Occultation, in astronomy, the passing of the moon or some other object of the solar system in front of a planet, star, or other celestial body, hiding it from view. The most common type of occultation is that of a star by the moon. An occultation of the sun by the moon is called a solar eclipse.
When a planet with an atmosphere passes in front of a star, the star dims before it disappears, because the planet's atmosphere absorbs some of the light passing through it. When the moon passes in front of a star, the star disappears suddenly without dimming, because the moon has no atmosphere.
Not only light, but also radio waves and other forms of electromagnetic radiation are blocked during an occultation. When the moon occults a celestial body that is a source of radio waves, the reception of the radio waves stops at the same time the body is hidden from view. Occultations have thus helped astronomers identify various celestial bodies as sources of radio waves.