William Lassell, the discoverer of Triton
The orbit of Triton (red) is opposite in direction and tilted −23° compared to a typical moon's orbit (green) in the plane of Neptune's equator.
Animation of Triton
The Kuiper belt (green), in the Solar System's outskirts, is where Triton is thought to have originated.
Artist's impression of Triton, showing its tenuous atmosphere just over the limb.
Clouds observed above Triton's limb by Voyager 2.
Interpretative geomorphological map of Triton
Triton's bright south polar cap above a region of cantaloupe terrain
Cantaloupe terrain viewed from 130,000 km by Voyager 2, with crosscutting Europa-like double ridges. Slidr Sulci (vertical) and Tano Sulci form the prominent "X".
Tuonela Planitia (left) and Ruach Planitia (center) are two of Triton's cryovolcanic "walled plains". The paucity of craters is evidence of extensive, relatively recent, geologic activity.
NASA illustration detailing the studies of the proposed Trident mission
Neptune (top) and Triton (bottom) three days after flyby of Voyager 2
thumb|Close up of the volcanic province of Leviathan Patera, the caldera in the center of the image. Several pit chains extend radially from the caldera to the right of the image, while the smaller of the two cryolava lakes is seen to the upper left. Just off-screen to the lower left is a fault zone aligned radially with the caldera, indicating a close connection between the tectonics and volcanology of this geologic unit.
thumb|Dark streaks across Triton's south polar cap surface, thought to be dust deposits left by eruptions of nitrogen geysers
thumb|Two large cryolava lakes on Triton, seen west of Leviathan Patera. Combined, they are nearly the size of Kraken Mare on Titan. These features are unusually crater free, indicating they are young and were recently molten.

Triton is the largest natural satellite of the planet Neptune, and was the first Neptunian moon to be discovered, on October 10, 1846, by English astronomer William Lassell.

- Triton (moon)

In 1846, Lassell discovered Triton, the largest moon of Neptune, just 17 days after the discovery of Neptune itself by German astronomer Johann Gottfried Galle, using his self-built instrument.

- William Lassell

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Flamsteed House in 1824

Royal Observatory, Greenwich

Observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to the north.

Observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to the north.

Flamsteed House in 1824
Royal Observatory, Greenwich c. 1902 as depicted on a postcard
Greenwich Observatory (Latinized as "Observatorium Anglicanum Hoc Grenovici prope Londinum"), as illustrated in Johann Gabriel Doppelmayr's map of the southern celestial hemisphere, ca. 1730
Royal Observatory, Greenwich
Dome of the Greenwich 28 inch refractor telescope and tree
The Airy Transit Circle, used for over a century (1851–1953) as the reference point when charting the heavens and determining times, thus earning for it the epithet "the centre of time and space"
The building housing the origin of the Greenwich Prime Meridian
Laser projected from the observatory marking the Prime Meridian line
Laser at night
Shepherd Gate Clock at Royal Greenwich Observatory
One of the hyper-accurate timekeepers at the observatory
The time ball is the red ball on a post – when it drops a certain time is signalled. This allowed clocks to be set from afar with great accuracy, particularly the chronometers of ships on the River Thames below, prior to sailing. The observatory would first determine the time by stellar observations.
Dome of the Great Equatorial Building overlooking Greenwich Park
21st-century view of the Altazimuth Pavilion
Standard lengths on the wall of the Royal Observatory, Greenwich, London – 1 yard (3 feet), 2 feet, 1 foot, 6 inches (1/2-foot), and 3 inches. The separation of the inside faces of the marks is exact at an ambient temperature of 60 °F and a rod of the correct measure, resting on the pins, will fit snugly between them.
Aerial view of the Royal Greenwich Observatory, Herstmonceux site in East Sussex; the dome that formerly housed the Isaac Newton Telescope is the single dome to the right. The telescope was moved to La Palma in the Canary Isles in 1979.
Former Royal Greenwich Observatory, Herstmonceux, East Sussex
Greenwich House at Cambridge
The Queen's House (centre left) at Greenwich, with the Royal Observatory on the skyline behind, in 2017.
The Magnetic Pavilion, 1900
Tourists flock to the Observatory museum, 2009
The centuries-old Flamsteed House overlooking Greenwich Park in London. The statue at left is of Major General James Wolfe, who died capturing Quebec in 1759, and was buried in St Alfege Church, Greenwich.

The Lassell two-foot reflector was a famous metal-mirror telescope that had been used to discover the Moons Triton and Hyperion.

The scheme of Neptune's ring-moon system. Solid lines denote rings; dashed lines denote orbits of moons.

Rings of Neptune

Observing program proposed by André Brahic and Bruno Sicardy from Paris Observatory, and at Cerro Tololo Interamerican Observatory by F. Vilas and L.-R.

Observing program proposed by André Brahic and Bruno Sicardy from Paris Observatory, and at Cerro Tololo Interamerican Observatory by F. Vilas and L.-R.

The scheme of Neptune's ring-moon system. Solid lines denote rings; dashed lines denote orbits of moons.
A pair of Voyager 2 images of Neptune's ring system
A Voyager ring image shown at increased brightness to bring out fainter features
Arcs in the Adams ring (left to right: Fraternité, Égalité, Liberté), plus the Le Verrier ring on the inside

Neptune's rings are named after astronomers who contributed important work on the planet: Galle, Le Verrier, Lassell, Arago, and Adams.

The first mention of rings around Neptune dates back to 1846 when William Lassell, the discoverer of Neptune's largest moon, Triton, thought he had seen a ring around the planet.

Photograph taken by NASA's Voyager 2 in 1989


Eighth and farthest-known Solar planet from the Sun.

Eighth and farthest-known Solar planet from the Sun.

Photograph taken by NASA's Voyager 2 in 1989
Galileo Galilei
Urbain Le Verrier
A size comparison of Neptune and Earth
Combined colour and near-infrared image of Neptune, showing bands of methane in its atmosphere, and four of its moons, Proteus, Larissa, Galatea, and Despina
Bands of high-altitude clouds cast shadows on Neptune's lower cloud deck.
The Great Dark Spot (top), Scooter (middle white cloud), and the Small Dark Spot (bottom), with contrast exaggerated.
Four images taken a few hours apart with the NASA/ESA Hubble Space Telescope Wide Field Camera 3
Neptune (red arc) completes one orbit around the Sun (centre) for every 164.79 orbits of Earth. The light blue object represents Uranus.
A diagram showing the major orbital resonances in the Kuiper belt caused by Neptune: the highlighted regions are the 2:3 resonance (plutinos), the nonresonant "classical belt" (cubewanos), and the 1:2 resonance (twotinos).
A simulation showing the outer planets and Kuiper belt: a) before Jupiter and Saturn reached a 2:1 resonance; b) after inward scattering of Kuiper belt objects following the orbital shift of Neptune; c) after ejection of scattered Kuiper belt bodies by Jupiter
Natural-colour view of Neptune with Proteus (top), Larissa (lower right), and Despina (left), from the Hubble Space Telescope
Neptune's moon Proteus
A composite Hubble image showing Hippocamp with other previously discovered inner moons in Neptune's ring system
Neptune's rings
In 2018, the European Southern Observatory developed unique laser-based methods to get clear and high-resolution images of Neptune from the surface of Earth.
A Voyager 2 mosaic of Triton
The appearance of a Northern Great Dark Spot in 2018 is evidence of a huge storm brewing.<ref>{{cite web |title=A storm is coming |url=https://www.spacetelescope.org/images/potw1907a/ |website=spacetelescope.org |access-date=19 February 2019 |language=en |archive-url=https://web.archive.org/web/20190220062857/https://www.spacetelescope.org/images/potw1907a/ |archive-date=20 February 2019 |url-status=live }}</ref>
The Northern Great Dark Spot and a smaller companion storm imaged by Hubble in 2020<ref>{{cite web|url=https://hubblesite.org/contents/news-releases/2020/news-2020-59.html|title=Dark Storm on Neptune Reverses Direction, Possibly Shedding Fragment|author1=Michael H. Wong|author2=Amy Simon|publisher=Hubblesite|date=15 December 2020|access-date=25 December 2020|archive-date=25 December 2020|archive-url=https://web.archive.org/web/20201225153808/https://hubblesite.org/contents/news-releases/2020/news-2020-59.html|url-status=live}}</ref>
The Great Dark Spot, as imaged by Voyager 2
Neptune's shrinking vortex<ref>{{cite web|title=Neptune's shrinking vortex|url=http://www.spacetelescope.org/images/potw1808a/|website=spacetelescope.org|access-date=19 February 2018|archive-url=https://web.archive.org/web/20180219125043/http://www.spacetelescope.org/images/potw1808a/|archive-date=19 February 2018|url-status=live}}</ref>

Its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 13 known moons were located telescopically until the 20th century.

Triton was discovered by William Lassell just 17 days after the discovery of Neptune itself.