Why does the Moon appear red when it is low on the horizon?

When the Moon is near the horizon during moonrise or moonset, its light has to pass through a much thicker layer of the Earth's atmosphere than when it's directly overhead. As a result, the blue and green components of the Moon's light are scattered and dispersed by the atmosphere, while the longer wavelengths (red and orange) are less affected and can reach your eyes.

This image was taken on October 30, 2023, as it broke the horizon. It is said to be a Waning Gibbons Moon at 94% full.

As the Moon rises in the sky, the natural unaltered appearance returns as pale gray as illuminated by the Sun.