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Lunar eclipse: The science behind the blood moon and the reddish glow

The total Lunar eclipse on Friday will last about an hour and 43 minutes with the moon in perfect alignment with the sun and Earth.

science Updated: Jul 24, 2018 16:49 IST
Tejas Narayan
Tejas Narayan
Hindustan Times, New Delhi
Blood moon,Lunar eclipse,Eclipse
In this file photo taken on January 31, 2018 the ‘super blue blood moon’ is seen over Los Angeles, California. (AFP File Photo)

Stargazers in most parts of the world will get to see the longest total lunar eclipse of the century on Friday. And this one is extra special because it’s also the ‘Blood Moon’ when the moon will get a reddish glow

The total eclipse will last about an hour and 43 minutes with the moon in perfect alignment with the sun and Earth. But while the internet is buzzing with excitement over the one-of-a-kind total lunar eclipse on Friday, the science behind eclipses is regretfully neglected, reducing the phenomenon to a one-day cosmic wonder.

Few people ask why a total eclipse results in a red moon. Or why is a shadow red instead of dark and why lunar eclipses always coincide with the full moon.

Lunar eclipses occur when the earth is situated directly between the sun and moon, blocking sunlight from reaching the moon’s surface and casting a shadow over it.

The moon orbits the earth about 12 times a year, but we don’t get eclipses every time it is the farthest from the sun because the orbital planes of the earth and moon differ slightly, intersecting at about a five-degree angles.

Although this difference appears negligible, it is more than enough for sunlight to bypass the earth completely and bounce off the moon, resulting in a fully illuminated moon surface, or a full moon.

However, at special points called lunar nodes, the orbit of the moon intersects the orbital plane of the earth. When a full moon happens near a lunar node, the earth blocks enough sunlight to form a shadow on the moon, resulting in an eclipse.

The closer the full moon is to the node, the more of the earth’s shadow will cover the moon and for a longer period. If the full moon occurs far enough from the node, we instead see partial or penumbral eclipses as the moon only passes through the edge of the earth’s shadow.

Since eclipses happen near lunar nodes, every lunar eclipse is accompanied by a solar eclipse two weeks before or after it. The upcoming eclipse’s twin, however, is not visible outside of the Arctic.

But if the moon is completely covered by the darkest part of the earth’s shadow (called the umbra) during a total lunar eclipse, why do we see more of the moon than we would if it were not illuminated at all? The reason we can see lunar eclipses is that small amounts of sunlight refract, or bend, around the earth through its atmosphere, just enough to land on the moon.

This light tinge is red because of an interesting property of light: colours with shorter wavelengths, such as blue, are refracted at sharper angles, while reds, which have longer wavelengths, do not diverge as far from their original path. These low-frequency waves refract just enough to pass the earth and land on the moon, which is why lunar eclipses appear red.

The higher frequency waves, which manifest as blues, greens and violets, deviate farther from their paths and land on the earth itself. We see this light as though it is coming from directly above us – if you have ever wondered why the sky is blue, it is this refracted light that we see that causes this phenomenon.

First Published: Jul 24, 2018 16:48 IST