Sky Search: How to Find Neptune

Last week we looked at the planet Uranus. Interestingly, the discovery of that planet in 1781 eventually led to the discovery of yet another new world, some 65 years later. Since it appeared bluish in color, the new planet was named Neptune after the god of the sea.

But in contrast to Uranus, which is right on the threshold of naked-eye visibility, Neptune is much too faint to be perceived without any optical aid. It is slightly smaller than Uranus, with a diameter of 30,800 miles (49,600 kilometers), and it currently lies at a distance of 2.74 billion miles (4.41 billion kilometers) from Earth.

At magnitude +7.9, Neptune is more than six times dimmer than Uranus. (On this scale, larger numbers represent dimmer objects.)

Nonetheless, if you have access to a dark, clear sky and carefully examine our map, you should have no trouble in finding it with a good pair of binoculars. Neptune is now among the stars of Capricornus, but with a telescope, trying to resolve Neptune into a disk will be more difficult than it is with Uranus. You're going to need at least a four-inch telescope with a magnification of no less than 200 power just to turn Neptune into a tiny bluish dot of light.

Gas world

Voyager 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 Jupiter's 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using the Hubble Space Telescope suggest that the Dark Spot seen by Voyager 2 has dissipated, yet it has apparently been replaced by another.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 thirteen moons, one of which, Triton, has a tenuous atmosphere of nitrogen. At nearly 1,700 miles in diameter, it is larger than Pluto.

Because it is moving in a retrograde (backward) orbit, there has been some suggestion that Neptune may actually have captured it in the distant past. Those who have access to a telescope of 12 inches or more might even be able to get a glimpse of Triton, very close to Neptune itself.

Perturbing Uranus

Neptune's 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, were independently working on this problem.

Neither knew what the other was doing, but ultimately, both men had 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 his directed.

Johann Galle and Heinrich d'Arrest at Berlin did exactly as instructed and found the new planet in less than an hour. In the year 2010, Neptune will have completed one full trip around the Sun since its discovery.

"Almost" discoveries

A strange fact about Neptune is that it was very nearly discovered by none other than Galileo with his crude telescope more than two-and-a-half 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 further 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 too, stumbled across it without realizing what it really was. 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.