Neptune

Provenance-Backed Scientific Measurements

Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.

J2000 Equatorial Sky Chart & Culmination Altitude

The chart below plots Neptune — Outermost Ice Giant Reference Data at RA 23h 58m 00.00s, Dec -01° 35′ 00.0″ within Pisces, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.

Astrophysical Context & Structure

Found on the night of September 23, 1846, by Johann Gottfried Galle and Heinrich d'Arrest at the Berlin Observatory within one degree of the coordinates calculated independently by Urbain Le Verrier (and John Couch Adams) from unexplained irregularities in Uranus's motion, Neptune was the first world discovered by mathematical prediction before being recognized at the eyepiece. Orbiting 30.07 astronomical units from the Sun, it takes 164.79 years to complete a circuit—having finished its first full orbit since discovery in July 2011.

Although Neptune is 3 percent smaller in diameter than Uranus (49,528 kilometers at the equator), it is 18 percent more massive (17.15 Earth masses), making it the densest giant planet in the Solar System at 1.638 grams per cubic centimeter. Unlike quiescent Uranus, Neptune radiates 2.61 times as much thermal energy as it absorbs from the distant Sun. That internal heat engine drives the fastest sustained winds in the Solar System—reaching 2,100 kilometers per hour—along with transient anticyclonic dark vortices (such as the Great Dark Spot imaged by Voyager 2 in 1989) and high-altitude methane-ice cirrus clouds.

Neptune's largest moon, Triton, is a 2,707-kilometer cryovolcanic world orbiting in a retrograde, inclined 5.88-day path—clear evidence that it is a captured Kuiper Belt dwarf planet. Beyond its orbit, Neptune's gravity sculpts the architecture of the trans-Neptunian belt, locking Pluto and the Plutinos into a stable 3:2 mean-motion resonance.

Practical Visual Observing Guide

• At magnitude V ~ 7.8, Neptune is never visible to the naked eye, but is easily located in 10x50 binoculars or a small finder scope.
• Resolving its tiny 2.3-arcsecond azure disk requires at least 150x–200x magnification in a 4-inch to 6-inch telescope under steady seeing.
• Its largest moon, Triton (V = 13.5), orbits up to 16 arcseconds from the planet and is a rewarding target for 8-inch to 12-inch amateur telescopes.

Catalog Cross-Identifications & Aliases

Common Aliases: Le Verrier's Planet · Poseidon · Sol VIII

Related Objects, Comparisons & Guides

Scientific Sources & Dataset Provenance

• NASA/JPL Solar System Dynamics & Planetary Fact Sheets (DE441 (2021/2026)): Park, R. S., et al. 2021, AJ, 161, 105 (The JPL Planetary and Lunar Ephemerides DE440 and DE441) — NASA Jet Propulsion Laboratory / GSFC