Blue moons and blood moons. While some of these names are meant to coincide with other astronomical or worldly event — the strawberry moon is so named because it’s the time when strawberries are usually ready to harvest in certain parts of the world — there’s no arguing that moonlight sometimes seems to change color right before our eyes.
But this doesn’t really make much sense when you think about it. After all, the Moon isn’t putting off any light itself. Instead, it’s just reflecting light from the sun. So, then, why does moonlight change color? The answer is simpler than you think.
Why moonlight changes colors
The way that our Moon absorbs and reflects light all comes down to science. Samples taken from Earth’s satellite suggest that the Moon is mostly made up of gray rock called anorthosite. Sure, there are darker areas — made up of basalt, analysis has shown — but overall, it’s the same uniform gray across most of the lunar surface. And, it turns out, gray absorbs energy in pretty equal terms. This means that it takes a tiny bit of every part of the light spectrum, so to speak, and then reflects the rest of the light back down to Earth, which is what gives our moon its iconic grayish-white appearance.
But the state of our atmosphere can greatly affect how much of that light reaches our eyes here on Earth. Depending on various conditions, different wavelengths of light might reach our eyes from night to night. When the Moon is positioned high in the sky, the path light travels through the atmosphere is relatively short. This means that the light doesn’t scatters very much, making all that white light reach the surface.
However, when the Moon is closer to the horizon, the angle that the light travels through the atmosphere changes significantly. This can change what light scatters, and which wavelengths make it through, giving the moonlight a different color entirely. And because moonlight contains a smattering of all visible wavelengths, the atmosphere can affect which of those wavelengths gets through. This, in turn, can change what color the Moon appears to be in our eyes.
A blood moon or lunar eclipse, as Live Science explains, sees all of the blue light wavelengths absorbed by our atmosphere as the light bounces through our atmosphere and out to the Moon. Since the blue light is absorbed, only the red and orange wavelengths make it through. So, that is what is reflected back to us, and that’s why lunar eclipses have a red-orange appearance.
It’s an interesting reminder of just how complex light really is, and how it can literally change how we see the world. It is also a reminder that the Moon, though a bit dull in color normally, has a lot to offer when it comes to understanding the science that guides our universe.
