On the morning Tuesday August 28 there will be an eclipse of the Moon, visible in part over most of North America. The full eclipse will be visible west of the Rockies; the rest of the continent will see the beginning of the eclipse, but the Moon will set before it is finished.

Lunar eclipses are caused by the shadow of the Earth falling on the Moon. They can only occur at Full Moon, but not at every Full Moon because usually the Earth's shadow passes above or below the Moon. A couple of times a year, Sun, Earth, and Moon line up exactly, and we get a lunar eclipse.

The Earth's shadow has two parts, called the penumbra and the umbra.

The umbra is the central part of the shadow, where the Sun's light is completely blocked except for the dim reddish light refracted through the Earth's atmosphere, the light of all the world's sunrises and sunsets.

The penumbra is the outer part of the shadow, where at least some of the light comes directly from the Sun. The whole effect is like a bull's eye: the dark central umbra surrounded by the lighter penumbra.

Many eclipses are called "partial" because the Moon is never completely immersed in the umbra, but this eclipse is total, with the Moon passing almost through the centre of the umbra. This makes it one of the longest total lunar eclipses possible, with totality lasting a full 90 minutes.

Lunar eclipses take place at exactly the same time, wherever you observe them from, but appear to take place at different times because of the rotation of the Earth, causing the local times to be different. This table shows the local times for the major time zones across North America. It also shows the local times of moonset for major cities in each time zone, which will cut the eclipse short.

I've shaded the times of parts of the eclipse which won?t be visible in a particular time zone because the Moon will be below the horizon. The important events in an eclipse are as follows:

• P1: Penumbral first contact: when the very first shading appears on the Moon.
• U1: Umbral first contact: when the dark central umbra first touches the Moon.
• U2: Umbral second contact: total eclipse begins.
• Greatest: Greatest eclipse: when the Moon is closest to the middle of the umbra.
• U3: Umbral third contact: total eclipse ends.
• U4: Umbral fourth contact: when the umbra leaves the Moon.
• P4: Penumbral fourth contact: very last shading leaves the Moon.

Notice how more of the eclipse is visible the farther west you go. People in the Pacific time zone will see the entire eclipse, while people in the Atlantic zone will see a partially eclipsed Moon set.

In every case, the Sun will begin to rise just as the Moon sets, as always happens at Full Moon.

The Eclipse in Starry NightŪ

Starry NightŪ can depict the lunar eclipse really well. I've set the location to New York, NY, which automatically shows the view from Central Park. The time is set to 5:30 a.m., with the Moon about a third of the way into the umbra. Under Options:Solar System:Planets-Moons I can set Starry NightŪ to display the Earth?s shadow and outline the edges of the umbra and penumbra. This is the result:

By advancing time, I can see the Moon move deeper into the shadow until it sets just to the left of the twin skyscrapers at right, deep in eclipse.

Observing the Eclipse

A lunar eclipse is an event for every stargazer! Whether you observe it with your unaided eyes, with a binocular, or with a small telescope, you will have a beautiful view. Because this eclipse carries the Moon particularly deep inside the Earth?s shadow, we can expect deep and beautiful colors.

It's interesting to see how dark the Moon gets during totality by comparing it to the brightness of the stars; here it helps if you normally wear strong glasses so that, by taking them off, both the Moon and the stars will appear as fuzzy blobs!

If you're observing with high power binoculars or a small telescope, you can record the times when craters are immersed in shadow and when they emerge; timings to the nearest tenth of a minute are adequate.

Every eclipse is a major photo opportunity, and this one will be particularly so because, for much of the continent, the Moon will be low in the western sky.

Ordinary cameras can make as good pictures as telescopes of a lunar eclipse, but remember, if you normally keep a filter over your lens, to remove it for the eclipse. Filters often cause unwanted reflections, and I once ruined a set of eclipse pictures by leaving my filter on!

Be sure to post your eclipse reports and images on the Starry NightŪ Yahoo Group for all of us to enjoy:

• tech.groups.yahoo.com/group/StarryNight/