Galileo discovers Neptune!
A headline that could have been printed in 1613, but Neptune’s actual discovery wasn’t announced until over two hundred years later.
During December of 1612 and the following January, Galileo was hard at work with his telescope observing Jupiter and its moons. In his logbook, he sketched the planet and the movement of its satellites. He also included the positions of background stars that shared the field of view. During the course of his observations, the location of one of those stars changed. Galileo noticed the shift in position, but stayed on task studying Jupiter.
Galileo’s moving star was, in fact, the planet Neptune. But, with his efforts directed toward the movement of Jupiter’s moons, the significance of the odd little star’s motion remained buried in his copious notes.
Later in 1781, William Herschel discovered Uranus. At the time, it held the title of outermost planet. To aid astronomers tracking Uranus, Alexis Bouvard published tables predicting the orbital motion of Uranus in 1821. But, over the years, it became clear that the actual motion of the planet deviated from Bouvard’s predictions. Curiously, at one point along its orbital path, Uranus appeared to slow down. What could cause this change?
One explanation suggested that Uranus had been struck by a comet. But, according the Newton’s laws of gravity, it was more likely that some unseen body lurked nearby and its mass perturbed Uranus’ orbit.
To solve the puzzle, scientists turned to math instead of empirical observation. Working independently of each other, John Crouch Adams and Urbain Le Verrier spent much of the 1840’s making calculations to predict the location of the mystery object. Several attempts to find the new planet failed until 1846 when Le Verrier sent a message to Johann Galle at the Berlin Observatory. Armed with Le Verrier’s calculations, on September 23rd 1846, Galle looked through his telescope and spied an object that did not match any of the star positions plotted in his atlas.
This month marks 163 years since the discovery of Neptune: a triumph of physics and mathematics. During 2009, as we commemorate the International Year of Astronomy and the 400th anniversary of Galileo’s first astronomical use of a telescope, we once again find Neptune in the same celestial neighborhood as Jupiter. But, the fun doesn’t stop there. Because one Neptune year equals 165 years on Earth, if you catch sight of Neptune before mid-2011 you can observe it during the same year as Johann Galle.
