A blood moon happens during a total lunar eclipse, when the Earth is directly between the Sun and the Moon, blocking the Sun's light from reaching the Moon. The Moon passes through the Earth's shadow, specifically the umbra, and although direct sunlight is blocked, some sunlight still reaches the Moon but passes through the Earth's atmosphere first. This light is filtered by the atmosphere, where shorter blue wavelengths are scattered out and longer red wavelengths pass through, causing the Moon to take on a reddish or blood-like color. This phenomenon is why the Moon appears red during the eclipse, known popularly as a blood moon. The redness varies depending on the Earth's atmospheric conditions at that time, such as dust or clouds, which can make the color appear darker or lighter.
