Diana Hannikainen, Observing Editor
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J. Kelly Beatty, Senior Editor
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The first total lunar eclipse of 2018 will be a predawn spectacle for western North America.
No one has seen a total eclipse of the Moon since September 27-28, 2015, and now the 2½-year-long drought is finally ending — at least for those living near the West Coast and most everywhere around the Pacific Rim. In North America, a total lunar eclipse occurs before dawn on Wednesday, January 31st.
Weather permitting, those in western North America should see much if not all of the eclipse. Alaska, Hawai'i, Yukon, most of British Columbia, the Northwest Territories, and parts of Nunavut should witness every stage of the eclipse starting well before dawn. The entire eclipse is also visible on the evening of January 31st from eastern and northern Asia, New Zealand, and most of Indonesia and Australia.
According to Sky & Telescope magazine, anyone west of a line running from Chicago to New Orleans will see at least the first part of the Moon's 76-minute-long plunge through Earth's deep shadow. "The farther west you are, the higher the Moon will be in the predawn sky when the eclipse starts — and the more of it you'll see," explains Diana Hannikainen (HAHN-ih-ky-nen), S&T's Observing Editor.
For the eastern U.S. and Canada, the Moon sets (and the Sun rises) as the early stages of the eclipse get under way. Those on the East Coast will see, at best, a dark "bite" extending about halfway across the lunar disk.
See the table below and diagrams at the end for times of key events during January 31st's eclipse. The stages of the eclipse occur simultaneously for everyone who can see the Moon, but the clock times for you depend on your time zone:
A second total lunar eclipse occurs this year on July 27th. It's observable only in the Eastern Hemisphere: Africa, the Middle East, and central Asia. But don't despair: The next total lunar eclipse visible everywhere in North America occurs on January 21, 2019.
What to Look For
This eclipse promises to be a grand one, since it happens one day after the Moon passes its perigee, or the point in its orbit closest to Earth. The Moon will appear about 13% larger in diameter (what has come to be called a "supermoon") than it does when it's at apogee (farthest from Earth). That might not be enough for anyone but dedicated Moon watchers to notice. Still, given the Moon's above-average size and the fact that it's the second full Moon in January (a "blue Moon"), this eclipse is going to get a lot of publicity.
A lunar eclipse happens when the Sun, Earth, and a full Moon form a near-perfect lineup in space. The Moon gradually glides into Earth's shadow, until the entire lunar disk turns from white to an eerie dim orange or red. The total phase of the eclipse will last 1 hour 16 minutes, with its midpoint at 5:30 a.m. PST (13:30 Universal Time). Then events undo themselves in reverse order, until the Moon returns to full brilliance. The whole process for this event will take more than 4 hours.
You only need your eyes to see the drama unfold, but if you have binoculars or a backyard telescope, they'll give a much-enhanced view.
The events that happen to a shadowed Moon are more complex and interesting than many people realize. A total lunar eclipse has five stages, with different things to watch for at each.
(1) The penumbral stage begins when the Moon's leading edge enters the pale outer fringe of Earth's shadow: the penumbra. But the shading is so weak that you won't see anything of it until the Moon is about halfway across the penumbra. Watch for a slight darkening on the Moon's lower-left side as seen from North America.
The penumbra is the region where an astronaut standing on the Moon would see Earth covering only part of the Sun's face. The penumbral shading becomes stronger as the Moon moves deeper in.
(2) The second stage is partial eclipse. This begins much more dramatically when the Moon's leading edge enters the umbra, the cone of Earth's shadow within which the Sun is completely hidden. With a telescope, you can watch the edge of the umbra slowly engulfing one lunar feature after another, as the entire sky begins to grow darker.
The partial phase lasts just over an hour. As its end approaches, only a final bright sliver remains outside the umbra. By this time, the rest of the lunar disk should already be showing a dim, foreboding reddish glow.
(3) The third stage is total eclipse, beginning when the last rim of Moon slips into the umbra. But the Moon won't black out completely: it's sure to glow some shade of intense orange or red.
"That red light you see is sunlight that has skimmed and bent through Earth's atmosphere and continued on through space to the Moon," says Alan MacRobert of Sky & Telescope. "In other words, it's from all the sunrises and sunsets that ring the world at the moment.
"Picture it from the point of view of an astronaut standing on the Moon," says MacRobert. "They would see the dark Earth in the sky thinly ringed with brilliant orange from the Sun hidden behind it. The ring is bright enough to illuminate the ground at the astronaut's feet an eerie red."
The red umbral glow can be quite different from one eclipse to the next. Two main factors affect its brightness and color. The first is simply how deeply the Moon goes into the umbra as it passes through; the center of the umbra is much darker than its edges.
The other factor is the state of Earth's atmosphere along the sunrise-sunset line. If the air is very clear, the eclipse is bright. But if a major volcanic eruption has recently polluted the stratosphere with thin global haze, a lunar eclipse will be dark red, ashen brown, or occasionally almost black.
In addition, blue light is refracted through Earth's clear, ozone-rich upper atmosphere above the thicker layers that produce the red sunrise-sunset colors. This ozone-blue light tints the Moon also, especially near the umbra's edge. The result can be a subtle mix of changing blue, gray, and even green.
(4) As the Moon continues moving along its orbit, events replay in reverse order. The Moon's edge re-emerges into sunlight, ending totality and beginning a partial eclipse again.
(5) When all of the Moon escapes the umbra, only the last, penumbral shading is left. By about 30 or 40 minutes later, nothing unusual remains.
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