The celestial teapot will lose part of its handle tonight — one of its stars will be hidden behind the Moon. The disappearing act will be visible across most of the United States. The event is known as an occultation — from a Latin word that means “to hide.” Such an event is possible because the star is close to the ecliptic — the Sun’s path across the sky. The Moon stays close to the ecliptic as well. So when the geometry is just right, the Moon crosses in front of the star, so the star winks out. Astronomers can learn a lot about a star system by watching an occultation. If they know the star’s distance, then the amount of time it takes for the star to vanish can reveal its size. And if the light drops in steps, then it means the system has two stars or more. Tonight’s occultation is of Tau Sagittarii. It’s at the lower corner of the handle of the teapot outlined by some of the bright stars of Sagittarius. The star is about 120 light-years away. It’s past the end of its “normal” life
