What is an Astronomical Occultation?

Astronomical occultation occurs when one celestial body eclipses, or blocks the light from, another celestial body from the Earth's perspective. Moons, planets, stars, small satellites, and asteroids can all interact to form an astronomical occultation with respect to a position on the Earth's surface. Many databases and organizations keep track of the orbits of our moon and various asteroids to predict visible occultations in the coming months. Amateur astronomers can observe these events to determine information such as the accuracy of that body's orbit or the topography of the moon.

Occult means "to hide." This is what occurs when the moon obscures a bright star for a brief period of time, for instance. That astronomical occultation would be visible to some locations of the Earth. The International Occultation Timing Association keeps records and publishes schedules of various kinds of astronomical occultation for the whole world. They track the Earth's position and orientation in the solar system with respect to the moon, orbiting asteroids, and distant stars. In 1977, astronomical occultation was the first method we used to see the rings of Uranus.

In popular astronomy, astronomical occultation provides events that hobbyists can analyze. They can actually produce new information without huge telescopes or sophisticated computers. All a backyard astronomer needs is a global positioning system, a 4-6" (10-15 cm) telescope, and a video-recording device. They can use these simple instruments to find smaller satellites orbiting an asteroid, find companions in binary star systems, determine the diameter and shape of larger asteroids, and improve scientists' records on positions of stars.

Amateur astronomers can even map the topography, the mountain ranges and deep valleys, near the polar regions of the moon. That's made possible by grazing occultation, where a star passes just along the edge of the moon, rather than direct occultation, where the star completely disappears behind the moon's body. During a grazing astronomical occultation, a star will be hidden by tall lunar mountains, then visible in a hollow valley, then obscured again by higher ground. Using mathematical calculations, you can determine the elevation of those mountains and the depth of the valleys with results impossible to conclude with even the most powerful telescopes.