) 12:25 a.m.
F Moon leaves penumbra 1:24 a.m.
*Source: Sky & Telescope, January 2000, p 110

No special equipment is needed to observe the eclipse. Just step out and look up. Since the moon rises at 5:28 p.m. CST, it will be high in the sky by the time the eclipse begins. Of course, be sure to dress warmly and find a comfortable place to sit if you plan to stay outside and watch from beginning to end. Don't miss the show. The next lunar eclipse visible from Oklahoma won't occur until May, 2003!

Return to list of Terry L. Goforth articles

Last update:  January 20, 2000