Everyone with clear skies across the Americas will have a front-row seat Sunday night to a rare total eclipse of the super-harvest moon.
On the evening of September 27, three separate lunar events converge. The total eclipse coincides with the full moon nearest the fall equinox, known as the harvest moon. What's more, the moon is at its closest approach to Earth for the year, making it also a supermoon or perigee moon. That’s why it's being coined by some as a Super Harvest Blood Moon—a mouthful to be sure.
This confluence has happened only five times since 1900. According to NASA, the last time we saw this celestial triple combination was in 1982, and it won’t repeat until 2033.
The most spectacular part of the eclipse will be the totality phase, when Earth's shadow completely covers the moon and turns it an eerie red. The moon will dip into the deepest and darkest part of Earth’s shadow, or umbra, during the totality phase, which lasts as long as 72 minutes.
This weekend's blood moon will be the last in a series of four lunar eclipses, dubbed a tetrad, over the last two years. That pattern won’t repeat for another 20 years or so.
What Happens During an Eclipse?
In a lunar eclipse, Earth casts a shadow on the moon. This doesn't happen every time the moon makes its monthly trek around Earth, though; because the moon's orbit is tilted, it usually falls above or below Earth's shadow.
Total lunar eclipses, known popularly these days as blood moons, are even more rare. They happen only during a full moon, and only when the sun, Earth, and moon are precisely aligned so that our planet's shadow completely blankets the moon’s disk. This usually happens only twice a year, and can be seen from only one hemisphere of the Earth.
For thousands of years, eclipses of Earth’s lone natural satellite have garnered awe and fear. Now that science has explained the celestial mechanics at play, we can all simply enjoy the cosmic mechanics. (See lunar eclipse myths from around the world.)
This week's eclipse is even more special because the lunar disk will appear slightly larger than usual. The moon will be at perigee—its closest point to Earth—just 59 minutes before the height of the eclipse. This will make the lunar disk appear 13 percent larger than average.
What Makes the Moon Turn Red?
During the total eclipse, sunlight shining through the ring of Earth's dusty atmosphere is bent, or refracted, toward the red part of the spectrum and cast onto the moon's surface.
As a result, expect to see the lunar disk go from a dark gray color during the partial phase of the eclipse to a reddish-orange color during totality. The moon's color during totality can vary considerably depending on the amount of dust in the Earth's atmosphere at the time. Active volcanoes spewing tons of ash into the upper atmosphere, for instance, can trigger blood-red eclipses.
No one can predict exactly what color we'll see before each eclipse.
Can I See the Eclipse?
Skywatchers across eastern North America will get to see all the phases of this special sky show as the moon rides high in the eastern sky, while observers in the far western parts of the continent will see the moon begin to be gobbled up by Earth’s shadow as it rises in the east, just after local sunset.
Meanwhile, eclipse watchers in South America will see the show later in the night local time, and skygazers in Europe and most of Africa can watch during early morning hours local time on Monday the 28th.
Unfortunately, folks in Asia and Pacific Ocean will be on the wrong side of the planet when the eclipse is under way.
Unlike solar eclipses, lunar ones are safe to watch with the naked eye. (Related: "How to Safely Watch a Solar Eclipse.")
What If I Miss It?
If you miss this one, you're in for a bit of a dry spell. The next total eclipse will appear on January 31, 2018, and will be visible from western North America and the entire eastern hemisphere.
If you get clouded out this week, you can still check out the eclipse live online via science outreach venture Slooh. Their broadcast begins at 8 p.m. EDT (5 p.m. PDT) right here:
What Time Do I Watch?
The first part of the eclipse will be the partial phase, when the moon enters Earth's dark shadow (umbra) beginning at 9:07 p.m. EDT or 01:07 GMT. From that point, the dark umbral shadow will spread across the moon’s disk from left to right.
At 10:11 p.m. EDT, totality begins—when the moon is fully engulfed in the umbral shadow and turns a shade of orange-red. Totality will last as long as one hour and 12 minutes, with the rest of the visible eclipse ending at 12:27 a.m. EDT.