• Moiya McTier

Astronomy Fun Fact #91

If you're lucky enough to be standing outside at sunrise or sunset with an unobstructed view of the Sun as it dips below the horizon in a place with clean air and a few other just-right conditions, you can see something called the Green Flash.

The green flash is a phenomenon that happens either right before sunrise or right after sunset where you see a flash of green light on the edge of the Sun.

The Sun is emitting light at all wavelengths, but humans can only see a very narrow slice of this spectrum, and we call it the visual range, which ranges from red to blue light. When sunlight passes through Earth's atmosphere, molecules scatter it in different directions. Long wavelengths (like red) only get scattered a little bit, and shorter wavelengths (like blue) get scattered a lot. This is why the sky looks blue when viewed from most angles on the Earth's surface.

But when the Sun is just below the horizon, green light gets scattered enough that it's briefly visible right before (at sunrise) or after (at sunset) every other wavelength of light! You might think there should be a blue flash, but those wavelengths actually get scattered too much.

I myself have never seen a green flash, but it's definitely on my bucket list!


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