Geography

Red Moon


Sometimes we can look at the sky and come across a red moon. There are some situations that can cause a red moon. Among them, the most common cause occurs when the moon is low in the sky, soon after sunrise, or earlier when it is about to set below the horizon.

Like the sun, moonlight has to "go through" a greater amount of atmosphere when it is low near the horizon than when it is higher.

Earth's atmosphere can propagate sunlight, and since moonlight is only scattered sunlight, the atmosphere can propagate that as well. Red light can pass through the atmosphere and not dissipate much, while light at the blue end of the spectrum is more easily scattered.

When we come across a red moon, we are seeing the red light that has not dissipated, as the blue and green light has been scattered away. That's why the moon turns red, or rather, that's why we look at it this way.


The phenomenon happens because sunlight will be filtered through the earth before reaching the moon.

The second cause is if there is any kind of particle in the air. A forest fire or volcanic eruption can fill the air with tiny particles that partially obscure sunlight and moonlight. Again, these particles tend to scatter blue and green light, while allowing red light to pass through more easily. When we see a red moon high in the sky, it is possibly because there is a lot of dust in the air.

There is still a third cause for the moon to turn red, the phenomenon called total eclipse, when the earth is between the sun and the moon.