The first full Moon of 2019 meets Earth's shadow in a widely visible total eclipse on the evening of January 20–21. Here's a guide on what to expect.
A full 62 luxurious minutes of totality. That's what we can expect on the night of January 20–21 when the Full Wolf Moon does a slow dance through Earth's umbra (the innermost region of the shadow). The last total lunar eclipse over the Americas took place in the wee hours of January 31, 2018. This one will be more user-friendly for Western Hemisphere observers as it happens during evening hours.
If you're adept at comparing full Moon sizes, examine the Moon during the eclipse. Does it appear larger than normal? In fact it is! Perigee, when the Moon is closest to the Earth, occurs only about 14 hours after maximum eclipse. That makes this a supermoon, defined as a full Moon that comes within 90% of its closest approach to Earth. The average angular diameter of the full Moon is 31 arcminutes, but during the eclipse it will be 2.2 arcminutes wider. Let us know if you can see the difference.
There are so many ways to enjoy a lunar eclipse. With the naked eye you can watch the Moon's stately progress into and out of Earth's shadow and its amazing transformation in color and brightness. Binoculars make eclipse colors more vivid and give great 3D views of the totally eclipsed Moon suspended among the stars. A telescope will reveal subtle colors in Earth's shadow as well as "mini-eclipses" of craters and other lunar features as the encroaching shadow covers them one after another. I plan to cut back on photography to better experience the event unhurried in real time. Reality only happens once.
The fun starts early during the penumbral phase, when the Moon slips into Earth's penumbra, or outer shadow. Although invisible at first, you should start to notice the penumbral shading about 20–30 minutes in, when the translucent shadow tints the lower left (celestial east) edge of the Moon a subtle gray. Look for a blunted or "dirty" appearance.
The umbra comes next, announcing its presence as a dark and ever-growing bite into the lunar cheese wheel. Test your color perception by noting how many minutes into partial eclipse you first see color in Earth's shadow.
At some point during the partial phases, take your binoculars and examine the shadow's edge. Pretty fuzzy, isn't it? Earth's shadow has a soft edge for the same reason a tree casts a fuzzy shadow. Because the Sun is an extended disk rather than a point of light, light from one side of the disk spills into areas that the other side of the Sun can't reach and vice versa. This "spillage" softens and diffuses the shadow's edge. Only point sources like Venus can create sharp-edged shadows.
While you're watching, you'll also notice that the shadow is curved, a clue ancient observers used to deduce that Earth must be spherical. Aristotle gets a lot of bad-mouthing for his geocentric universe, but he slam dunked on the Earth's shape, writing in On the Heavens:
“The earth is spherical…in eclipses the outline is always curved: and, since it is the interposition of the earth that makes the eclipse, the form of this line will be caused by the form of the earth’s surface, which is therefore spherical.”
Two minutes before totality, only the Moon's edge still lingers in sunlight (and penumbral sunlight at that!) in striking contrast to the somber and beautiful reds and oranges that tint the rest of the lunar disk. If the Earth had no atmosphere the Moon would completely disappear within its shadow. Then we might only see it with averted vision under a rural sky as a black disk silhouetted against the feeble glow of the gegenschein.
We're rescued from this dire situation by sunlight that gets refracted around the circumference of the Earth and into the planet's shadow. Because the solar rays graze Earth's limb the same way they do at sunrise and sunset, the cooler colors are scattered away by the atmosphere, leaving the warmer ones to filter in and paint the Moon in gorgeous, warm colors, the intensity of which has much to do with the state of Earth's atmosphere. If relatively free of materials like volcanic aerosols, the eclipse will be bright, but if not, the Moon can sometimes grow so dark as to be difficult to find in a light-polluted sky.
Take a minute during totality and pretend you're an astronaut standing on Moon looking back at Earth. The first thing you'd notice is a big drop in temperature. NASA's Lunar Reconnaissance Orbiter measured declines on the order of 250°F during the June 2011 eclipse. Now, look around and you'll see the rocks glowing smoky orange, as the vibrant red edge of Earth's blackened disk slowly covers the Sun in a total solar eclipse.
Because the Moon crosses Earth's shadow well north of umbra center, the northern limb should be relatively bright throughout totality compared to the southern. Watch for subtle changes in illumination and hue as the Moon moves through the umbra, and by all means observe the event from a dark-sky site. That way you can fully appreciate the strange quietude that spreads across the land when moonlight is fully quenched, and the stars and Milky Way return in full force.
With the Moon's brilliance quenched, telescopic observers will have a chance to see the Moon occult several stars in the constellation Cancer. For more details, go to Eberhard Riedel's Grazing Occultation Maps site.
Fresh darkness will also reveal a fuzzy presence some 6° east of the ruddy Moon — the Beehive Cluster. Both will fit in wide-field binoculars. If you want to photograph the pair, use a 150-mm or shorter focal length, your widest lens setting, an ISO of 800-3200, and exposure of 15–30 seconds. For a complete guide to lunar eclipse photography, pay a visit to Fred Espenak's Mr. Eclipse site.
As totality ends, the lunar limb peeps back into filtered sunlight. The eclipse now runs in reverse as the Moon slowly extricates itself from Earth's shadow and stars go back into hiding.
From start to finish the whole affair will last more than 5 hours, though I doubt most of us will stay till the bitter end. One night long ago, I did. When I was finally ready for bed, I took one last look and saw a glaring, white Moon shining nearly overhead as if nothing had ever happened.