A total lunar eclipse is when the Moon passes into the Earth's umbra, or total shadow. This can only happen at a full Moon when the Sun, Earth and the Moon are exactly in line.
During the total phase of the eclipse, those under otherwise dark skies away from city lights should notice a distinct darkening of their surroundings as more stars emerge in the sky. The dimming of the Moon should present a beautiful photo opportunity as the Moon will have dimmed from -12Mag to -6. Dim stars around the Moon will be visible and the Milky Way should appear in the sky.
A lunar eclipse can be enjoyed with the naked eye. But optical aids offer the chance to better investigate craters on the Moon and the subtle shading and colouring of the eclipse.
If you plan to watch more than just a few minutes of the eclipse, seasoned observers recommend dressing warmer than you think necessary and bringing food and hot drinks. A chair or blanket will make the experience more enjoyable.
The various stages, fully described:
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 hour 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 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 on Earth and deep shadows of a brilliant moonlit night begin to fade away, at least for viewer's away from bright lights.
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 does not involve 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. Colours 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.
5) Less than five minutes to totality - Several minutes before (and after) totality, the contrast between the remaining pale-yellow sliver and the ruddy-brown coloration spread over the rest of the Moon's disk may produce a beautiful phenomenon known to some as the "Japanese Lantern Effect."
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.
7) Middle of totality - The Moon is now shining anywhere from 10,000 to 100,000 times fainter than it was just a couple of hours ago.
Since the Moon is moving well to the south of the centre of the Earth’s umbra, the gradation of colour and brightness across the Moon’s disk should be such that its upper portion should appear darkest, with hues of deep copper or chocolate brown. Meanwhile, its lower portion -- that part of the Moon closest to the outer edge of the umbra -- should appear brightest, with hues of reds, oranges and even perhaps a soft bluish-white.
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 the constellation of Aries, the Ram, with the beautiful little Pleiades Star Cluster, shining 25 degrees to the Moon’s east (left). Your fist on an outstretched arm covers about 10 degrees of sky.
The darkness of the sky is impressive for those in the countryside. The surrounding landscape takes 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 130 degrees Celsius. Since the Moon lacks an atmosphere, there is no way that this heat can be retained from escaping into space as the shadow sweeps by. Now, in shadow, the temperature on the Moon has dropped to 99 degrees below zero Celsius. That's a drop of 229 degrees Celsius 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.
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