Asteroids are always getting in the way. Every night, several of them pass in front of stars, briefly blocking the stars from view. Astronomers don’t mind, though. In fact, each of these occultations offers a chance to learn about the asteroids themselves. Occultations are visible across a narrow path — usually just a few miles wide. To be of greatest value, that path needs to cross over several telescopes. They don’t have to be at major observatories, though — many amateur astronomers contribute valuable measurements as well. By comparing the length of an occultation at different sites along the path, and adding observations close to the path that show no occultation, astronomers can learn quite a bit. A couple of years ago, for example, telescopes across the United States watched an occultation of two asteroids that form a binary. Several telescopes saw the occultation, while the rest saw nothing. From those observations, astronomers measured the size and shape of both asteroids with
