How can stellar occultations measure an asteroid’s size?

Stellar occultation is a direct method for measuring an asteroid's size by timing how long a star is obscured as the asteroid passes between it and Earth.1

To determine the size and shape, astronomers use the following process:

• Timing the Event: Observers measure the duration of the star's disappearance and reappearance.21
• Calculating Chords: By coordinating multiple observers at different locations across Earth, astronomers can record the occultation from various vantage points. Each location provides a "chord," or a cross-sectional measurement of the asteroid's path.21
• Determining Dimensions: Using the known distance and the asteroid's rate of motion, these chords are combined to map the asteroid's diameter as projected onto the plane of the sky.1

When numerous measurements are collected, they can reveal the asteroid's overall size and even its three-dimensional shape, such as identifying an elongated or cigar-like profile. Because this technique relies on rare alignments and provides only a single cross-section per event, it is most effective when multiple observers are spread out perpendicular to the asteroid's shadow track.21