What Is a Lunar Eclipse? (And Why It Turns Red)
A lunar eclipse is one of the easiest and most striking sky events to watch. No special glasses, no gear, just the moon slowly turning a deep red. Here is how it works.
Earth's shadow falls on the moon
A lunar eclipse happens at a full moon, when the sun, Earth, and moon line up almost exactly, with Earth in the middle. Earth blocks the sunlight and casts its shadow onto the moon. As the moon drifts into that shadow, it slowly darkens.
Why it turns red, not black
You might expect the moon to just go dark, but during a total eclipse it glows a rusty red. That is because a little sunlight bends through Earth's atmosphere and reaches the moon. The atmosphere filters out the blue light and lets the red through, the same effect that makes sunsets red. All the sunrises and sunsets on Earth at once are being cast onto the moon. That red glow is why it is nicknamed a Blood Moon.
How it differs from a solar eclipse
- A lunar eclipse is the moon passing into Earth's shadow. It is safe to look at with your eyes, happens at night, and is visible to half the planet at once.
- A solar eclipse is the moon blocking the sun. It is dangerous to look at without proper filters and is only visible from a narrow path.
Why not every month
The moon's orbit is tilted a little, so at most full moons it passes above or below Earth's shadow and nothing happens. The alignment only lines up a couple of times a year, which is what makes eclipses special.
A lunar eclipse always falls on a full moon, so you can see the full moons coming up on moontrack. New to eclipses? Read the names of the full moons for where "Blood Moon" fits in.