What Is a Blood Supermoon Lunar Eclipse and When Can I See It?

Later this month, a "blood moon" lunar eclipse will be visible in the night sky during the next full moon.

Full moons are a lunar phase that occurs roughly once every month when the moon is located opposite the sun in space, with the Earth in between.

During a full moon, the side of the moon that faces toward our planet is fully illuminated, appearing like a perfect circle.

Technically, the moon only turns full for a moment, which in May will occur on the night of the 15-16. According to the Old Farmer's Almanac, the moon will reach peak illumination at 12:15 a.m, ET on May 16, or 9:15 p.m. PT on May 15.

What is a blood moon?

"Blood moon" is a non-scientific term that is often used to refer to the moon during a total lunar eclipse. This is because the moon often takes on a reddish color during these events.

Total lunar eclipses occur when the moon is on the opposite side of the Earth from the sun, and the Earth's shadow falls on the moon.

"This only happens when the moon is at a full moon phase and the sun, Earth and moon are perfectly aligned," Tania de Sales Marques, a planetarium astronomer from the U.K.'s Royal Observatory, Greenwich, told Newsweek.

"We don't see a total lunar eclipse every month even though a full moon happens every month, and that's because the moon orbits the Earth on a tilted orbital plane, hence it misses the Earth's shadow most of the time," she said.

Total lunar eclipses progress in three stages—a penumbral eclipse where the moon enters the lighter shadow of the Earth (penumbra) and only a very light dimming is noticeable; a partial eclipse when the moon starts to enter the darker shadow of the Earth (umbra) and it dims noticeably; and finally the total eclipse phase, where the whole moon is in the Earth's umbra and it turns red.

A total lunar eclipse
The moon during a total lunar eclipse on May 26, 2021, as seen from Inner Mongolia, China. Photo by TPG/Getty Images

"After the total eclipse stage, the moon goes through the partial and penumbral eclipse stages again until it looks like the typical full moon in the sky," Marques said.

When the moon passes through the Earth's umbra, direct sunlight is blocked from illuminating its surface. The reason the moon turns red during these moments is that the only light that reaches the moon is sunlight that has scattered as it passed through the Earth's atmosphere.

"As light passes through the Earth's atmosphere, the blue wavelengths get more scattered than red wavelengths, so the only light reaching the moon is red light," Marques said. We see the same effect occur when the sky reddens at sunrise and sunset," Marques said.

The total phase of the eclipse will be visible across most of the Americas—except for the northwesternmost part of the continent—the western edge of Europe, western parts of Africa, and the Antarctic.

For those located in the Eastern time zone of the U.S., the penumbral stage of the eclipse will begin at 9:32 p.m. ET on May 15 and the whole event will be over by 2:50 a.m. ET on May 16, according to EarthSky. The total phase of the eclipse begins at 11:29 p.m. ET on May 15 and ends at 12:53 a.m. ET on May 16. The maximum eclipse will occur at 12:12 a.m. ET.

"As with any other sky-related event, an unobstructed view of the sky is preferable" to view the eclipse," Marques said, although no special equipment is required to see it. "Like with any other stargazing event, weather plays a crucial role in what we might be able to see. Let's hope for the best!"

What is a supermoon?

May's full moon will be very close to the perigee—the point in its orbit when it is closest to Earth. The distance between the moon and the Earth varies because the moon has an elliptical orbit around the Earth. In addition, the Earth is not right at the center of the moon's orbit, according to Marques.

"Supermoon" is another non-scientific term that is popularly used to describe any full moon that coincides with the moon reaching perigee. While there are several ways to define a supermoon, perhaps the most common definition refers to any full moon that occurs when the moon is at least 90 percent of perigee, according to the Old Farmer's Almanac.

If this definition is used, then the upcoming May full moon qualifies as a supermoon—the first of the year, in fact.