Did Galileo Spot Neptune Two Centuries Before Its "Discovery"?

By Eliza Strickland | July 13, 2009 7:17 am

NeptuneDoes Galileo Galilei deserve yet another notch in his belt? Besides discovering four of Jupiter‘s moons, studying sunspots, observing the phases of Venus, and examining the rough mountains and craters of the moon, Galileo may also have identified the planet Neptune more than two centuries before its official discovery, one researcher is arguing.

Its widely accepted that in 1612 and again in 1613 Galileo must have observed Neptune, although at the time he thought it was a star, spotted during his observation of Jupiter’s moons. But physicist David Jamieson from the University of Melbourne, Australia, says that history has judged Galileo incorrectly – and that his notebooks reveal that he knew he was looking at a planet after all [Nature blog].

We know that Galileo spotted Neptune through his telescope because his notebook drawings of the night sky in 1612 and 1613 include a mysterious “star” near Jupiter, in a spot where star catalogs show that no star should be. Jamieson explains that “this unknown star was actually the planet Neptune…. Computer simulations show the precision of his observations revealing that Neptune would have looked just like a faint star almost exactly where Galileo observed it” [LiveScience].

But could Galileo have determined that the “star” was actually a planet? “On the night of January 28 in 1613 Galileo noted that the ‘star’ we now know is the planet Neptune appeared to have moved relative to an actual nearby star,” says Jamieson…. Planets, of course, move across the starry background of the night sky, making it relatively easy to separate the two classes of object even if they appear similar in a fairly basic telescope [The Register]. While Galileo didn’t make any explicit reference to the possibility that he was seeing a new planet, Jamieson has one more piece of evidence: an unlabeled black dot on an earlier notebook entry from January 6, 1613. The dot is in the position where Neptune would have appeared that night. “I believe this dot could reveal he went back in his notes to record where he saw Neptune earlier when it was even closer to Jupiter but had not previously attracted his attention because of its unremarkable star-like appearance” [Nature blog], says Jamieson.

Jamieson wonders whether a chemical analysis of Galileo’s inks can reveal when the black dot was placed on the notebook page. He also, somewhat wistfully, suggests that Galileo could have sent a secret, coded message to a colleague to record the discovery, as he did when he observed the phases of Venus. Could a still-unscrambled anagram lurk in the archives somewhere? Until such a document is turned up, the astronomers who predicted Neptune’s location and finally found it in 1846 will continue to get credit.

Related Content:
DISCOVER: 20 Things You Didn’t Know About… Galileo
DISCOVER: He Charted the Moon Before Galileo, but You’ve Probably Never Heard of Him
DISCOVER: Neptune Rising explores the planet’s mysteries, including possible storms of diamonds

Image: NASA


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