occultations

An occultation is an astronomical event in which one body passes in front of another from an observer’s point of view, briefly blocking the background object’s light. The most common instances involve a star being occulted by a closer solar-system body—typically the Moon, a planet, or an asteroid. Because the foreground body covers only part of the star’s image for a short time, careful timing of disappearance and reappearance can reveal properties of the occulting object and, in some cases, of the background source.

Preparation and observation: Occultations are predicted from precise orbital data. The path of the occultation across the Earth is narrow, but the event can be observed by many observers along the path. Observers use telescopes, video, or photometric measurements to record the times of disappearance and reappearance. Combining data from multiple sites yields a two-dimensional silhouette of the occulting body and can reveal atmospheres, rings, or satellites.

Applications and notable results: Stellar occultations have been used to determine asteroid sizes and shapes with greater precision than direct imaging. They have detected tenuous atmospheres around outer Solar System bodies—most famously Pluto’s atmosphere in occultation data—and discovered rings around Chariklo in 2013. In addition, occultations can constrain stellar diameters and provide insight into the properties of the background stars.