The moon exits the Earth's umbral shadow after a full lunar eclipse in 2015. This month's partial lunar eclipse will be the longest such event in more than 500 years. Credit: Troy R. Bennett / BDN

PORTLAND, Maine — The longest partial lunar eclipse in 580 years coming up this week. It will last 6 hours. The last time humans watched Earth’s closest celestial neighbor go nearly dark that long was in the year 1440.

Back then, the Incas were still building Machu Picchu and Christopher Columbus was more than 50 years from making it to the Americas.

The good news is that Mainers will be able to see almost the entire celestial show. The bad news is that if it’s cloudy, they won’t see anything.

Simple bad weather can ruin any astronomical viewing event and there’s nothing to be done about it.

Regardless of weather, the eclipse will happen late Thursday into early Friday morning.

“If it’s clear, it’ll be great fun,” said John Meader, owner and educator at North Stars Planetarium in Fairfield. “But, statistically speaking, Maine is more often cloudy than not.”

Meader, an avid though realistic night sky watcher, said our skies are at least partially overcast 60 percent of the time.

Still, he’s hopeful.

Lunar eclipses occur when the moon crosses into Earth’s shadow. That is, when our planet is between the moon and sun. The darkness seen creeping across the moon’s surface during a lunar eclipse is Earth’s own shade.

Solar eclipses happen the opposite way, when the moon gets between Earth and the sun.

For this event, Earth’s outer shadow, called the penumbra, will begin dimming the moon at 1 a.m. Friday. At that stage, changes will be subtle and you may think nothing is happening.

“The penumbra is not the exciting part of an eclipse,” Meader said.

The real fun starts closer to 2:20 a.m. when the umbra, or visible shadow, starts inching across the illuminated lunar disk. When that happens, the moon will likely take on a dramatic, red tone.

The umbra will persist for more than 3 hours.

“I’m telling people to get up at 3:45 a.m. if they want to see the best part,” Meader said.

The eclipse will not be total — but close to it. At its height, around 4 a.m., Earth’s shadow will blot out 97 percent of the moon’s reflected light.

The whole eclipse event will run a full 6 hours, until 7 a.m. By that time, the sun will be up, so Mainers will miss the tail end of the celestial show.

The easiest way to see this particularly elongated eclipse is to just go outside and cast your gaze skyward. Unlike solar eclipses, you can look right at it with no fear of hurting yourself.

“All you need is your two eyes,” Meader said. “If you wear glasses, use them. You’ll enjoy the eclipse even more if you look through a pair of binoculars. You’ll catch more details.”

Lunar eclipses — full and partial — typically last a couple of hours, start to finish. This one is taking extra long because the moon will be nearly at apogee, or as far as it gets from Earth in its orbit. When it’s out that far, it slows down a bit. Thus it will take longer than normal to get through our shadow.

Though this is the longest partial eclipse in many centuries, it is not the longest eclipse overall during that time. Total lunar eclipses — some in just the past few years — have gone longer.

In 2018, for example, a total lunar eclipse lasted for 6 hours and 23 minutes. However, it was not visible in Maine. The next six-hour lunar eclipse you’ll be able to see here will be on March 14, 2025.

It will be total.

There will not be another partial eclipse nearly as long as this month’s, visible over Maine, in any of our lifetimes.

“The next really big event here will be the total eclipse of the sun in 2024,” Meader said.

That dramatic event is due to cut a dark, 50-mile wide swath right through western Maine. For more than 3 minutes, the daytime sky will go dark. Stars will appear, as if it were night.

If it’s not cloudy, of course.

Correction: A previous version of this morning misstated the projected size of the eclipse.

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Troy R. Bennett

Troy R. Bennett is a Buxton native and longtime Portland resident whose photojournalism has appeared in media outlets all over the world.