A blood moon is a popular term for a total lunar eclipse, during which the Earth's moon takes on a reddish color. This occurs when the Earth comes directly between the sun and the moon, blocking direct sunlight from reaching the moon's surface. However, some sunlight still reaches the moon indirectly after passing through Earth's atmosphere, which scatters shorter wavelengths of light (like blue) and allows longer red wavelengths to reach and illuminate the moon, giving it a reddish, orange, or brownish hue

. The term "blood moon" is not a scientific term but is widely used because of the moon's striking red appearance during these eclipses. Sometimes, the term also refers to a series of four total lunar eclipses occurring consecutively, known as a lunar tetrad, which is a rare phenomenon

. In summary, a blood moon is a total lunar eclipse where the moon appears red due to Earth's atmosphere filtering sunlight, and it happens only during a full moon when the sun, Earth, and moon align perfectly