(Photo: AP)

Skywatchers around much of the world have begun watching a complete lunar eclipse that is the longest of this century.

The so-called “blood moon,” when it turns a deep red, is visible at different times in Australia, Africa, Asia, Europe and South America when the sun, Earth and moon line up perfectly, casting Earth’s shadow on the moon.

The total eclipse lasts 1 hour and 43 minutes, with the entire event lasting closer to four hours.

Across Africa people turned to the sky, watching the reddish shadow slide up the moon’s surface. In Somalia, some hurried to mosques for special prayers often observed during lunar eclipses. In South Sudan, some dared to take photos in a war-torn country where using a camera in public is discouraged.

In Ethiopia’s capital, Addis Ababa, people at an open-air restaurant admired a rare clear view during the rainy season, comparing a live NASA webcast to what they saw above. Then clouds rolled in.

“Dem yelebesech chereka,” some murmured — Amharic for “blood moon.”

“The reason that the moon turns red is because atmospheric scattering causes red light to pass through the atmosphere and the composition of the atmosphere can change if volcanic eruptions or forest fires occur,” said Tom Kerss, an astronomer with the Royal Observatory Greenwich.

“And the density of dust increasing in the atmosphere can cause the moon to appear a particularly deep red, and indeed it has the same effect on our sunsets and sunrises.”

In a special treat, Mars is also at its closest approach to Earth this week since 2003, making it appear bigger and brighter.

Excited skywatchers on social media shared photos of the bright planet just to the right of the moon.

North America missed out on Friday’s lunar eclipse but can look forward to the next one on Jan. 21, 2019, according to NASA.