Why does the Moon often look red during a total lunar eclipse?

During a total lunar eclipse, the Moon appears red because of how Earth’s atmosphere interacts with sunlight. While the Moon is passing through the umbra—the darkest part of Earth's shadow—it is not completely obscured.1

Instead, a portion of sunlight passes through the edges of Earth's atmosphere. As this light travels through the atmosphere, it is refracted and scattered. The atmosphere filters out shorter wavelengths of light, allowing the longer, redder wavelengths to penetrate the shadow cone and reach the lunar surface.12

The resulting color, often described as a reddish-brown or coppery hue, can vary in intensity based on meteorological conditions on Earth. For instance, a higher concentration of dust or particles in the atmosphere can cause the Moon to appear even redder. This phenomenon is sometimes colloquially referred to as a "blood moon," though this is an unscientific term for a total lunar eclipse.123