OVERVIEW
] MOONIapetus is a moon of Saturn. It orbits Saturn at a mean distance of 3 561 700 . It has a mass of 3.02×10⁻⁴ and a mean radius of 734.3 . It was discovered in 1671.
Two-faced: one hemisphere as dark as coal, the other as bright as snow, with an equatorial ridge of mountains running around it like a seam.
- WHAT IT IS
- Moon of Saturn
- HOW FAR
- 3 561 700
- HOW BIG
- 734.3
- ONE ORBIT TAKES
- 79.3
Its average distance from Saturn, not from Earth.
Its mean radius: half the width across.
One trip round Saturn.
THE MODEL
] CATALOGUED RADIUSCALCULATED · NOT AN OBSERVATION · CATALOGUED RADIUS · DENSITY 1.09 G/CM³ · RADIUS 734 KM · LIT FROM WHERE THE SUN IS · SURFACE FROM A GLOBAL MAP · AT ITS REAL LONGITUDES
Drawn from the catalogued radius and from the same properties the picture at the top of this page uses, at the distance a flight to it would stop. The surface is a global map rather than a pattern — a mosaic of real images, wrapped on by latitude and longitude. It is Cassini's colour, stretched past what the eye separates and into the ultraviolet and the infrared, which is how the differences in the ice are made visible. Longitude zero on it is laid where longitude zero on the body actually is at the moment this page was built, so a feature is drawn where the feature is. The map comes from NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute, in the public domain. Drag it, or use the arrow keys, to look from another direction. The stars behind are the 498,631 this catalogue holds real distances for, at their real positions and drawn from where the camera actually is — so the constellations are wrong, and that is the point. This is the sky from there, not the sky from here.
IMAGES
] 4 FRAMES- NASA / JPL / SSI / Gordan Ugarkovic
CC BY-SA 2.5 - NASA / JPL / Space Science Institute
Public domain - Wikideas1
CC0 - Wikideas1
CC0
OBSERVING
] FROM WHERE YOU AREWhat it takes to see Iapetus for yourself. Everything in this part of the page is worked out for a real horizon, a real night and the equipment you say you have.
CATALOG DATA
] 25 FIELDSThe raw catalogue values, as the source published them, with the error bars where one was published — sorted by what each set of figures is about rather than by the order they happened to be stored in.
WHAT KIND OF THING IT IS
What the object is in itself, rather than how it happens to look from here.
- Mean density
- 1.0887 ±0.013
THE ORBIT
The shape of the path, where the object sits on it, and the instant those figures were true of. An orbit is a fit to observations and drifts away from the real one either side of its epoch.
- Orbits
- Saturn
- Orbital period
- 79.331002
- Eccentricity
- Inclination
- 7.6
- Longitude of ascending node
- 86.5
- Argument of periapsis
- 254.5
- Mean anomaly
- 74.8
- Element epoch
- 2451545.0000
- Elements referred to
- the local Laplace plane — where the orbit's own node precesses
- Reference plane pole (right ascension)
- 288.7
- Reference plane pole (declination)
- 78.9
- Apsidal precession period
- 1662.9
- Nodal precession period
- 3130.302
- Mean anomaly fitted to the ephemeris
- 220.68
- Mean anomaly rate
- 4.53752
- Apsidal precession rate
- 0.000592715
- Nodal precession rate
- −0.000314866
- Agreement with the ephemeris
- 1.18
WHEN IT WAS FOUND
Who first saw it, from where, and by what method.
- Discovered by
- G.D. Cassini
OTHER NAMES AND NUMBERS
The identifiers other catalogues file this object under, for anyone joining this data to theirs.
- Also known as
- Japetus
- JPL satellite code
- 608
- Wikidata item
- Q17958
- Wikipedia article
- Iapetus (moon)
WHERE THESE FIGURES CAME FROM
Which catalogue, paper or model each of the fields above was read from, where the source named one.
- Ephemeris
- SAT441
ALSO KNOWN AS
] 1 DESIGNATIONS- Common name Iapetus
CITE THIS PAGE
] DR1This page is live and its numbers move when an upstream refreshes, so the citation names the release the rows belong to rather than the day you read them. The release itself is frozen, republished as a bundle, and is what a data-availability statement should point at.
SpaceCatalog. Iapetus [catalogue entry]. Data release DR1 (2026-08-21). https://spacecatalog.org/object/iapetus
@misc{spacecatalog_iapetus,
author = {{SpaceCatalog}},
title = {{Iapetus}},
howpublished = {SpaceCatalog, data release DR1 (2026-08-21)},
year = {2026},
url = {https://spacecatalog.org/object/iapetus},
note = {Catalogue entry, data release DR1 (2026-08-21). Values from JPL Planetary Satellite Data.}
}The values above were taken from JPL Planetary Satellite Data — No licence stated; citation requested. Citing this page does not replace citing that catalogue.
That catalogue asks for this acknowledgement: Solar System Dynamics. (Downloaded 2026, September 13). Planetary Satellite Mean Elements; Planetary Satellite Physical Parameters; Planetary Satellite Discovery Circumstances. https://ssd.jpl.nasa.gov