Lunar eclipses explained

Find out why blood moons happen, and how people have interpreted lunar eclipses through the ages.

Published 21 Jan 2019, 14:32 GMT

Every so often, the silvery orb of the full moon undergoes a dramatic transformation. Darkness slips across the lunar face, and for a while, the entire moon may be coloured a deep blood red.

This is what's known as a total lunar eclipse, when the moon, the sun, and Earth line up in just the right way for the moon to be engulfed in Earth's shadow. This cosmic phenomenon has delighted or terrified humanity throughout recorded history, and astronomers have learned how to track celestial motions to predict when a lunar eclipse will occur years to centuries in advance.

Under a blood moon

Lunar eclipses happen when the moon moves behind Earth, from the sun's perspective. When the moon is completely swallowed up by the darkest part of the planet's shadow, we see a total lunar eclipse. Its peak, called totality, can last for up to an hour and 47 minutes, while the full eclipse can last about six hours from start to finish as the moon slips behind Earth and then emerges on the other side.

During totality, the lunar orb changes colour and becomes yellow, ruddy orange, or even a deep crimson, which is why a total lunar eclipse is sometimes called a blood moon. This ominous-looking effect appears because the moon does not generate its own light, and what we normally see as moonlight is really reflected light from the sun.

While most of that sunlight is blocked during a total lunar eclipse, some of it gets bent and scattered as it filters through Earth's atmosphere, so that only longer, redder wavelengths make it through. The exact color we see on the moon depends on how much dust is in Earth's atmosphere, since dust can absorb more of the filtered light and thus deepen the eclipsed moon's hue.

Different eclipse types

Total lunar eclipses can only happen when the moon is full, which means they can coincide with other full-moon phenomena, such as supermoons, blue moons, and harvest moons. But while we typically see one full moon a month, not every month delivers a lunar eclipse. That's because the moon's orbit around Earth is slightly tilted, and it only passes through the planet's cone-shaped shadow at specific times.

That tilt is why each eclipse can be seen at different times and from different places on Earth, although astronomers have noticed that lunar eclipses follow a long-term pattern called the Saros cycle that causes eclipses to share similar attributes through time.

What's more, not every lunar eclipse completely covers the moon's disc. During a partial lunar eclipse, only part of the moon passes through the darkest part of Earth's shadow, called the umbra, and the rest of the moon stays illuminated by the sun. And during a penumbral eclipse, all or part of the moon passes through the brighter part of Earth's shadow, and we see very little effect here on Earth.

Lunar eclipse myths

Today, lunar eclipses are cause for celebration, and many people host eclipse- viewing parties and even travel great distances to see one in person. But past eclipses have sparked a number of myths and legends linked to more colorful interpretations for the moon's vanishing act.

The Inca, for instance, told stories of a jaguar attacking the moon and turning it bloody, while the Mesopotamians believed the moon was being assaulted by demons. And the Batammaliba people in Togo and Benin interpret a lunar eclipse as the sun and moon working through a feud.

Thanks to our understanding of the ongoing dance of orbital dynamics, astronomers are able to calculate when eclipses must have happened in the past. By checking those dates against written accounts, scientists can determine when a total lunar eclipse must have happened during a moment of historical significance.

Enjoy total lunar eclipses while they last: Measurements of the distance between Earth and the moon show that our lunar companion is spiraling away from us at about 3.8 centimeters a year. In a billion years or so, the moon will be too far away to be fully enveloped by Earth's shadow, and blood moons will become the stuff of legend.

NASA: Lunar Eclipse Page
NASA: Periodicity of Lunar Eclipses
NASA: Lunar Eclipses of Historical Interest
NASA: Measuring the Moon's Distance
University of Rochester Department of Physics and Astronomy
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