While the moon usually appears to have a golden, white, or gray hue, certain astrological conditions can change the moon's color to a rusty red or orange color. The explanation for this phenomenon has to do with the way that the moon reflects and refracts the light from another planetary body—the sun.
The Sun's Reflection
The reason humans are able to spot the glowing moon in the night sky is because light from the sun is reflected off of the surface of the moon. As the moon makes its 29.5 day journey around the Earth, different parts of its surface face the sun and are able to reflect sunlight back to Earth. At any given time when the moon is visible in the sky it reflects between three and 12 percent of the light that reaches it from the sun.
Red Moon Causes
A red moon appears in three circumstances. During a lunar eclipse, the moon appears red as it passes through Earth's shadow. When the moon is low in the sky it also can take on a reddish hue. The third event that causes a red moon is the presence of some kind of micro particle in the air. This can happen after a volcanic eruption or forest fire.
In each of these circumstances, the moon appears red because it is reflecting red light that has been filtered through Earth's atmosphere. When light hits the atmosphere, it becomes scattered. Light at the blue end of the spectrum is easily scattered and diffused, while red light is more resistant to being scattered, and passes through the atmosphere more easily.
During a lunar eclipse, a full moon passes into the shadow of Earth, also called the Earth's umbra. When this happens, the Earth is positioned directly between the moon and the sun, and the sun's rays cannot directly reach the moon's surface. However, refracted red light in Earth's atmosphere is able to reflect off of the moon's surface, causing the moon to appear orange or red.
Near the Horizon
When the moon is positioned near Earth's horizon is often appears to have a red or orange hue. This is because the light must pass through a larger atmospheric area when the moon is lower. This causes blue light to be scattered, while red light remains visible.
When the amount of micro particles in the air increases, the moon can appear to be red or orange in color. Causes of these particles can include a volcanic eruption, forest fires, or air pollution. Micro particles can thicken Earth's atmosphere, making it difficult for reflected light to travel between the moon and the Earth. The thickened atmosphere scatters light at the blue end of the spectrum, while red light may still travel through, making the moon appear red.