On July 27, the longest total lunar eclipse of the 21st century will take place. It will be completely visible in East Africa, central Asia, and India, and it will be at least partly visible from New Zealand and South America.
During lunar eclipses, the Moon moves into the shadow that the Earth casts into space. This shadow leads to two possible types of eclipse – partial if the Moon is crossing the penumbra (Latin for half-shadow), or total when the Moon is in the umbra (the full shadow). The umbra is big enough for the Moon to cross it without going to its center. But this month there will be a central lunar eclipse and that’s why it'll be quite long.
What's more, there's another factor that plays a role in this long eclipse. The Moon is at its apogee, the furthest point on its monthly orbit that our natural satellite is from the Earth. At that distance, the Earth casts a wider shadow, so all these factors combined create a record-breaking lunar eclipse.
The Moon will be eclipsed for 1 hour and 43 minutes, the longest time since the year 2000. That eclipse was 1 hour and 46.4 minutes, only a handful seconds shorter than 1 hour and 47 minutes, the maximum possible lunar eclipse length. The two eclipses belong to the same Saros cycle, meaning they have the same geometry. These eclipses repeat every 18 years.
The eclipse will peak at 8.22pm UTC (4.22pm EDT). Billions of people will witness the Moon getting progressively darker before taking the characteristic crimson color of a total lunar eclipse. The Moon turns red because the only light reaching it and reflecting back on its surface passes through the Earth’s atmosphere, so it takes the typical hue of sunsets and sunrises.
An often-asked question when it comes to eclipses is, why don’t they happen every month? The answer is not complicated. The Earth, Sun, and Moon do not all orbit the same plane, so they are only aligned once in a while. The Moon’s orbital plane is actually tilted about 5 degrees with respect to the Earth-Sun plane.