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This shows the subtle effects of the penumbral shadow at upper left edge during the eclipse of November 1994

 

 


Eclipse sequence November 1993

A total lunar eclipse occurs when the moon enters the shadow of Earth and gets cut off from all direct sunlight. This can only happen at full moon when it is directly opposite the sun as seen from Earth. If the moon's orbit was in the same plane as the Earth's, a total eclipse would happen every month. The orbit is tipped about 5° however, so the moon is more often out of alignment with the sun and Earth when it is full.

The Earth constantly casts a shadow out into space directly away from the sun. The darker inner shadow, the umbra, forms a cone of darkness where no direct sunlight hits. Around that is the penumbra, a lighter semi-shadow where varying amounts of sunlight falls. The diagram below shows the shadow in relation to the moon, and a hypothetical lunar eclipse with the moon traveling directly through the shadow's center.


Though technically the eclipse begins when the moon first enters the penumbra (P1), it would be difficult to see any darkening at such an early stage. The shadows are much softer-edged than shown, and the outer penumbra is still very bright. You should see some slight darkening effects after the moon travels about two-thirds of the way into the penumbra. The show really begins when the moon first contacts the umbra (U1). Totality occurs when the moon's trailing edge contacts the umbra (U2). This is when all direct sunlight is blocked from the lunar surface. The contact point names (P1, U1, etc.) are references used in eclipse event data like the map at the bottom of the page.

At left is a totally eclipsed moon showing some interesting phenomena that needs a bit of explanation. Such as, why is there so much color if the umbral shadow has no light? Although no direct sunlight hits the lunar surface, there is indirect light that is refracted into the shadow by Earth's atmosphere. This sunlight basically skims past the Earth through a long stretch of air, and much like the light we see at sunset, has the shorter wavelengths of blue and green light scattered, letting only the longer wavelengths of reddish light pass. If we were on the moon looking back at Earth at the time of totality, we would see a bright ring of orange-red light encircling the planet.

Then there is the matter of the uneven illumination on the lunar surface. This is mostly due to the moon's path through the shadow. If it went directly though the center of the umbra, and the Earth's atmosphere was completely uniform, the eclipsed moon would appear evenly lit. That is rarely the case however, and in the eclipse pictured above (from September 1996) the moon passed near the edge of the umbra where the shadow is a little brighter. When viewing a lunar eclipse with the naked eye, you will not see such spectacular bright colors at totality. Binoculars will show the rich hues much better, and of course photography captures them best. The image above was from my film days. It was taken on the richly saturated Velvia film. The image below is from the August 28, 2007 eclipse photographed with a digital camera for a better color rendition and contrast control.


The two outer images in the composite above were taken minutes before and after totality when there was a small sliver of direct sunlight on the lunar edge. The middle exposure was taken at mid-eclipse. Notice how you can see the size and shape of the Earth's shadow in relationship to the moon. The position of the light on the moon's lower edge shows that it didn't traveled through the center of the umbra, but rather a little closer to the bottom.

A total eclipse of the moon is not as spectacular as one of the sun. The good thing about the lunar variety is that they are much more frequent for any given location on Earth. A lunar eclipse occurs out in space and you merely have to be in sight of the moon to see it. During a typical eclipse about half the world can witness totality. Fewer people get to see a total solar eclipse because they occur for very small areas on the Earth's surface where the sun and moon alignment are perfectly centered.

It is worthwhile seeing a variety of lunar eclipses that occur in different parts of the sky and times of night. When seen in twilight, like the one pictured at far left, the totally eclipsed moon will appear quite dim and ghostly. That image is from South Mountain Park in Phoenix on the evening of November 8, 2003. The image at near left is from the event of September 26, 1996. The dark umbral shadow had just started across the lunar surface at moonrise. I took this photo a few minutes later. Notice the effects of the lighter penumbral shadow on the central part of the moon. I really like this image because it not only shows the Earth's shadow crossing the moon, but also its deep blue band climbing higher into the atmosphere. The clearest view of totality's coloration effects can be seen when mid eclipse occurs at midnight with the moon high in a dark sky.

Map courtesy of Fred Espnak, NASA GSFC. For more information visit the NASA
Lunar Eclipse Page

Full page of eclipse data
from which this map came

December 10, 2011 Eclipse Map

The next total lunar eclipse is on June 15, 2011. North America will not get to see any of this one since it is centered on about 60° E longitude where Africa and the Middle East enjoy ring side seats. A second total eclipse occurs on December 10, 2011 which parts of the U.S. will get to see. (Follow link for more detail about this eclipse in Arizona). Below is a handy map from the NASA website showing the circumstances of the event. People viewing from the area in white can see the entire eclipse, while those in locales marked in varying shades of gray will enjoy only part of the celestial show. Here is where you get to use the contact point names (P1, U1, etc.) from the diagram above.

Take the U1 point for instance, that all important time when the moon first contacts the dark umbral shadow. The U1 line crosses the map in two places—one for locations where the moon is rising and another for when it is setting. People viewing along the U1 line in the Eastern U.S. would see this eclipse phase just as the moon is going below the horizon, and no more. The U1 line going through parts of the Middle East and to the north would see this phase at moonrise, and could enjoy the rest of the eclipse as the moon climbs higher in the sky.

Mid totality, the point of greatest eclipse, occurs half-way between the U2 and U3 lines. Observers located between there in the moonset section of the map, such as in the Southwestern U.S. and up through central Canada, would get to witness the first half of the eclipse before the moon dropped out of sight. Saudi Arabia, Turkey, Eastern Europe and north would see the last half of the eclipse starting with totality peeking over the eastern horizon.


All images are copyrighted by Frank Zullo. Please do not use without written permission.