Untitled DocumentVoyager 2 passed Neptune in 1989 and showed it to possess a deep-blue atmosphere, with rapidly moving wisps of white clouds. Also evident was a Great Dark Spot, rather similar in nature to Jupiters famous Great Red Spot.
Recent observations of Neptune using the Hubble Space Telescope suggest that the Dark Spot seen by Voyager 2 has dissipated; yet has apparently been replaced by another.
The atmosphere of Neptune is apparently composed chiefly of hydrocarbon compounds.
Based on the rotation rate of its magnetic field, a rotation rate of 16.1 hours has been assigned to Neptune. Voyager 2 also revealed the existence of at least three rings around Neptune, composed of very fine particles.
Neptune has 11 known moons [the most recent three were announced in January 2003]. One of them, Triton, has a tenuous atmosphere of nitrogen and at nearly 1,700 miles (2,740 kilometers) in diameter is larger than Pluto. Because Triton is moving in a retrograde orbit -- backward in relation to the planet's spin -- there has been some suggestion that Neptune may actually have captured it in the distant past. Astronomers with access to a telescope of 12 inches or more might even be able to get a glimpse of Triton, very close to Neptune itself.
Odd discovery
Neptunes discovery came about from long-term observations of Uranus. It seemed to astronomers that some unknown body was somehow perturbing Uranus orbit.
In 1846, two astronomers, Urbain J.J. Leverrier (1811-1877) of France and John Couch Adams (1819-1892) of England independently were working on this problem. Neither knew what the other was doing, but ultimately, both men figured out the probable path of the supposed object that was disturbing the orbit of Uranus. Both believed that the unseen body was then in the constellation of Aquarius, the Water Carrier.
Adams was a student at Cambridge University, England, and he sent his results to Sir George Airy (1801-1892), the Astronomer Royal, with specific instructions on where to look for it.
For some unknown reason Airy delayed a year before starting the search.
In the meantime, Leverrier wrote to the Berlin Observatory requesting that they search in the place he directed. Johann Galle and Heinrich dArrest at Berlin did exactly as instructed and found the new planet in less than an hour.
Interestingly, in the year 2010 Neptune will have completed one full trip around the Sun since its discovery.
Galileo almost found Neptune
A strange fact about Neptune is that it was very nearly discovered by none other than Galileo with his crude telescope more than 2 centuries earlier.
It was while observing Jupiter and its system of four large satellites on Dec. 28, 1612 that, in the same field of view, Galileo unknowingly recorded Neptune as an eighth magnitude star.
Just over one month later on Jan. 27, 1613, Galileo recorded two stars in his telescope field, one of which was Neptune. The next night when he looked again, he noted that the two stars seemed farther apart. If he had only continued to keep watch in the following nights he would have almost certainly would have realized that one of the "stars" was indeed moving.
But Galileo should not be faulted for not recognizing Neptune, for later observers stumbled across it without realizing what it really was, too. Among them was the skilled French astronomer, Lalande (1795); the English astronomer, John Herschel (1830); and the Scottish astronomer, Von Lamont, just days before Neptune was actually discovered in 1846. All thought it was nothing more than an ordinary star.
And yet, if only Galileo had followed through with his observations, the eighth planet would have been discovered before the seventh!
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.
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