In the early evening on Wednesday Feb. 20 the moon will slide through the shadow earth casts in the solar system giving us a chance to observe a total lunar eclipse.
The eclipse will be at least partly visible from locations covering about three quarters of the earth’s surface. The eclipse will not be visible at all for observers in Australia and northward through most of China, Japan and eastern Russia. Observers in the America’s have the best view; we will be able to see the eclipse from start to finish.
These diagrams are from NASA’s eclipse website. Much more information about this week’s eclipse, and about future eclipses, is available at NASA’s eclipse page.
The sun will set in West Michigan at 6:23 pm on the day of the eclipse. The full moon, however, in anticipation of its transit west to east through earth’s dark shadow will rise in the east a few minutes earlier, at 6:08 pm.
The eclipse will begin just after 7:30 pm when the moon starts to slip into the faint part of earth’s shadow called the penumbra. For the hour or so the moon moves in earth’s penumbra it will be difficult to see anything special is happening, but the situation will change at 8:43 pm when the moon will first encounter the dark part of earth’s shadow called the umbra. When the moon moves into earth’s umbra it will look as though Stephen King’s Langoliers are eating up the moon.
The moon will be completely in earth’s umbra for about 50 minutes between 10:00 and 10:51 pm. During this time, the moon will appear a dull, copper red, flanked by the planet Saturn to the left and the bright star Regulus in the constellation Leo to the right.
The moon will then slip out the other side of earth’s shadow, through the not-totally-dark penumbra and into the bright sunlight again by 1:17 am.
Take time to watch Wednesday’s eclipse if weather permits because while we have a good view of this one, we won’t be able to see another until December, 2010.
So what’s the difference between the umbra of earth’s shadow and its penumbra? You can see for yourself by conducting a simple experiment on a sunny day. Hold a ball in the sun so that it casts a shadow onto the floor or a wall. If you examine the shadow carefully you will see that its edge – the dividing line between bright and dark – is not perfectly sharp. Also, if you move the ball away from the wall you will see that the dark part of the shadow gets smaller, as you probably expected, but that its edge gets fuzzier.
The totally dark part of a shadow is its umbra. The umbra of a sun shadow is the region of space behind a shadow-casting object that light from the sun cannot reach. Consider the situation from a different perspective: an observer in the umbra of a sun shadow is not able to see the sun at all.
The fuzzy part of a shadow is its penumbra. The penumbra of a sun shadow is the region of space behind a shadow-casting object that some, but not all, rays of the light from the sun can reach. An observer in the penumbra of a sun shadow is able to see part, but not all, of the sun.
Earth’s shadow at the distance of the moon is really fuzzy. The umbra is about 2.5 times the diameter of the moon and the penumbra is almost 5 times the diameter of the moon. When the moon is in the penumbra it looks normal, but just a bit dimmer than usual. When the moon is in the umbra it looks very, very dark.
