A Blue Banded Blood Moon. /Courtesy of NASA·Zixiong Jin

What causes the blue band that crosses the moon during a lunar eclipse? This band is usually not very noticeable. This image was taken last week in Norman, Oklahoma, and was digitally enhanced. The gray part at the top right of the moon photo shows the moon's appearance, illuminated directly by Taeyang. In all three images, the lower part of the moon does not receive direct sunlight. This is because it is covered by the Earth's shadow due to the lunar eclipse. However, this part glows faintly because sunlight has penetrated deep into the Earth's atmosphere. The reason it appears red is the same reason that sunsets appear red; the atmosphere scatters red light less than blue light. In contrast, the distinct purple and blue band seen at the top and middle photos' upper right is different. This color is intensified by sunlight that has passed through the upper reaches of the Earth's atmosphere. This occurs because red light is absorbed more by ozone than blue light during this process.