Viewer's Guide: Oct. 27 Total Lunar Eclipse

Post-Event: Pictures of the eclipse are here.

Enthusiastic skywatchers try to never miss a total eclipse of the Moon. The spectacle is often more beautiful and interesting than one would think. The next opportunity is Wednesday, Oct. 27.

The Moon, normally lit by sunlight, will enter and later emerge from the shadow of Earth. Much of the change will be obvious. But secondary phenomena may be overlooked.

This chronology includes some of the things you might expect to see. Probably not all of those mentioned will occur because no two eclipses are exactly the same. But many will, and those who know what to look for have a better chance of seeing it!

The Stages: What to Expect

1) Moon enters penumbra - The shadow cone of the Earth has two parts: a dark, inner umbra, surrounding by a lighter penumbra. The penumbra is the pale outer portion of the Earth's shadow. Although the eclipse begins officially at this moment, this is in essence an academic event. You won't see anything unusual happening to the Moon - at least not just yet.

The Earth's penumbral shadow is so faint that it remains invisible until the Moon is deeply immersed in it. We must wait until the penumbra has reached roughly 70 percent across the Moon's disk. For about the next 50 minutes the Full Moon will continue to appear to shine normally although with each passing minute it is progressing ever deeper into the Earth's outer shadow.

2) Penumbral shadow begins to appear - Now the Moon has progressed far enough into the penumbra so that it should be evident on the Moon's disk. Start looking for a very subtle light shading to appear on the Moon's left portion. This will become increasingly more and more evident as the minutes pass; the shading appearing to spread and deepen. Just before the Moon begins to enter the Earth's dark umbral shadow the penumbra should appear as an obvious smudge or tarnishing of the Moon's left portion.

3) Moon enters umbra - The Moon now begins to cross into the Earth's dark central shadow, called the umbra. A small dark scallop begins to appear on the Moon's left-hand (eastern) limb. The partial phases of the eclipse begin; the pace quickens and the change is dramatic. The umbra is much darker than the penumbra and fairly sharp-edged.

As the minutes pass the dark shadow appears to slowly creep across the Moon's face. At first the Moon's limb may seem to vanish completely inside of the umbra, but much later, as it moves in deeper you'll probably notice it glowing dimly orange, red or brown. Notice also that the edge of the Earth's shadow projected on the Moon is curved.

Here is visible evidence that the Earth is a sphere, as deduced by Aristotle from lunar eclipses he observed in the 4th Century BC. Almost as if a dimmer switch was slowly being turned down, the surrounding landscape and deep shadows of a brilliant moonlit night begin to fade away.

4) 75 percent coverage - With three-quarters of the Moon's disk now eclipsed, that part of it that is immersed in shadow should begin to very faintly light up . . . similar to a piece of iron heated to the point where it just begins to glow.

It now becomes obvious that the umbral shadow is not complete darkness. Using binoculars or a telescope, its outer part is usually light enough to reveal lunar seas and craters, but the central part is much darker, and sometimes no surface features are recognizable. Colors in the umbra vary greatly from one eclipse to the next, Reds and grays usually predominate, but sometimes browns, blues and other tints are encountered.

6) Total eclipse begins - When the last of the Moon enters the umbra, the total eclipse begins. How the Moon will appear during totality is not known. Some eclipses are such a dark gray-black that the Moon nearly vanishes from view.

At other eclipses it can glow a bright orange. The reason the Moon can be seen at all when totally eclipsed is that sunlight is scattered and refracted around the edge of the Earth by our atmosphere. To an astronaut standing on the Moon during totality, the Sun would be hidden behind a dark Earth outlined by a brilliant red ring consisting of all the world's sunrises and sunsets.

The brightness of this ring around the Earth depends on global weather conditions and the amount of dust suspended in the air. A clear atmosphere on Earth means a bright lunar eclipse. If a major volcanic eruption has injected particles into the stratosphere during the previous couple of years, the eclipse is very dark. But, as of this writing, no such eruption has happened, so the betting is that this eclipse will be bright.

Observers away from bright city lights will notice a much greater number of stars than were visible before the eclipse began. The Moon will be in the constellation of Aries, the Ram. Well above and to the right of the Moon will be the Great Square of Pegasus while the beautiful Pleiades Star Cluster will sit far off to the Moon's left.

The darkness of the sky is impressive. The surrounding landscape has taken on a somber hue. Before the eclipse, the Full Moon looked flat and one-dimensional. During totality, however, it will look smaller and three-dimensional - like some weirdly illuminated ball suspended in space.

Before the Moon entered the Earth's shadow, the temperature on its sunlit surface hovered at 266 degrees Fahrenheit (130 degrees Celsius). Since the Moon lacks an atmosphere, there is no way that this heat could be retained from escaping into space as the shadow sweeps by. Now, in shadow, the temperature on the Moon has dropped to minus 146 degrees Fahrenheit (99 degrees below zero C). A drop of 412 degrees Fahrenheit (229 degrees C). in less than 90 minutes!

8) Total eclipse ends - The emergence of the Moon from the shadow begins. The first small segment of the Moon begins to reappear, followed again for the next several minutes by the "Japanese lantern effect."

9) 75 percent coverage - Any vestiges of coloration within the umbra should be disappearing now. From here on, as the dark shadow methodically creeps off the Moon's disk it should appear black and featureless.

10) Moon leaves umbra - The dark central shadow clears the Moon's right hand (western) limb.

11) Penumbra shadow fades away - As the last, faint shading vanishes off the Moon's right portion, the visual show comes to an end.

12) Moon leaves penumbra - The eclipse "officially" ends, as it is completely free of the penumbral shadow.

• Top 10 Eclipse Facts
• Eclipse News, Lore and Photos
• Local Weather Forecast
• Oct. 27 Eclipse Coincides with Game 4 of the World Series!

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Oct. 27, 2004 Eclipse Webcasts Planned (weather permitting)

• Astronet (Netherlands and Belgium)
• Saros Group, Canary Islands (off Africa)
• Klipsi's eclipsec@m, Switzerland
• Universidad de Sonora, Mexico
• UAI (Italy)
• East Antrim Astronomical Society (Northern Ireland)
• Asociaci?n Argentina "Amigos de la Atronom?a" (Argentina)
• University of North Dakota
• Parssky (Iran)
• Grupo de F?sica & Astronomia - IP&D (Brazil)

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Starry Night software brings the universe to your desktop. Map the sky from your location, or just sit back and let the cosmos come to you.

Joe Rao, a veteran of 12 total lunar eclipses, serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.