Happy New Year! May you have as pleasant a start to the new year as Giuseppe Piazzi did on January 1, 1801. That night the Italian monk and amateur astronomer was at work in his observatory on the island of Sicily compiling a catalog of the stars in the constellation Taurus. Using an earlier catalog as his reference, he noticed a star that was out of place and not included among its pages. The next night he returned to check it and to his surprise, the star had moved!
Piazzi suspected it might be a “new star” but still cautious, he observed it on a third and even a fourth night – noting it had moved the same distance each night – before sending news of the discovery by letter to colleagues at other European observatories.
He initially reported the object as a comet, similar to what William Herschel did when he discovered Uranus back in 1781, but added in a letter to an astronomical friend:
“I have announced the star as a comet. But the fact that the star is not accompanied by any nebulosity (haziness) and that its movement is very slow and rather uniform has caused me many times to seriously consider that perhaps it might be something better than a comet. I would be very careful, however, about making this conjecture public.”
Piazzi wrote a similar letter to Johann Bode, director of the Berlin Observatory. Because Napoleon’s invasion of Italy disrupted communications at the time, Bode didn’t get the letter until March 20, nearly 3 months later. But one look at the figures convinced him Piazzi had found something brand new.
Could it be the missing planet astronomers had hypothesized must lie between the vast and apparently empty gap between Jupiter and Mars?
Unfortunately, Piazzi had been the only person to observe the new object before it’s conjunction with the sun and disappearance in the solar glare. When predicted to reappear in the morning sky several months later, astronomers were ready with their telescopes, but no one could find the object.
Because of bad weather and illness, Piazzi hadn’t made enough observations earlier that year to allow astronomers of the day to establish an orbit. Without an orbit, predicting the “planet’s” position after solar conjunction was impossible.
A young German mathematician named Carl Friedrich Gauss became aware of the problem and worked for months to produced a new, more accurate method of orbit determination. He applied it to Piazzi’s handful of positions and confidently predicted where to look for the new “planet” in December 1801. And wouldn’t you know it, one Baron Franz Xaver von Zach , a Hungarian astronomer, found it within 1/2 of a degree of Gauss’ predicted position on December 31, 1801, almost exactly one year after Piazzi’s discovery.
Zach was famous for organizing a group of astronomers nicknamed the Celestial Police, whose mission was to find the putative planet predicted by Bode’s Law to lie between Jupiter and Mars.
Piazzi named the new object Ceres Ferdinandea, after the Roman goddess of agriculture and grain crops and King Ferdinand IV of Naples and Sicily. The Ferdinand was later dropped (thank you!) leaving us with Ceres.
Ceres became the first – and largest – of the asteroids, a collection of small, rocky bodies orbiting in a broad belt between Mars and Jupiter. Stirred up by Jupiter’s gravity, they never glommed together to form a planet.
Ceres was recently reclassified as as a dwarf planet, joining erstwhile-planet Pluto and icy asteroids Eris, Makemake and Haumea. Three more asteroids – Pallas, Juno and Vesta – were discovered in the next few years. In the early 19th century, it was believed these objects were actual planets; you’ll see them referred to as such in books of the time.
As the discovery pace quickened in the 1840s and beyond, astronomers soon realized there were too many to be considered in the same class as planets. Instead they named them asteroids from the Latin aster or “star”, referring to their appearance in the telescope. It’s estimated there are between 1.1 and 1.9 million asteroids in the main belt 0.6 miles across and larger and millions more smaller ones. While these numbers sound impressive, if you could crunch all the asteroids in the main belt together they’d only form a sphere less than half the size of the moon!
Ceres’s story continues into the present. After departing Vesta this fall, NASA’s Dawn space probe will visit the dwarf planet for a year starting sometime in early 2015. Based on density estimates, astronomers suspect it harbors water beneath its crust. The Hubble picture reveals a pink surface and mysterious white spot.
Piazzi, we’ve only begun to know the world you found that New Year’s night.