This is your last chance to see a total lunar eclipse until 2021.
This weekend, a total lunar eclipse will give skywatchers a chance to see the moon turn a dull red as it slips into Earth's shadow.
The eclipse, which will occur overnight on Jan. 20-21, will be the first lunar eclipse of the year and the last total lunar eclipse until 2021. It will be visible in North and South America as well as parts of western Europe and Africa.
January's full moon is sometimes dubbed a "wolf moon" in the folklore tradition because it occurs at a time of year when hungry wolves howled outside villages. And since the moon will be at its closest point to Earth in its elliptical orbit around our planet this weekend, it will be a "supermoon." As a result of these special cases — and because lunar eclipses are also known as "blood moons" — some are calling this eclipse a "super blood wolf moon."
During a lunar eclipse, sunlight falling on the surface of the moon is blocked by Earth as it passes between the sun and the moon. As the shadow starts to fall on the lunar surface, it looks as if a bite has been taken out of the moon — a phase known as a partial eclipse.
This weekend, the partial eclipse will begin at 10:33 p.m. ET. Totality, when the moon reddens as it slips completely within Earth's shadow, will follow at 11:41 p.m. ET.
Here are five things to know about the eclipse.
1. Though this will be a supermoon lunar eclipse, you probably won't be able to tell. Since a supermoon appears only a tiny bit bigger in the night sky than a moon at other points in its orbit, this eclipse won't appear appreciably different than others.
"If you take a baseball and tennis ball, set them side by side and look at them from a distance of about 20 feet, they'll look pretty similar," said Patrick Hartigan, an astrophysicist at Rice University in Houston. "That's about the difference between the largest possible supermoon and the smallest possible moon."
"If you have binoculars or a small telescope, you might get a better view, but you don't really need any of those things," Hartigan said, adding that this is one skywatching experience that's great for children.
"I would really encourage it," he said. "This is a fun natural phenomenon and a good way to illustrate geometry and motions in the sky, so it's a lovely educational experience for kids."
3. If you were on the moon during the eclipse, the view would be out of this world. Lunar eclipses are dramatic events when viewed from here on Earth. Viewed from the moon, they're even more spectacular.
If you were standing on the moon as Earth began to block the sun's light, darkness would fall around you. But if you looked up, you'd see a ring of light in the darkened sky, as sunlight illuminated the rim of atmosphere surrounding Earth's disk, according to the Lunar and Planetary Institute. In essence, you'd be seeing a solar eclipse.
4. If you miss this one, you won't be able to see another total lunar eclipse until 2021. The next total lunar eclipse visible in the U.S. will occur on May 26, 2021. Another total lunar eclipse visible in the U.S. will occur on May 16, 2022.
Like this weekend's eclipse, both of these eclipses will feature supermoons — though totality for both won't arrive until the early morning hours. "If you miss both of these, and you want a supermoon, and you want it to be in the early evening, you're going to have to wait until 2050," Hartigan said.
5. Like all solar and lunar eclipses, this one acts a bit like a timemachine. Lunar and solar eclipses occur in cycles, and each eclipse differs subtly in the way Earth, the sun and the moon are aligned. This means each eclipse is essentially identical to others that have occurred previously.
Observing this weekend's lunar eclipse is similar to observing "one that was happening long ago in historical times," Hartigan said.
Hartigan said this weekend's total lunar eclipse is part of a cycle of lunar eclipses that began on Oct. 25, 1874. The last identical one occurred on Jan. 9, 2001, and the next identical one will occur Jan. 31, 2037. Identical eclipses will occur every 18 years until July 26, 2325, when the cycle ends.
"These eclipses go back long before the dawn of the written word, and they'll go all the way into the future," Hartigan said. "Who knows what this world will be like in 2325? Maybe they'll think back to us in 2019 — if there are even still humans around."
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