On the night of January 1, 1801, Giuseppe Piazzi, a priest in Palermo, Italy, was mapping the stars in the sky. Over three nights, he’d look at and draw the same set of stars, carefully measuring their relative positions.
That night, he measured the stars. The next night, he measured them again. To his surprise, one had moved. The third night, the peculiar star had moved again. This meant it couldn’t be a star at all.
It was something new, the first asteroid ever discovered, which Piazzi eventually named Ceres. Asteroids are bits of rock and metal that orbit the Sun. At over 900 kilometers across, Ceres is a very large asteroid. But through a telescope, like Piazzi’s, Ceres looked like a pinpoint of light similar to a star. In fact, the word asteroid means star-like. You can tell the difference between stars and asteroids by the way they move across the sky. Of course, Piazzi knew none of that at the time, just that he had discovered something new. To learn about Ceres, Piazzi needed to track its motion across the sky and then calculate its orbit around the Sun.
So each clear night, Piazzi trained his telescope to the heavens. Night after night, he made careful measurements, but from his observations he learned that Ceres was only visible in the sky during the day. It would take another year and a lot of astronomers to nail down Ceres’ path, but we haven’t lost track of it since.
Today, we can do something that Piazzi could only dream of: send spacecraft to study asteroids up close. One spacecraft called Dawn journeyed billions of kilometers over four years to the main asteroid belt. There, it visited Ceres and another asteroid, Vesta. Dawn’s stunning images transformed Piazzi’s dot of light into a spectacular landscape of craters, landslides, and mountains.
From the TED-Ed Lesson The first asteroid ever discovered – Carrie Nugent