Why Does the Moon Turn Red During a Total Lunar Eclipse?
Total lunar eclipse of July 16, 2000.
Credit: Fred Espenak/NASA's Goddard Space Flight Center

Tonight, an oversized ruby-colored sphere will rise in the sky as a total lunar eclipse turns the normally pallid moon scarlet.

Since the moon is in perigee, meaning our lonely satellite is at the place in its orbit where it is closest to Earth, tonight's full moon will appear larger than normal. The so-called supermoon is also at the perfect spot in its orbit so the alignment between the sun, Earth and moon will be perfect … for a total lunar eclipse: At about 8:11 p.m. EDT (0011 GMT), the moon will tiptoe into the outer portion of Earth's shadow, becoming totally bathed in the darkest part of that shadow at 10:47 p.m. EDT (0247 GMT), with the total lunar eclipse ending at about 1:22 a.m. EDT (0522 GMT).

But rather than "lights out," the moon will become engulfed in a light-orange to blood-red glow. [Rare 'Supermoon' Lunar Eclipse Tonight: Skywatching Tips]

Here's why: Picture yourself standing on the moon (lots of dust and craters at your feet), looking down on Earth during the spectacular night-sky event. When the Earth is directly in front of the sun — blocking the sun's rays from lighting up the moon — you'd see a fiery rim encircling the planet.

"The darkened terrestrial disk is ringed by every sunrise and every sunset in the world, all at once," according to NASA. Even though our planet is way bigger than the sun, our home star's light bends around the edge of Earth. This light gets reflected onto the moon.

But not before it travels through our atmosphere, which filters out the shorter-wavelength blue light, leaving the reds and orange unscathed to bathe the moon's surface. And voila, a red moon.

The moon will change various shades during different stages of tonight's supermoon lunar eclipse, going from an initial grayish to orange and amber. Atmospheric conditions can also affect the brightness of the colors. For instance, extra particles in the atmosphere, such as ash from a large wildfire or a recent volcanic eruption, may cause the moon to appear a darker shade of red, according to NASA.

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