The discovery of Neptune remains one of the more colorful and controversial tales in the history of observational astronomy.
To recap: In the decades after the discovery of Uranus in 1781, an irregularity in Uranus's orbital motion led astronomers to suspect that the gravitational pull of another planet, farther out, was affecting it. In the 1840s the brilliant French mathematician Urbain Jean-Joseph Le Verrier calculated the putative perturber's location.
Le Verrier then enlisted the aid of Johann Galle at Berlin Observatory. With the blessing of director Johann Encke and aided by an eager young observer named Heinrich d'Arrest, Galle slewed a 9-inch (23-cm) refractor to the predicted position on September 23, 1846, and found Neptune in just a half hour's time. Our solar system doubled in size overnight.
The successful prediction of Neptune's position has long been co-credited to John Couch Adams in England. But recently discovered documents from the time make it clear that Adams deserves much less of the credit. His predictions were vague and conflicting, sending British searchers at the Cambridge University Observatory on a six-week wild goose chase hunting the planet during the summer of 1846. Adams's equal billing with Le Verrier (arranged by the British a few months later) was a diplomatic nicety to avoid a rift in the touchy relations between England and France; see Secret Documents Rewrite the Discovery of Neptune.
Fast forward 164 years. Neptune is now nearing the completion of its first complete circling of the Sun since being discovered. Its 1-year "birthday" is still more than a year away (mark your calendars: July 12, 2011), but planet-watcher John Westfall notes that today marks a milestone of sorts for this distant blue-tinged world: it's situated near the Aquarius-Capricornus border exactly where Galle and d'Arrest spotted it.
For Sky & Telescope's guide to finding Neptune (and Uranus) in the coming months, click here.
"The hardest part of finding Neptune's discovery position was getting an accurate time for the discovery," Westfall tells Sky & Telescope. "The time I used was apparently when Galle and d'Arrest actually checked the star field against their copy of the star chart," which he estimates was roughly 00:15 Universal Time on September 24, 1846.
As Westfall points out, Neptune comes back to its discovery position only once as seen from the Sun, but five times as seen from Earth (thanks to two retrograde loops during the next two years. So if it's cloudy tonight, you can follow in history's footsteps by observing the planet on any of these upcoming dates: July 17, 2010; October 27, 2011; and November 22, 2011. (On the fifth date, next February 11th, Neptune will be too close to the Sun to see.
Of course, none of these dates would matter if Galileo had recognized Neptune's true nature when he unknowingly spotted it near Jupiter on December 28, 1612, and on January 27, 1613. Another couple of nights' observing, and he might have realized that the "fixed" background star lurking nearby (which he labeled fixa) was itself moving.
Imagine the consequences of that altered history. Suspecting that something must orbit between Saturn's 9½ astronomical units from the Sun and Neptune's 30 a.u., astronomers might have undertaken a search for Uranus long before William Herschel stumbled upon it in 1781.
Moreover, aided by two extra centuries' tracking of Neptune's orbital motion, outer-planet motions would have been fully understood long ago. So William Pickering and Percival Lowell might never have predicted the existence of a "Planet X" even further out. So there'd have been no need for Clyde Tombaugh to search for it — and who knows when Pluto might have been discovered?