March 30, 2020

Astro-Challenge: Spotting Triton.

Triton as seen from Voyager 2. (Credit: NASA/JPL).

This week’s Astro-Challenge is definitely for the “been there, done that” crowd, a hunt that will put that gia-normous light bucket you’ve got sitting in your backyard to good use. August 22nd, the planet Neptune reaches opposition. And yes, we know you’ve spotted the grey-blue world at magnitude +7.9 with no problem, but have you ever seen its elusive moon, Triton? Now is a good time to try and check this fascinating moon off of your “life list.”

But first, the “Wow” factor stuff… discovered on October 10th, 1846 by William Lassell only 17 days after the discovery of the planet Neptune itself, this moon is 2,706 kilometers in diameter, making it larger than Pluto and 7th in size in terms of solar system satellites. We’ve seen Triton close-up exactly once during the Voyager 2 flyby on August 25th 1989, which caught 40% of its terrain at a passage 130,000 kilometers distant. It revealed a variegated, ‘cantaloupe-like’ terrain, a world that is geologically active and perhaps subject to cryo-volcanism much like Cassini has observed on Saturn’s moon, Enceladus. Interestingly, Triton is the only large moon with a retrograde orbit in the solar system, and was probably captured by Neptune from the Kuiper Belt somewhere in its past. Triton also bears some intriguing similarities to Pluto, and it will be interesting to compare the two once New Horizons completes its flyby of the Pluto-Charon system in 2015. Models of the interior suggest a differentiated core-mantle-crust that is subject to heating via radioactive decay and tidal stress from nearby Neptune, which in turn fuels said cryo-volcanism, and may even make for a sub-surface ocean like the one theorized to exist beneath the crust of Jupiter’s moon Europa possible… this could also put Triton on the short list of places in the solar system that may harbor life. Triton also has the highest albedo of any object in the solar system, reflecting back 60-95% of the sunlight falling on its methane-ice laced with ammonia snow-covered surface. Triton’s departure from Neptune’s orbital plane varies over a period of 678 Earth years from a minimum of 127° to 173°, and is currently about 130° wide, close to that lower figure. This means that Triton can indeed transit Neptune from two points along the planet’s 164.79 year orbit, which happened last in 1951-55 and will happen again starting in 2033 A.D. (catching such an event would be a true challenge!).  Triton presented its widest orbit to us in 1994, and is still reasonably open to us 17 years later. Triton also has a synchronous rotation facing Neptune like our Moon does with the Earth, and its hemispheres alternate facing the Sun for 82 Earth year periods.


Neptune at opposition, 2011: a wide-field view. (Created by the Author in Starry Night).

So, just how difficult is it to spot Neptune’s largest Moon? Well, Lassell used a 24” Newtonian reflector on discovery; however, mirrors of the day where made of speculum metal with only about 40-60% reflectivity newly finished; at about magnitude +13.5, a minimum aperture of 8” under dark skies should be able to duplicate the feat. The best bet is to catch the moon at greatest elongation, east or west of the planet. 2.4” arc seconds in diameter, Neptune will look like a grayish-blue disc, barely discernable and refusing to focus to a star-like point. Triton never strays from Neptune more than about 18” max, roughly the apparent size of the globe of Saturn from Earth.

Neptune’s current position in the constellation Aquarius near the Capricornus border is;

Right Ascension: 22h 7.3’ minutes

Declination: -12degrees 9.4’ minutes

Center the coordinates and sweep the field, or locate the star Iota Aquarii and search about 2 degrees to the northeast.

Triton orbits Neptune once every 5.877 days, which means a shot at greatest elongation comes around not quite every 3 days… some of the best chances over the next month are as follows;


19 AUG


22 AUG
24 AUG
27 AUG
30 AUG
02 SEP
05 SEP
08 SEP
11 SEP
14 SEP
17 SEP

(Ephemeris created by Ed Kotapish using pds-rings Neptune Tracker tool).

Triton approaching greatest elongation the night of August 24th…(Created by Author in Starry Night).

Thanks goes out to Ed Kotapish for working these out… note that good elongations for North American observers are in the offing for the evenings of August 19th, 22nd, & 24th; Triton is in an open, slow moving orbit, meaning that its worth trying for a sighting 12 hours + or – of these times. An occulting bar eyepiece may help hide the “glare” of Neptune… good luck!

The Astro-word for the week is Roche Limit. This is the point at which an approaching body would disintegrate due to the tidal forces enacted upon it by a larger mass by overcoming its own internal gravitation holding the smaller body together. This was the case that led to the creation of Saturn’s ring system, and indeed, it is now known that every large gas giant world including Neptune has just such a ring system. There for, such a disintegration event must be a common occurance. Lassell also made note of a ring system around Neptune during his discovery of Triton (!) but it is doubtful that he sighted the tenuous rings that Voyager 2 spotted on its reconnaissance of the system. Curiously, some thoughts are out there recently that even Pluto may have rings, a very planet-like thing to possess…hey, I’m just sayin… This concept is important to Triton because its orbit is not entirely stable. In about 3.6 billion years (about the time our Sun will start thinking about fusing heavier elements and flaring up into a red giant) Triton will approach its Roche limit as drag forces it to spiral in towards Neptune and it will undergo its own unique Armageddon, shredding apart into a new ring system for denizens of our future solar system to enjoy!



  1. [...] -      24th: Neptune reaches opposition, a good time to try your hand at Neptunian moon spotting. [...]

  2. [...] discovered by a deliberate systematic search in 1846, and the first Neptunian moon discovered was Triton spotted by William Lassell on October 10th, 1846, just 17 days after the planet itself was found. [...]

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