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 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. 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.
John Herschel, the son of William Herschel, who found Uranus in 1781, also saw Neptune. However, it took the work of early astrophysicist Urbain Jean-Joseph Le Verrier to predict that Neptune existed. 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.
Neptune is termed an “ice giant” as it has a rocky core like Uranus and is unlike the much larger gas giants Jupiter and Saturn, and it also has much more water ice and vapor in its atmosphere than the true “gas giant” planets.
Neptune isn’t observed often by most astronomers, though it’s worth taking a look at and fairly easy to locate.
When to Observe Neptune
Neptune is currently (as of 2022/3) 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 to become 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, pixelated blobs of detail, such as storms, in its cloud tops.
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 same size as Pluto, but it is very reflective, which makes it very bright even though it is very far from the Sun. Since Neptune isn’t very bright, Triton won’t be able to hide in its light, unlike the moons of Saturn and Uranus, which are also not very bright. Triton orbits Neptune backwards, or clockwise instead of counterclockwise as seen from above. It is likely 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 from 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-inch 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.