The briefest eclipse of the century

The eclipse is to be witnessed worldwide today

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 the 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 could last anywhere from 9 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 the 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 rarer. They happen only during a full moon, and only when the sun, the earth, and the 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, towards 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 grey colour during the partial phase of the eclipse to a reddish-orange colour during totality. The moon's color during totality can vary considerably depending on the amount of dust in the earth's atmosphere at that time. Active volcanoes spewing tons of ash into the upper atmosphere, for instance, can trigger blood-red eclipses.