Western North America will have a front-row seat on Saturday as the full moon gets painted red in the briefest eclipse this century.
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 only skirt the deepest and darkest part of Earth’s shadow, or umbra, and totality could last anywhere from nine to 12 minutes.
This weekend's blood moon will be the third of four lunar eclipses, dubbed a tetrad, over the course of two years. The pattern won’t repeat for another 20 years or so. The first and second happened in April and September 2014, and the last of the tetrad will grace our skies on September 28, 2015.
During a lunar eclipse, Earth comes between the moon and the sun, casting a shadow. This lineup doesn't happen every time the moon makes its monthly trek around Earth, though, because the orbit is tilted and usually keeps the moon out of Earth's shadow.
Total lunar eclipses, known 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 ballet.
What makes the moon turn red?
During the total phase of the 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?
Western North America will have a good view of this special sky show, while observers in the Plains and on the East Coast will have to be content with seeing only the first half as the moon begins to be gobbled up by Earth’s shadow.
For early risers in the eastern half of North America, the full moon will be sinking below the western horizon in a quickly brightening dawn sky just as the total eclipse is getting under way. This means New England will unfortunately miss this one entirely.
From western South America only the early stages of the eclipse—as Earth’s shadow begins to cover the moon—will be visible.
Meanwhile, eclipse watchers in Asia, India, western China, Russia, Australia, and the Pacific region will get a chance to see the second half of the show. The partial phase of the eclipse unfolds to the east just after local sunset.
Unfortunately, folks in Africa, Europe, and the Middle East 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, the next total eclipse will appear on September 28, 2015, and will be visible from the Americas, Europe, and parts of Africa.
And if clouds ruin your view this week, you can still check out the eclipse online via live webcasts in celebrations of Global Astronomy Month by science outreach ventures Virtual Telescope Project and Slooh.
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 3:15 a.m. PDT. From that point, the dark umbral shadow will spread across the moon’s disk from left to right.
At 4:54 a.m. PDT, totality begins—that's when the moon is fully engulfed in the umbral shadow and turns a shade of orange-red. Totality will last as long as 12 minutes, with the rest of the eclipse ending at 6:45 a.m. PDT.