Neptune reaches opposition next week, giving amateurs the chance to track its unique, backwards-orbiting moon Triton.

Worlds of gas and ice
Neptune and its largest moon Triton (1,700 miles in diameter) photographed on September 25, 2010. Neptune's blue color is obvious. Triton, like Pluto, is pink, but too faint to show color in amateur telescopes.
Damian Peach

I've gotten into the habit lately of thinking Saturn's the only remaining evening planet. Far from it. Two more planets have quietly joined the skyscape in recent weeks — Uranus and Neptune. Because both require optical aid, we often forget they're there. Neptune reaches opposition on August 31 when it will shine at magnitude +7.8 and present a tiny disk just 2.3″ across. Uranus follows suit the night of October 11.

Neptune and Earth compared
Neptune, the fourth largest planet, is nearly four times Earth's size. Methane in its atmosphere gives it a characteristic blue color.

Voyager 2, which flew by Neptune in August 1989, sent back photos of an aquamarine globe blemished by a huge, spinning storm called the Great Dark Spot. The planet owes its remarkable blue hue to absorption of red light by methane in the atmosphere. I've found that Neptune's color varies depending on how long you stare at it and how tired your eyes are. Sometimes it's an obvious blue, other times a washed-out green or greenish-yellow. What do you see?

Galileo was the first to see Neptune on December 27, 1612, while observing Jupiter and its moons. The two planets happened to be in close conjunction at the time, with Neptune in the same field of view. Using only an inch-and-a-half telescope and 30x, he spotted the planet and recorded its position on two occasions, but apparently mistook it for a faint star.

Any pair of 7x binoculars mimics Galileo's view of Neptune, which looks like a slow-crawling "star" just west of the 4th magnitude Lambda Aquarii this season. A 3-inch or larger telescope at 75x hints of its non-stellar nature, and if you increase the magnification to 150x or higher, a tiny blue disk emerges. On nights of good seeing, when the disk focuses crisply, you can sense how remote the planet — nearly four times the size of our own — must be to look so minute. Is this how the Earth, suitably magnified, appears from Neptune, I wonder?

Neptune — wide and tight
The combination map shows the sky facing southeast in late August–early September around 10 o'clock local time from mid-northern latitudes. Your keystone star is Lambda Aquarii, circled in red. Once you've found it, use the detailed chart to spot Neptune in binoculars and telescopes. Neptune's position is shown for August 26, 2015. Click to download detailed charts for Neptune and Uranus.
Sky & Telescope / Stellarium

Neptune is the only planet discovered by mathematical prediction rather than staring us in the face or through accidental telescopic discovery. French mathematician Urbain Leverrier sent his predictions (based on irregularities in Uranus' motion) of the planet's location to German astronomer Johann Galle, who found it after only a half hour of looking on September 23, 1846.

And Then Came Triton

17 days later on October 10, another amateur, William Lassell, nabbed Triton, following a suggestion made by astronomer John Herschel that a search for moons might prove fruitful. One could argue that beer was crucial to the discovery of Triton. Lassell ran a successful brewery business and used some of the money to build the telescope that revealed the first of the planet's 14 known moons.

Icy weirdness on par with Pluto
Covered in nitrogen frost and tholins (dark organic compounds that form when solar UV alters methane gas), Triton is one of the coldest places in the Solar System. Its surface displays a peculiar netted or "cantelope" terrain and active cryovolcanoes.

No one bothered to formally name Triton until Neptune's second satellite, Nereid, was discovered in 1949, calling it simply "the satellite of Neptune." Named for the son of Poseidon, the Greek equivalent of Neptune, Triton resides 2.8 billion miles from the Sun. It's so cold there that nitrogen condenses as frost, brightening the moon's crust and making it an effective reflector of light. Triton turns back an average of 76% of the light it receives from the Sun, which helps to chill its surface down to –391° F (–235° C), nearly identical to that of Pluto. Brrr!

Never one to play hide-and-seek
Triton's steep inclination, paired with its retrograde motion around Neptune, make it appear to orbit in a broad ellipse without ever passing in front or behind the planet. It completes one orbit in 5.9 days. South is up.
Bob King

Triton shines at magnitude 13.5, well within the range of 8-inch and larger telescopes under dark skies. That and the fact that it's the only large moon in the Solar System that revolves around its host planet backwards — or retrograde — makes it a fascinating observing target. If you've looked at Jupiter and Saturn through a telescope, you know that their moons orbit from east to west in the same direction as the planets rotate. Likewise Earth and its Moon.

Triton on the run
Observers can easily watch Triton circle about Neptune from one night to the next during its 5.9-day period. This panel shows the moon's positions (south up) around 10 p.m. CDT on the nights shown. Trtion's separation from Neptune varies from about 10″ to 17″. 
Sky and Telescope / Bob King

Not Triton. You've got to see it with your own eyes to believe it. The moon circles Neptune from west to east every 5.9 days, inclined 157° to the planet's equator. Currently, Triton's steep tilt makes it trace out a nearly circular path, so it's visible nearly every clear night. No planet to get in the way! You'll have lots of opportunities to see Triton with Neptune well placed now through December.

Quick trip to Triton
Click and you'll be taken to Sky and Telescope's Triton Tracker, where you can see where the moon is at any date and time.

I've spotted Triton faintly with averted vision in my 10-inch scope at 254x, nestled right next to Neptune like a close, unequal double star. My 15-inch makes easy work of it. Remember to use high magnification and averted vision.

Once you've seen Triton a few times and familiarized yourself with its appearance and relation to Neptune, you'll always remember the way back. If you're not sure or want to confirm your observation, just use the Sky & Telescope Triton Tracker. The tracker will also help you determine the times the moon reaches its greatest separation from Neptune and is easiest to see.

The question remains of Triton's independent nature. Why does it rotate in the opposite direction of Neptune's rotation? Some astronomers think it's a refugee from the Kuiper Belt captured by Neptune's gravity. It may even have been paired with another asteroid, which Neptune peeled away and flung to parts unknown while keeping Triton for itself.

We do know this. Neptune's gravity acts as a break on the contrary-orbiting Triton, slowing it down and making it drop closer and closer to the planet. One day, millions of years from now, Neptune's gravity will tear it to pieces. What remains may spread into a series of rings of shattered rock and ice. A glorious end to the story of a wayward moon!

Tracking planetary satellites? Try the Sky & Telescope JupiterMoons and SaturnMoons apps!


Image of SNH


August 28, 2015 at 4:11 pm

Just a couple weeks ago I spotted Triton for the first time in my 10" SCT using 395x - and it was pretty darn easy! I liked your article and hope that it encourages many more people that haven't tried to spot Triton to do so. One reason I found it so extremely easy the first time was because it was at greatest elongation, a fact I found out two days later when it was closer in to the planet and a bit harder! My next challenge: spotting one of Uranus's moons. Wish me luck!

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