An unusually brief total eclipse of the Moon will be visible before dawn this Saturday, April 4th, from western North America. The eclipse happens on Saturday evening for Australia and East Asia.
The total eclipse of the Moon on Saturday, April 4th, will be the third in the current "tetrad" of four in a row at half-year intervals.
As was the case last October 8th, this lunar eclipse favors westerners in the U.S. and Canada. And once again many will need to look low in the west as dawn brightens — lower, in fact, than last time.
But in another way this time will be different. This eclipse will be just barely total — in fact, you may get the impression that it never becomes quite total at all. The Moon's north-northeastern limb squeaks so slightly inside the umbra (dark inner portion) of Earth's shadow that it will remain much brighter than the deep red we can expect across the rest of the Moon's face.
So borderline is this eclipse that different authorities give durations of totality ranging from 5 to 12 minutes long — depending on the definition chosen for the exact edge of the umbra, which is actually somewhat fuzzy.
The map, diagram, and timetable here show what to expect at your location and when. Weather permitting, those near the West Coast will see the total phase of the eclipse with the Moon fairly high in a dark sky before dawn even begins. Skywatchers in the Plains states will find dawn brightening and the Moon sinking low in the west around totality. For easterners, the Moon sets (and the Sun rises) during the partial phase before totality begins. New England misses this one altogether.
Meanwhile, as seen from Hawai'i or New Zealand, the eclipse happens deep in the night and high in the sky. For Australia, Japan, China, and Southeast Asia, it comes on Saturday evening local time.
|Total Eclipse of the Moon, April 4, 2015|
|Eclipse event|| UT
|Penumbra first visible?||9:35||5:35 a.m.||4:35 a.m||3:35 a.m.||2:35 a.m.||11:35 p.m. (4/3)|
|Partial eclipse begins||10:15||6:15 a.m.||5:15 a.m.||4:15 a.m.||3:15 a.m.||12:15 a.m.|
|Total eclipse begins||11:54||7:54 a.m.||6:54 a.m.||5:54 a.m.||4:54 a.m.||1:54 a.m.|
|Mid-eclipse||12:00||—||7:00 a.m||6:00 a.m.||5:00 a.m.||2:00 a.m.|
|Total eclipse ends||12:06||—||7:06 a.m.||6:06 a.m.||5:06 a.m.||2:06 a.m.|
|Partial eclipse ends||13:45||—||—||7:45 a.m.||6:45 a.m.||3:45 a.m.|
|Penumbra last visible?||14:25||—||—||—||7:25 a.m.||4:25 a.m.|
Where to Watch the Eclipse Online
|Can't see the eclipse from where you are? Then check out one of these live webcasts:|
What to Watch for During April 4th's Lunar Eclipse
A total lunar eclipse has five stages, with different things to watch for at each. You only need your eyes to see this celestial drama unfold, though the view is certainly better through binoculars or a small backyard telescope.
The first 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 the penumbra until the Moon is about halfway across it. Watch for a slight darkening to become apparent on the Moon's celestial southeastern side (likely on the left or lower left as seen in the sky, depending on where you are). The penumbral shading becomes stronger as the Moon moves deeper in.
The second stage is the partial eclipse. This begins much more dramatically when the Moon's leading edge enters the umbra, Earth's inner shadow, where no direct sunlight reaches. With a telescope you can watch the edge of the umbra slowly engulf one lunar feature after another. Also note how the entire sky begins to grow darker, as what had been a full Moon gradually disappears.
On April 4th, it'll take a good hour and a half before only a final bright sliver remains outside the umbra. By this time, the rest will already be showing a foreboding reddish glow.
The third stage is total eclipse, beginning when the last rim of Moon slips into the umbra — depending on how the edge of the umbra is defined! The edge is not sharp, since Earth has a semitransparent atmosphere. In a grazing instance like this, so critical is the adopted definition that the U.S. Naval Observatory's Astronomical Almanac (which S&T uses) lists totality as lasting 12.3 minutes, whereas Fred Espenak's Fifty-Year Canon of Lunar Eclipses says 8.6 minutes. The French national almanac lists it as 4.7.
Most of the Moon is sure to glow some shade of intense orange or red. That red light shining onto the Moon is sunlight that has skimmed and bent through Earth's atmosphere: that is, from all the sunrises and sunsets that ring the world at any given moment.
Two factors affect a lunar eclipse's color and brightness. 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 second factor is the state of Earth's atmosphere along the sunrise-sunset line. If the air there is very clear, the eclipse is bright. But when a major volcanic eruption has recently polluted the stratosphere with thin haze worldwide, a lunar eclipse will be dark red, ashen gray, or occasionally almost black.
In addition, blue light refracted through Earth's clear, ozone-rich upper atmosphere can also add to the scene, especially near the umbra's edge, creating a subtle mix of changing colors. Time-lapse videos may show large "flying shadows" in the umbra, caused by changing cloud-shadowing effects around the sunrise-sunset line as Earth moves and turns.
And then, as the Moon continues eastward along its orbit, events replay in reverse order. The Moon's edge re-emerges into sunlight, ending totality and beginning stage four: a partial eclipse again. When all of the Moon escapes the umbra, only the last, penumbral shading is left for stage five. By about 45 minutes later, nothing unusual remains.
Here's Alex Mellinger's cool time-lapse video of the total lunar eclipse last October, tracked on the reference frame of Earth's shadow:
The Moon's face is rich with geologic wonder. To track down all the details with your telescope, you'll want to have Sky & Telescope's laminated Moon map at your side.