Why did people panic when the Moon changed color?

On July 27, some people get a rare celestial treat: the longest total lunar eclipse of the century. As the Earth snuggles into perfect alignment between the moon and sun, its shadow will completely cover the moon. Rather than appearing as a black spot in the heavens, however, our sole natural satellite will exude a blood-red hue.

Yes, it's the blood moon. And while skywatchers today will look up to the heavens in awe (if they're in the right spot), people long ago took a change in lunar hue as a sign of doom — a good reason to panic.

For instance, folklore suggests that Christopher Columbus knew a lunar eclipse would happen on Feb. 29, 1504, and used this to his advantage.

Columbus and his men had been trapped on an island, now known as Jamaica, for over six months. As time wore on, the initial generosity that the indigenous people, the Arawak, had shown faded. As famine loomed, Columbus turned to an almanac published by a German astronomer and mathematician, Johannes Müller von Königsberg, also known as Regiomontanus, only to find out that a total lunar eclipse was forthcoming, according to Space.com.

So, Columbus told the Arawak that his god was angry that they were withholding food from the invaders. Columbus' god, the European told the Arawak chief, would therefore make the moon disappear or become "inflamed with wrath" in three days. [10 Coolest Moon Discoveries]

When the blood moon indeed rose that eerie Sunday night, it reportedly terrified the Arawak. They agreed to provide Columbus and his men with anything they needed, just so long as he asked his god to bring back the regular moon.

Normally, sunlight hits the moon directly and that's why it's typically bright and white. But during an eclipse, the Earth moves directly between the sun and the moon and filters out some of that light, according to NASA. When the white sunlight hits the Earth's atmosphere, it scatters the blue light but pushes the red light through straight to the moon — and so, a blood moon rises.

The amount of light that gets through to the moon, and thus the shade of red or orange that the moon turns, depends on other factors like how much dust and water are in the Earth's atmosphere that day. It can even depend on temperature and humidity, according to NASA.

"There have been a few times in history, some of them fictional, some of them supposedly real, where people have taken advantge of the predictions of lunar eclipses to control an indigenous population," Duane Hamacher, a cultural astronomer at Monash University in Australia, told Live Science. And there are many, many more stories of indigenous populations being terrified of eclipses in the sky, he added.

"The sky is normally very predictable," Hamacher said. "So, when something comes along that is quite unusual and doesn't fit into that [predictable schedule] very well, it could often cause a sense of awe or even fear."

Some indigenous populations of Australia, for example, associated the color red with "evil," "blood" or "fire", according to Hamacher. There's "not that much stuff in the sky that is the color red," Hamacher said. But the populations considered it an omen when that shade did appear on something in the sky, such as meteors, auroras or the moon, he said.

The blood moon for aboriginal people, for example, "was commonly seen as the moon man covered in blood, rising from the dead," Hamacher said.

Some ancient Mesopotamian myths portrayed a lunar eclipse as the result of an attack from seven demons, and the Inca may have viewed a lunar eclipse as a jaguar attacking the moon. To save the moon and the people of Earth who might also become victims, the Inca would shake spears at the moon, make a lot of noise and beat their dogs to make them howl, according to National Geographic.

"Different cultures assign different kinds of meaning to the world around them," Hamacher said. "Now that pretty much anywhere in the world people know the scientific explanation of how lunar eclipses occur … we know there's no need for fear."

Indeed, further knowledge transformed that fear into fascination as time wore on. And instead of yelling, beating animals and banging pots together, we now lock our curious eyes with a moon "inflamed with wrath," knowing that the deep red color is a trick of our world's atmosphere.

This month's ominous moon will reach its peak at 4:21 p.m. EDT on July 27, and the totality will last for a total of 1 hour and 43 minutes. The eclipse won't be visible in the U.S., but it will be visible, at least partially, in most parts of the Eastern Hemisphere. The total eclipse can be seen from Africa, the Middle East and central Asia, according to Space.com.