This Friday, we’ll experience the longest lunar eclipse of the 21st century, one that won’t be eclipsed (ha) until the 22nd century. But why will it be so long? Let’s find out.

On July 27, a total lunar eclipse will be visible for a whopping 1 hour and 43 minutes from most of Africa, the Middle East, India, Australia, and parts of Europe. It’ll peak at about 8.22pm UTC, but unfortunately won’t be viewable from the US. The next in the US will be on January 21, 2019.

A total lunar eclipse occurs when the Moon passes into the deepest part of Earth’s shadow, the umbra. Here, it's hit by sunlight that has been heavily scattered by Earth’s atmosphere towards the red end of the spectrum. As a result, the Moon turns a red color.

The length of the eclipse is dictated by where the Moon is in its orbit. It doesn’t always follow the same path through the umbra, so sometimes it’ll pass closer to the middle than others owing to its angle with Earth’s orbit, noted Live Science. In this case, it’ll go right through the middle. This follows a repeating 18-year cycle, known as a Saros cycle.

Another factor is how far the Moon is from Earth. The Moon’s orbit around our planet changes from its closest point, perigee, of about 363,000 kilometers (226,000 miles) to its furthest, apogee, which is about 406,000 kilometers (252,000 miles) away.

The further it is, the smaller it appears and the slower it is moving. And it just so happens that this total lunar eclipse will take place during lunar apogee, on the same date of July 27.

Then there’s also Earth’s position to consider, too. This lunar eclipse will occur just after Earth reaches its furthest point from the Sun, known as aphelion, in early July. This makes Earth’s umbral shadow both longer and wider, keeping the Moon inside the umbra for longer.

So these factors combined mean that the total lunar eclipse next week will be the longest since July 16, 2000, which lasted 1 hour, 46 minutes, and 24 seconds, just shy of the maximum possible length of 1 hour and 47 minutes. The July 27 eclipse won’t be beaten this century, with the next longest of 1 hour, 46 minutes, and 6 seconds occurring on June 9, 2123.

Now all you need is to hope for clear skies on the night of the eclipse. Fingers crossed.

(H/T: Live Science)