While it might seem like a sign you need a new glasses prescription, it's really caused by ice crystals high up in the atmosphere.
"These small ice crystals actually refract the light from the moon," said Kara Lamb, an associate research scientist in the Department of Earth and Environmental Engineering at Columbia University.
Those ice crystals accumulate in cirrus clouds — clouds of pure ice in the stratosphere, up to 31 miles (50 kilometers) above ground.
The halo is always the same size, no matter where you are in the world or what the weather is like that day
If you were to measure it — which you can do by holding your hand out wide with your thumb on the moon and your outstretched pinky on the halo — you'd see it was around 22 degrees wide.