The longest partial lunar eclipse of the century will occur early Friday, when 97% of the moon will be covered by the darkest part of the Earth’s shadow, with just a small sliver visible at the climax of this rare celestial event.

       The eclipse will last three hours, 28 minutes and 23 seconds, and will be visible in the Chicago area from 1:18 a.m. to 4:47 a.m. Friday, assuming weather conditions are favorable with few clouds.

       For night owls and early risers interested in learning more about the moon, the Adler Planetarium will livestream portions of the eclipse on YouTube as part of the museum’s Sky Observers Hangout program. Viewers will also be able to ask astronomy experts questions during a live moderated chat.

       The moon is seen from Portage Park in Chicago on Jan. 31, 2018, just before a super blue blood moon eclipse. (Jose M. Osorio / Chicago Tribune)

       “It should be pretty striking,” said Michelle Nichols, the planetarium’s director of public observing. “It’s a reason to get out and see the sky — and connect to it. People have been observing lunar eclipses for as long as people have been looking up at the sky, and they have tried to ascribe meaning to them. It’s a good excuse to sit back and think about the millennia upon millennia that people have been looking at this same phenomenon, wondering what is that and what does it mean?”

       Here are seven things to know about Friday’s partial lunar eclipse.

       1.During a lunar eclipse, the Earth aligns exactly between the sun and a full moon, and the moon is covered by the Earth’s shadow.

       “Normally the sun shines on the Earth and the Earth casts a shadow onto space,” Nichols said. “Just like the sun shines on you and you cast a shadow behind you. And then if the moon’s orbit lines up with where the shadow is, when the moon goes into the shadow, that’s what we call a lunar eclipse.”

       The Earth’s shadow has a lighter outer part called the penumbra and a darker inner part called the umbra, she said. When the moon is covered completely by the darkest part of the shadow, it’s a total lunar eclipse. When only part of the moon is covered, it’s a partial eclipse.

       2. The partial eclipse will likely be viewable from your backyard. There’s no need to travel to view the moon.

       “There’s no place that’s particularly better than any other place,” Nichols said. “It truly will depend on the weather. If you have a clear sky or even clear-ish sky from where you are you can absolutely see this from home.”

       The only exception might be an area where the sky is covered with particularly large trees or buildings. She added that light pollution usually doesn’t impede viewing.

       “Because it’s the moon, it’s bright,” she said.

       3. Weather forecasters say cloud conditions seem favorable for local viewing. Meteorologists are calling for clear skies early Friday.

       While clouds are expected on Thursday, most of the cloud coverage is forecast to clear up into the evening through early Friday morning.

       “Our best guess is that we’re going to be clear,” said Matt Friedlein, meteorologist with the National Weather Service’s Chicago office. “The trend looks positive for us to lose the clouds.”

       But those viewing the eclipse outdoors should dress warmly: It will be quite cold, with temperatures in the mid-20s and wind chills in the teens, he said.

       “It’ll be some of the chillier temperatures we’ve seen so far this autumn,” Friedlein said.

       4. The eclipse can be seen by the naked eye. There’s no need for any technology or special devices to view the moon, Nichols said. Special safety glasses aren’t required, unlike a solar eclipse, which can be dangerous to view without eye protection.

       “That’s the great thing about lunar eclipses,” she said. “You don’t need special equipment to see them. You just need your eyes.”

       For those who want to use binoculars or a small telescope, Nichols recommends bringing the equipment outside about 20 to 30 minutes before the eclipse.

       “Because you’ll be taking them from inside a really warm environment outside to a very cold environment … and your equipment might fog up or freeze up,” she said. “So get your equipment acclimated to the outdoors.”

       5. In the Chicago area, the eclipse will occur overnight. At about 1:18 a.m., the moon will be about two-thirds of the way up in the sky and just to the southwest, when it will start to pass into the umbra, the darkest portion of the Earth’s shadow, Nichols said.

       “The shadow will appear to creep across the moon, but that’s actually the moon moving into the shadow itself,” she said.

       About 3:03 a.m., the moon will be “almost due west “and about a third of the way up in the sky, she said.

       “That’s when it’s most fully within the umbra,” she said. “So more than likely what it may look like is the rest of the moon is a dark gray, maybe a dark brick red — we don’t know what the color will be until we see it — but that little sliver that’s still in the penumbra will be a little lighter than the rest of the moon.”

       About 4:47 a.m., the moon will be a little more to the northwest, and that’s when it will leave the darkest part of the Earth’s shadow, Nichols said.

       6. The eclipse will be visible to much of the world. Depending on the weather, the eclipse is expected to be viewable from multiple continents, including North America, South America, Australia and parts of Europe and Asia.

       “Know that there are literally millions of people doing the exact same thing … at the exact same time as you are,” Nichols said. “And that is a connection that is rare.”

       7. The next partial eclipse of a longer duration is centuries away. A partial eclipse on Feb. 8, 2669, is predicted to take three hours, 30 minutes and two seconds.

       But other partial eclipses of shorter lengths — as well as total lunar eclipses — happen periodically. Two total lunar eclipses will occur next year: one on May 15 and another on Nov. 8.

       eleventis@chicagotribune.com

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