A special celestial event is on the calendar for this Sunday night. North and South America will get the best view of the super “blood moon”, as it’s known, but Europeans and Africans will also be able to watch (weather permitting). So, let’s break down the hype, starting with the eclipse.
Unlike a solar eclipse, when the moon gets between Earth and the sun, a total lunar eclipse occurs when Earth aligns to block the sun’s light from the moon. That can only happen when the moon is on the opposite side of Earth from the sun. About once a month, a full moon is visible when it nears that far point and shines brightly as Earth covers up most of the sun. But approximately once a year, as the moon travels along its tilted axis, it ends up directly behind Earth and is thrust into near darkness.
- At 9:30 p.m. ET on Sunday, the moon will start to creep into the part of Earth’s shadow known as the penumbra. Not much darkening will be visible yet, according to NASA.
- By 10:33 p.m. ET, you should see Earth’s shadow start to move across the surface of the moon, growing larger and larger and larger until it completely covers it up.
- 11:41 p.m. ET will mark the totality of the eclipse, as the moon is fully shaded by Earth. That’s where the “blood” comes in. There’s no violence involved, instead, the term comes from a reddening of moon, as light leaks around the edge of Earth.
The moon will stay bathed in our colorful shadow for over an hour before orbiting back out into the sunlight.
As NASA explains, Earth casts a red shadow because of how our atmosphere scatters light. During our days, sunlight coming in from up high is scattered. Short blue light waves are bounced around, leaving us with blue skies. But when the sun comes in at an angle – at sunset, sunrise or from the perspective of the moon on the other side of Earth – the sun’s reddish light becomes dominant. That light can also change based on dust, pollutants or other particles in Earth’s atmosphere.
So what about the “super” part of Sunday’s show in the sky? The moon’s orbit around Earth is not perfectly round, so as it circles Earth it is sometimes closer and sometimes farther away, by a distance of about 26,000 miles. That mean that when the moon is at its closest point to Earth – known as the perigee – it can appear up to 17 percent larger than it does at the farthest point in its orbit. Sunday’s eclipse will almost coincide with that perigee, meaning that the moon won’t just be redder than usual, it will also look a tiny bit wider.
Why It’s Important
Lunar eclipses have long played an important role in understanding Earth and its motions in space.
In ancient Greece, Aristotle noted that the shadows on the Moon during lunar eclipses were round, regardless of where an observer saw them. He realized that only if Earth were a spheroid would its shadows be round – a revelation that he and others had many centuries before the first ships sailed around the world.
Earth wobbles on its axis like a spinning top that’s about to fall over, a phenomenon called precession. Earth completes one wobble, or precession cycle, over the course of 26,000 years. Greek astronomer Hipparchus made this discovery by comparing the position of stars relative to the Sun during a lunar eclipse to those recorded hundreds of years earlier. A lunar eclipse allowed him to see the stars and know exactly where the Sun was for comparison – directly opposite the Moon. If Earth didn’t wobble, the stars would appear to be in the same place they were hundreds of years earlier. When Hipparchus saw that the stars’ positions had indeed moved, he knew that Earth must wobble on its axis!
Lunar eclipses are also used for modern-day science investigations. Astronomers have used ancient eclipse records and compared them with computer simulations. These comparisons helped scientists determine the rate at which Earth’s rotation is slowing.
Image: A so-called “blood moon” seen during a total lunar eclipse in western Germany, on Sept. 28, 2015.