Red Moon and Blood Moon: Optical and Astronomical Phenomena

Origin of the Red Moon and Blood Moon

A lunar eclipse occurs when the Earth positions itself between the Sun and the Moon, casting its shadow on the satellite. The red moon or blood moon results from the scattering of sunlight in the Earth's atmosphere. Red rays, less dispersed by Rayleigh scattering, illuminate the Moon as it passes through the Earth's shadow, giving it its characteristic hue.

The color varies depending on atmospheric conditions: an atmosphere loaded with dust or volcanic ash intensifies the red tint, while a clearer atmosphere can produce an orange or brownish Moon. This phenomenon is therefore an indirect indicator of the composition and density of particles suspended in the Earth's atmosphere.

Associated Optical and Astronomical Phenomena

During a total eclipse, sunlight undergoes several physical processes before reaching the Moon: refraction in the Earth's atmosphere, absorption by aerosols, and selective scattering of short wavelengths. The light spectrum that reaches the Moon is thus dominated by red and orange, explaining the appearance known as the blood moon. Partial eclipses do not always exhibit this phenomenon because only part of the Moon enters the shadow.

Sources: Espenak & Meeus, 2006; NASA Eclipse Web Site; Scientific literature on atmospheric scattering.