This coming Sunday night, as January 20 becomes the early morning of the 21, the moon will become completely engulfed in Earth’s shadow. In North America, we are well positioned to view the total lunar eclipse in its entirety, as are the people in South America, Europe, and Africa.
What is a lunar eclipse?
Lunar eclipses occur when the moon is full and in alignment with both the Earth and sun. Pretend you’re back in grade school science class and picture the sun as a basketball, the Earth next as a baseball, and the moon on the right side of Earth as a Ping-Pong ball. A total lunar eclipse happens when the entire moon falls into the shadow that the Earth casts while it's backlit by the sun. Like so:
How long do they last?
This time around, the partial, or penumbral, phase begins at 9:36 p.m. Eastern time on January 20. The full eclipse will begin at 11:41 p.m. and last for just over an hour, until 12:43 a.m. This will be the best viewing time, when the moon is in the umbra, or the full shadow the Earth.
The last total lunar eclipse, , was the longest of this century, clocking in at one hour, 42 minutes, and 57 seconds. But this year’s is special for another reason: The moon will be near to the closest point in its orbit to Earth, which will make it bigger and brighter—also called a supermoon. Of the 12 or 13 full moons each year, only three or four of them bulk up this way. The next total lunar eclipse .
Why isn't there an eclipse every night?
The reason every full moon isn’t an eclipse (and why there's not a lunar eclipse every night) is because the moon’s orbit is tilted about five degrees off Earth’s orbit. More often than not, Earth’s shadow misses the moon altogether. During a partial eclipse, the Earth, sun, and moon are aligned such that the Earth's shadow lands on part of the moon, but doesn't cover the whole thing.
In fact, we're lucky we get to see eclipses at all. It's a coincidence that Earth and the moon are just the right size and distance from each other to make it happen.
Ever since the moon formed about 4.5 billion years ago, it has been moving away from Earth at a rate of about . We are fortunate to be living at a time when the moon is at just the right distance for the Earth’s shadow to cover it completely, but not too much. In another few billion years, the arrangement of celestial bodies will render total eclipses a thing of the past.
Why does the moon turn red?
Don’t be surprised if the moon looks like a rusty old penny in the sky when the whole thing is going down. During a total eclipse, the only light reflected back to us from the moon has been refracted, or bent, around the curvature of the Earth and through the atmosphere. The green-to-violet portion of the light spectrum gets filtered out, while the red, orange, and yellow wavelengths remain. This is the same phenomenon that reddens the sun as it approaches the horizon at sunset. Take away Earth’s atmosphere and the moon would go black.
Is that why it's called a "blood moon"?
Yes. That name comes from astrology, as do the other nicknames for the various full moons of the year. The January 2019 eclipse is being called "super wolf blood moon," because the full moon in January is called the "wolf moon" and the moon will be close to Earth and appear large in the sky (thus, a "supermoon").
A total lunar eclipse is a good excuse to stay up late, so dig out the binoculars, blankets, and hot drinks. And while you wait for the penumbral phase to shift into the umbral phase, see who can say "perigee syzygy" five times fast.