What causes the moon to look red?

We look upward and see the moon through a blanket of atmosphere that reaches many hundreds of miles above our heads. These airy gases are made of widely separated molecules of matter. Between us and the top of the atmosphere there are zillions of them. When we look low in the sky, our sight must travel through a much longer sloping slice of the atmosphere. And all these dancing particles of airy gases tend to play tricks with light with sunlight and starlight and light from the moon. They tend to sheer off wave lengths of light and scatter them around so that our eyes see them as colors. The color of the moon that reaches our eyes has been changed by the gaseous air above. Often it takes on a rosy glow as it rises and sets low in the sky. This is because more zillions of air molecules can separate out the longer wave lengths of reddish color. The larger particles in a misty sky also cause the moon to blush red. Misty hazes and high fogs also play tricks with moonbeams and create ...  more

As more of the atmosphere is between the viewer and the moon, the blue spectrum light gets filtered out, leaving only the red light from the moon.  more