Why Does the Moon Turn Red?

Few sights in the night sky stop people in their tracks like a deep red moon. For thousands of years it was read as an omen, a warning written across the heavens, a “blood moon” foretold in old prophecies. The truth is gentler, and in its own way just as remarkable. A red moon is sunlight itself, bent and filtered by the Earth’s atmosphere before it ever reaches the moon. Here is why it happens, and the two everyday situations that most often turn the moon red.

The idea is an ancient one. The prophet Joel wrote that “the sun shall be turned into darkness, and the moon into blood, before the great and terrible day of the Lord come” (Joel 2:31). For much of history a red moon was read not as astronomy but as prophecy.

A red moon low on the horizon

A red or orange moon is a common sight even on an ordinary night. Watch the moon as it rises or sets and you will often catch it glowing amber, especially when it is full. When the moon sits low in the sky, its light has to travel through a much thicker slice of atmosphere to reach your eyes. That thick air scatters away the shorter blue wavelengths and leaves the warm reds and oranges behind, exactly as it does for the setting sun.

This is why a rising harvest moon so often looks like a great orange lantern balanced on the horizon. As it climbs higher and its light passes through less air, the color fades back to its familiar pale silver.

Smoke, dust, and ash

Sometimes the moon turns red even when it rides high overhead. The culprit is usually something in the air. Smoke from distant wildfires, dust carried on desert winds, and ash from volcanic eruptions all fill the atmosphere with fine particles that scatter blue light and let red through. On nights like these a red moon can be striking, but it is also a quiet sign that something is in the air, whether smoke, dust, or ash.

Is it safe to look at?

Happily, yes. A red moon is completely safe to look at with the naked eye. You need no filters, no telescope, and no special equipment at all. You can simply stand outside and take it in, or bring out a pair of binoculars for a closer look at the warm light pooling across the craters.

So the next time the moon hangs red above you, you can enjoy it for exactly what it is. Not an omen, but a trick of light, the Earth’s atmosphere painting the moon in the colors of a sunset. To see what phase the moon is in tonight, or when the next full moon arrives, visit our home page and full moon calendar.