The night sky is a wondrous, awe-inspiring thing, and even more so when something out of the ordinary, like a lunar eclipse, happens.

Total lunar eclipses occur at least twice a year, but they aren’t visible from everywhere on Earth. They occur when Earth is directly between the sun and the moon, causing Earth to cast its shadow on the moon, according to EarthSky. During the period of totality, when Earth is directly between the sun and the moon, the moon has a red hue, which is why we call lunar eclipses blood moons. Sometimes, when lunar eclipses happen during a supermoon, we have even more star-studded names for the phenomenon, like super blood moon total lunar eclipse. Sounds pretty spectacular, right?

Read on to learn more about lunar eclipses.

There are three different kinds, but they aren’t all much to look at

Not all lunar eclipses are created equal. There are three different types of eclipses — total, partial and penumbral — and they all appear different in the night sky. A total lunar eclipse is the real show stopper, when the moon, Earth and sun are perfectly aligned and Earth casts a shadow on the entire visible surface of the moon, Space.com reports.

During a partial lunar eclipse, Earth and the moon don’t line up perfectly. The Earth will still cast a shadow on the moon, but it won’t cover it, so it instead looks like there is bite taken out of the moon, according to Space.com. Total lunar eclipses include partial eclipses both before and after the eclipse reaches totality, so you can view this bite out of the moon during a total lunar eclipse as well.

A penumbral eclipse occurs when the moon passes through Earth’s penumbra, which is the outer part of its shadow, according to NASA. When this happens, the moon will appear more dim than usual. Visually, however, these eclipses aren’t much to look at. Unless you’re a well-informed moon observer, you likely wouldn’t even notice it is happening.

They give the moon an eerie hue

Lunar eclipses aren’t called blood moons for nothing. A lunar eclipse makes the moon red in the night sky because of how light from the sun passes through Earth’s atmosphere, according to NASA. It’s only during the totality of the eclipse that the moon looks red, and it’s for the same reason the sky looks blue.

It’s because of a principle called Rayleigh scattering. Sunlight moves in waves, and the different colors of light — red, orange, yellow, green, blue, indigo and violet — have different properties. When red light travels through the atmosphere, it bends, or refracts, on each side of Earth, and this allows the red light to be cast on the moon during an eclipse, EarthSky reports.

A lunar eclipse will not always appear the same shade of red because it depends on the current conditions in Earth’s atmosphere, according to EarthSky. More clouds or dust in the atmosphere will make the moon look more red, while a cleaner atmosphere will make it look less red. Too much debris in the atmosphere, however, can dampen out this effect. During a total lunar eclipse soon after the eruption of a volcano in the Philippines, so much dust clouded the atmosphere that the lunar eclipse was almost blotted out.

They used to induce fear

Our understanding of the solar system is much greater today than it was in previous generations, enough so that we know an eclipse is going to occur long before it does. Centuries ago, though, lunar eclipses weren’t well understood, and they were a cause for concern. People didn’t understand the naturally occurring phenomena that allow eclipses to occur, and seeing the moon turn blood red in the night sky was frightening, Space.com reports. Many believed that a god was using the eclipse as a sign.

As frightening as a lunar eclipse may have been, solar eclipses were even more so. The blotting out of the sun in the middle of the day was thought to be a bad omen and a trigger for tragic events, Time reports. In the 1600s, people in England believed a solar eclipse may have triggered the Great Plague of London. Centuries earlier, in 585, a war between the Lydians and the Mendes was ended because the two sides believed a solar eclipse was a godly sign that their actions were wrong.

While it wasn’t until relatively recently in the span of human existence that eclipses have become well understood and predictable, the idea of them as a natural phenomenon has been suspected since about 500 B.C., Time reports. An ancient Greek historian claimed a philosopher was able to predict solar eclipses, and Greek philosopher Aristotle was also said to have understood what caused eclipses to occur.

They're predictable today

Since we understand why and how lunar eclipses occur, we no longer fear them and what they might mean. We even know when they will occur months, years, even decades ahead of time. That’s right. You can look up when a lunar eclipse will occur in 2049 on the NASA Eclipse Website; you can even find out when eclipses occurred in 1901 if you want.

Even though we know when they will occur, they don’t always occur at the same time. And we don’t always have the same number each year. A single calendar year will always have at least two lunar eclipses and two solar eclipses, but sometimes there are more, EarthSky reports. Remember, though, that not all eclipses can be seen from everywhere on Earth.

The most eclipses that can occur in a single year is seven, although that is rare. We last experienced seven eclipses in one year in 1982, and it will happen again in 2038. For seven eclipses to occur in a single year, the first must be in early January, allowing for the last eclipse to occur in late December, EarthSky reports. The reason eclipses don’t happen at the same time every year is because their occurrence is based on the lunar cycle, or lunar months, which lasts 29.53 days. Our months last between 28 and 31 days, so while the lunar cycle remains constant, it does not always match up the same on our calendar.

They won’t be happening forever

Lunar eclipses have been occurring for millennia, but they won’t last forever. This is because the moon is slowly drifting away from Earth, and at some point it will no longer be close enough to Earth to cast a full shadow on our home planet, NASA reports.

The moon is moving away from Earth ever so slowly, so the demise of lunar eclipses isn’t exactly imminent. It’s only drifting away at a rate of about 1.5 inches per year, about the same rate at which our fingernails grow, the BBC reports. At the rate the moon is moving away from Earth, it will take about 600 million years before it is too far away to fully cast a shadow on Earth, ending lunar eclipses.

The moon is moving away from Earth because of the tides here on our planet. The gradual movement is thought to have been occurring since the moon first formed about 4.5 billion years ago. While the movement is slight, it is making Earth slow down, the BBC reports. Fossils already show evidence of this slowdown. Scientists examining coral fossils have been able to determine that billions of years ago a day lasted about 19 hours compared to 24 hours today.