Neptune is the more massive of the two ice giant planets, the fourth-largest planet in our Solar System, and the eighth and most distant from the Sun. Neptune is the fourth largest planet in our solar system by diameter and the third largest by mass. It measures approximately 49,528 kilometers in diameter, which is about four times the size of Earth. In terms of mass, Neptune weighs a staggering 17 times more than our home planet.
Neptune is technically visible to the naked eye under perfect conditions, but in practice, it’s too dim, and you’ll need a telescope or binoculars to see it. The planet was, amazingly, first observed by Galileo in 1612 and 1613, and he even noticed that it moved across the sky, but he could not resolve its disk with his weak telescope and dismissed its motion as an observational error.
Other observers – including John Herschel, the son of William Herschel (who discovered Uranus in 1781), saw Neptune, but it took the work of early astrophysicist Urbain Jean-Joseph Le Verrier to predict Neptune’s existence, and Johann Gottfried Galle became the first person to both observe and recognize Neptune for what it was – a new planet – in 1846. Le Verrier also went on to postulate the existence of a planet closer to the Sun than Mercury, termed “Vulcan”, which was only disproven upon the discovery of Einstein’s theories of general relativity.
Due to its distance from the Sun, only one spacecraft, NASA’s Voyager 2, has visited Neptune, and it flew by in 1989. No missions to Neptune are currently planned by any country or space agency, as Uranus is quicker and easier to get to and remains similarly unexplored by spacecraft.
Neptune isn’t observed often by most astronomers, though it’s worth a look and fairly easy to locate.
When to Observe Neptune
Neptune is currently (as of 2023/4) in the constellation of Pisces, the Fish, and will remain there for quite some time thanks to its 187-year orbital period around the Sun. Neptune reaches opposition in late September for the next few years, and for Northern Hemisphere observers, it gets very high in the sky.
Neptune doesn’t change significantly in angular size, distance, or brightness due to its distance from the Sun; it varies in distance from us by just two astronomical units, whereas it is 30 astronomical units from the Sun on average (1 AU average is the distance from the Earth to the Sun). Neptune hovers around magnitude 8, theoretically visible to experienced astronomers with the naked eye under ideal conditions, but in practice relegated to binoculars, finder scopes, and telescopes due to its dimness – and more often than not, also due to light pollution.
Neptune is in the constellation Pisces right now, and its bluish color is easy to confuse with that of bright stars close by. It’ll show up in a magnifying finder or binoculars easily, but consulting a very accurate paper star chart or an app with real-time location information on where Neptune is in the sky is key to success. Neptune’s color is a little more of a deep blue than any star’s, but you might be hard pressed to tell the difference. In a telescope, you need at least 80x magnification to see that Neptune is “fuzzy” compared to nearby stars.
Observing Neptune through a Telescope
Neptune’s angular size is tiny, reaching no more than 2.5 arc seconds in diameter. Jupiter’s moons are only slightly smaller in apparent diameter, and the smallest telescopes (3” or less) actually lack the resolving power to perceive Neptune’s disk at all. Bad atmospheric conditions can also blur Neptune, making it unrecognizable as obviously “fatter” than surrounding stars. On a good night with a large telescope, however, Neptune’s dark blue disk can be resolved. Neptune’s rings are far too faint and small to be seen in any amateur telescope, and even gigantic telescopes like the Hubble, James Webb, or Keck telescopes don’t resolve much more than blurry and pixelated blobs of detail, such as storms, in its cloud tops. As of 2023, some observers have noted a small whitish spot in the planet’s atmosphere, which may match up with one of the whitish spots seen in the September 2022 JWST photo of the planet. It will require at least a 12” telescope and a magnification of 250x or greater to spot this and any other atmospheric details.
Neptune’s moon Triton is arguably easier to see than the disk of Neptune itself. At magnitude 13.5, Triton is brighter than Pluto or any of the moons of Uranus and within the range of 6-8” telescopes, even under fairly light-polluted skies. Triton is about the size of Pluto but extraordinarily reflective, making it quite bright despite its distance from the Sun. Thanks to Neptune’s relatively low brightness, you’ll have no issues with Triton hiding in its glare, unlike the similarly dim moons of Saturn and Uranus. Triton orbits Neptune backwards (clockwise instead of counterclockwise as seen from above) and is probably a captured dwarf planet like Pluto.
Triton being originally a separate planetary body is a theory that would explain why Neptune lacks any remotely large other moons – the next largest, Proteus, is ⅙ the size of Triton and probably accumulated debris orbiting Neptune after Triton arrived. The third largest moon of Neptune – and the most easily seen besides Triton – is Nereid, which is slightly smaller than Proteus and takes 1 year to orbit around Neptune, though you’ll need a 25” or larger telescope to see it at all on account of its apparent magnitude hovering around 19. Proteus is simply out of reach of amateur telescopes, as are all of the other smaller and dimmer Neptunian moons.
Conjunctions and Occultations of Neptune
Neptune frequently appears in conjunction with other planets in the night sky owing to its proximity to the ecliptic plane. These events can provide thrilling opportunities for stargazers and astronomers to catch a glimpse of this distant, icy giant. Occasionally, these are close conjunctions where both objects will fit in the same telescopic field of view. For instance, on April 3rd, 2024, it appears near Venus less than half a degree away – less than the apparent diameter of the moon—low in the pre-dawn sky. On April 28, 2024, it will be very close to Mars.
The nimble inner planets pass by Neptune rather quickly, but throughout mid-2025, the ice giant will be around a degree from Saturn, and neither will appear to move much owing to their distance and slower orbital velocities.
Neptune can also be occulted, or blocked, by the Moon, though for many people only a close conjunction between the two bodies will be observed. Several of these events will occur in 2024, with occultations visible from Oceania.
Neptune enters a conjunction with the Sun every 367 days between oppositions, when it is unobservable and at its furthest from us. This period, known as the superior conjunction, lasts a few weeks. During this time, Neptune is located on the far side of the Sun from the Earth, making it lost in the Sun’s glare and impossible to observe.
Color Filters for Observing Neptune
Filters are not of any value on Neptune and may dim the view too much.
Neptune is named after the Roman god of the sea, reflecting its deep blue color, reminiscent of the ocean. Analogous to the Greek god Poseidon, the name was proposed by Le Verrier, and it follows the Greco-Roman mythology naming convention used for other planets in the solar system.
Like Uranus, Neptune is classified as an “ice giant”, a type of gas giant planet, and is primarily composed of various ices and rock. Its upper atmosphere is mostly hydrogen and helium, with a small amount of methane. Deeper within the planet, there is a thicker layer of molecular hydrogen, helium, water, ammonia, and methane ices” (though they are all quite hot and are either in a liquid or gas state in Neptune’s interior; the “ice” terminology is due to relict naming conventions). At its core, Neptune has a solid center composed of rock and metal. These different layers give Neptune its unique characteristics and make it an intriguing subject of study in the fields of astronomy and planetary science. The vibrant blue color around Neptune is caused by the presence of methane in its upper atmosphere. Methane absorbs red light and reflects blue light back into space, which results in the planet’s deep azure appearance when viewed from a distance.
Neptune, like all the planets in our solar system, has a tilt in its axis. With an axial tilt of about 28.32 degrees, Neptune’s tilt is similar to Earth’s, which is about 23.5 degrees. This tilt gives rise to seasons on Neptune, much like we experience on Earth, although each season lasts for several of Earth’s decades due to Neptune’s long orbital period around the sun. Saturn and Neptune are the only giant planets with an Earth-like axial tilt; Uranus’ is extreme at 98 degrees, while Jupiter’s axis rotation is only tilted by a few degrees relative to its orbit around the Sun.
Neptune holds the title of the windiest planet in our solar system. The planet’s swift rotation and extreme distance from the sun give rise to its exceptionally severe weather, with wind speeds reaching up to 2,100 kilometers per hour (around 1,300 miles per hour). These super-strong winds continuously shape and reshape the planet’s atmospheric features, such as the Great Dark Spot. Neptune’s constantly churning atmosphere keeps away methane haze, a phenomenon that occurs on other worlds with methane-rich atmospheres such as Uranus and Titan, producing an orange-yellow layer in the upper atmosphere – thus causing Uranus’ greener color despite its otherwise-similar composition to Neptune.
Three distinct, faint rings encircle Neptune. Unlike the prominent and highly visible rings of Saturn, Neptune’s rings are faint and made up of dust particles mixed with small chunks of ice. The rings are named Adams, Le Verrier, and Galle in honor of the planet’s co-discoverers. None are visible with an amateur telescope; even the James Webb Space Telescope shows them as thin slivers.
Neptune is accompanied by a system of 14 known moons, all named after various water deities in Greek and Roman mythology. The largest and most notable is Triton, which is the only Neptunian moon massive enough to be spherical. Triton is unique in the solar system because it orbits Neptune in the opposite direction to the planet’s rotation, a retrograde orbit that suggests it was likely a dwarf planet captured by Neptune’s gravity and was not originally part of the planet’s system. Triton’s capture is likely the reason Neptune has no other large moons; any others would have been ejected out into orbit around the Sun or flung into Neptune during the encounter.