It's a headline that could have been printed in 1613 — "Galileo discovers Neptune" — but Neptune's actual discovery wasn't announced until more than 200 years later.

During December 1612 and January 1613, Galileo was 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. 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 odd little star's motion was buried in his copious notes.

In 1781, William Herschel discovered Uranus. At the time, it held the title of outermost planet. To aid astronomers in their observations, Alexis Bouvard published tables predicting the orbital motion of Uranus in 1821. But over the years, it became clear that the motion of the planet deviated from Bouvard's predictions. Curiously, at one point along its orbital path, Uranus slowed 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 1840s 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
Sept. 23, 1846, Galle looked through his telescope and spied an object that did not match any of the star positions plotted in his atlas.

Today marks 163 years since the discovery of Neptune — a triumph of physics and mathematics. This year as we commemorate the International Year of Astronomy and the 400th anniversary of Galileo's investigation of the night sky, we once again find Neptune in the same celestial neighborhood as Jupiter. Because one Neptune year equals 165 years on Earth, if you catch sight of Neptune before 2011, you can observe it during the same Neptunian year as Johann Galle.

Come join members of the Capital City Astronomy Club this Friday evening at the Galisteo Basin Preserve for a look at Neptune. The star party is co-hosted by the Commonweal Conservancy and the Santa Fe Conservation Trust. The events begins at
6 p.m. with hot cider and cookies. Dress warmly, bring along a picnic dinner and enjoy the sunset. Admission is $5 and advance purchase is required. For more information and to make your reservation contact the Santa Fe Conservation Trust at 505-989-7019.

Peter Lipscomb is the lead guide for Astronomy Adventures and works to promote sensible and energy efficient lighting practices. He keeps a blog of stargazing tips and events at www.eyesfullofstars.com.