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Image courtesy NASA/JPL-Caltech Origin of name
Neptune is named after the Roman god of the sea.

Discovered by
Johann Gottfried Galle in 1846

Named after characters from Roman or Greek mythology associated with Neptune or Poseidon or the oceans.



Physical Properties


Property Unit Value Earth = 1
Equatorial radius, 1 bar level km 24 764 3.883
Polar radius, 1 bar level km 24 341 3.829
Flattening   0.01708 5.10
Mass  (x1024) kg 102.43 17.147
Volume  (x1010) km3 6254 57.74
Mean density kgm-3 1638 0.297
Gravity at equator, 1 bar level ms-2 11.15 1.14
Escape velocity kms-1 23.5 2.10
Magnetic field at equator  (x10-4) T 0.14 0.46
Magnetic dipole moment  (x1017) Tm3 ~ 2.1 ~ 25
Tilt of dipole axis ° 46.8  
Dipole field centre offset from planet centre (equatorial radii)   0.55  
Number of natural satellites   13  


Orbital Parameters


Property Unit Value
Perihelion  (x106) km 4444.45
Aphelion  (x106) km 4545.67
Mean distance from the Sun  (x106) km 4495.06
Mean distance from the Sun AU 30.05
Eccentricity   0.0113
Sidereal orbit period y 164.79
Sidereal rotation period
Magnetic coordinates (Voyager 2)
h 16.11
Length of day h 16.11
Orbit inclination to Ecliptic ° 1.769
Mean orbital velocity kms-1 5.43
Maximum orbital velocity kms-1 5.50
Minimum orbital velocity kms-1 5.37
Axial tilt ° 28.32


  • Sidereal orbit period
    The time taken by the planet to complete exactly one orbit around the Sun with respect to the celestial sphere.
  • Sidereal rotation period
    The time in which the planet rotates around its axis exactly 360° with respect to the celestial sphere.
  • Length of a day
    Defined as the time between two successive sunrises over the meridian.
  • Magnetic dipole moment
    Calculated as the ratio of the magnetic field strength at the equator divided by the cube of the equatorial radius.


Last Update: 1 September 2019
13-Jun-2024 07:25 UT

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