Since the demotion of Pluto in 2006 to the status of "dwarf planet," the most distant of the eight classical planets is Neptune, located at an average distance of 2,795,084,800 mi (4,498,252,900 km) from the sun.
But have you ever actually seen Neptune?
Unfortunately, thanks chiefly to its great distance it's much too faint to be perceived with the unaided eye. Currently shining at magnitude +7.9, it's nearly four times dimmer than the faintest star visible with the unaided eye. On this scale, larger numbers represent dimmer objects, and about the dimmest things visible to the naked eye are around magnitude 6. 5 under very dark, rural skies.
Normally to find Neptune you would need to have access to a very dark, clear sky and very carefully examine a sky chart or star atlas; an attempt to locate Neptune can then be made using a small telescope or good binoculars.
But this week, using good binoculars or a small telescope, you'll have a great opportunity to easily locate Neptune using another planet: brilliant Jupiter, which will engage Neptune in the first three close conjunctions, an unusual "triple conjunction" between these two gas giants.
Typically, Jupiter and Neptune pair off about every 12 or 13 years, but triple conjunctions are less frequent.
For that to happen, both planets must arrive at opposition – that point in the sky directly opposite to the sun – at virtually the same time. This year, Jupiter will come to opposition on August 14; Neptune on August 17. For several months before and after these dates, the two planets will appear to temporarily move backwards against the background stars (called "retrograde motion") and in the process will pass each other not just once, but three times. The last Jupiter/Neptune triple conjunction was in 1971, while the next will be during 2047-2048.
The first of this year's three pairings will come on May 27; the closest of the three that occur in 2009, with Jupiter passing just 0.38-degree south of Neptune [Map].
That's roughly equal to three-quarters the apparent diameter of the Moon. So if you've never seen the most distant planet from the sun, you'll have an excellent opportunity on this morning, using Jupiter as your guide. With binoculars or a telescope, focus first on Jupiter then search just above it for a tiny bluish "star"; this will be Neptune. Keep in mind that Neptune will appear only about 1/13,000 as bright as Jupiter.
The next two conjunctions between these planets will be on July 9 and December 21.
Where are they now?
Jupiter can be found glaring low above east-southeast horizon at around 2 a.m. local daylight time, the brightest “star,” in the sky at that hour; a nighttime object that certainly attracts attention even from within brightly-lit cities and invites inspection the moment you set up a telescope.
Better to wait, however, until a couple hours later for it to gain some altitude above the horizon haze.
Jupiter has the largest apparent disk of any bright object in the sky after the Moon and the sun. Its dark belts and bright zones with their subtle markings resolves into a series of red, yellow, tan and brown shadings in most telescopes, and of course its four large and bright moons can be followed for hours, even in steadily held binoculars. Through a telescope you can watch as they speed in front of Jupiter, throwing their shadows on the planet, or vanish behind its disk or suddenly becoming eclipsed by its shadow.
In contrast to Jupiter, trying to resolve Neptune into a disk will be much more difficult. 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 blue dot of light.
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 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 eight moons, one of which, Triton has a tenuous atmosphere of nitrogen and at nearly 1,700 mi (2,700 km) in diameter, 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.
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 independently were 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.
Interestingly, next year Neptune will have completed one full trip around the sun since its discovery.
Galileo saw it!
A strange fact about Neptune is that it was very nearly discovered by none other than Galileo with his crude telescope more than 2 and a half centuries earlier.
It was while observing Jupiter and its system of four large satellites on December 28, 1612 that, in the very same field of view, Galileo unknowingly recorded Neptune as an eighth magnitude star. Just over one month later on January 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!
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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.