A total lunar eclipse occurs when the moon is completely submerged in Earth's dark, inner shadow, called the umbra. If the moon is only partly covered by the umbra, or only enters the outer shadow (called the penumbra), it is considered a partial lunar eclipse. The total eclipse is said to "begin" when the moon is fully covered by the umbra; this phase is also called "totality." [Super Blue Blood Moon 2018: When, Where and How to See It]
To help prepare for the upcoming eclipse Wednesday morning (Jan. 31), which NASA has dubbed a "Super Blue Blood Moon" eclipse due to its occurance near perigee and during the second full moon of January, Space.com has prepared a step-by-step chronology of the eclipses' major phases and some of the secondary phenomena you might expect to see. Not all of the events mentioned in the chronology will necessarily occur, because no two eclipses are the same. But many will, and those who know what to look for have a better chance of seeing them!
In the above timetable, all times are for a.m. on Jan. 31. When dashes are provided, it means that the moon has set and is no longer visible. Below are the numbered stages listed above, fully described:
1. Moon enters Earth's 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 Earth's shadow. Although the eclipse begins officially when the moon enters the penumbra, 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 about 70 percent across the moon's disk. So for about 40 minutes after the "start" of the partial eclipse, the full moon will continue to appear to shine normally, although with each passing minute it is progressing ever deeper into Earth's outer shadow.
2. Penumbral shadow begins to appear: Now the moon has progressed far enough into the penumbra that the shadow 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 evident as the minutes pass, with the shading appearing to spread and deepen. Just before the moon begins to enter Earth's dark umbral shadow, the penumbra should appear as an obvious smudge or tarnishing of the moon's left portion.
3. Moon enters Earth's 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, or it's apparent edge. 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 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 fourth century B.C. 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 by the umbra, the part of it that is immersed in shadow should begin to very faintly light up— like a piece of iron heated to the point where it just begins to glow. It now becomes obvious that the umbral shadow does not create complete darkness on the lunar surface. Using binoculars or a telescope, the shadow's outer part is usually light enough to reveal lunar maria 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.
5. Less than 5 minutes to totality: Several minutes before (and after) totality, the contrast between the remaining pale-yellow sliver of the moon's surface and the ruddy-brown coloration spread over the rest of the 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. Sometimes the fully eclipsed moon is such a dark gray-black that it nearly vanishes from view. But it can also 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 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 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 to the south of the center of Earth's umbra, the gradation of color 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 in the dim constellation of Cancer, the Crab, and positioned almost midway between the backward question-mark pattern of stars known as the Sickle of Leo well to its east (upper left) and the "twin stars," Pollux and Castor of Gemini well to the west (the moon's lower right). 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 Earth's shadow, the temperature on its sunlit surface hovered at about 266 degrees Fahrenheit (130 degrees Celsius). Since the moon lacks an atmosphere, there is no way this heat could be prevented from escaping into space as the shadow sweeps by. Now, in shadow, the temperature on the moon has dropped to minus 146 degrees F (minus 99 C); a drop of 412 degrees F, or 229 degrees C, in less than 150 minutes!
8. Total eclipse ends: The emergence of the moon from the umbral 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 signs of the eclipse come to an end.
12. Moon leaves penumbra: The eclipse "officially" ends, as it is completely free of the penumbral shadow.
Editor's note: If you capture an amazing photo of video of the Jan. 31 total lunar eclipse and would like to share it with Space.com for a story or gallery, send images and comments to: email@example.com.