Aplanetary coordinate system (also referred to asplanetographic,planetodetic, orplanetocentric)^[1][2] is a generalization of thegeographic,geodetic, and thegeocentriccoordinate systems forplanets other than Earth.Similar coordinate systems are defined for other solidcelestial bodies, such as in theselenographic coordinates for theMoon.The coordinate systems for almost all of the solid bodies in theSolar System were established byMerton E. Davies of theRand Corporation, includingMercury,^[3][4]Venus,^[5]Mars,^[6] the fourGalilean moons ofJupiter,^[7] andTriton, the largestmoon ofNeptune.^[8]Aplanetary datum is a generalization ofgeodetic datums for other planetary bodies, such as theMars datum; it requires the specification of physical reference points or surfaces with fixed coordinates, such as a specific crater for the reference meridian or the best-fittingequigeopotential as zero-level surface.^[9]

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The longitude systems of most of those bodies with observable rigid surfaces have been defined by references to a surface feature such as acrater. The north pole is that pole of rotation that lies on the north side of theinvariable plane of the Solar System (near theecliptic). The location of the prime meridian as well as the position of the body's north pole on the celestial sphere may vary with time due to precession of the axis of rotation of the planet (or satellite). If the position angle of the body's prime meridian increases with time, the body has a direct (orprograde) rotation; otherwise the rotation is said to beretrograde.

In the absence of other information, the axis of rotation is assumed to be normal to the meanorbital plane;Mercury and most of the satellites are in this category. For many of the satellites, it is assumed that the rotation rate is equal to the meanorbital period. In the case of thegiant planets, since their surface features are constantly changing and moving at various rates, the rotation of theirmagnetic fields is used as a reference instead. In the case of theSun, even this criterion fails (because its magnetosphere is very complex and does not really rotate in a steady fashion), and an agreed-upon value for the rotation of its equator is used instead.

Forplanetographic longitude, west longitudes (i.e., longitudes measured positively to the west) are used when the rotation is prograde, and east longitudes (i.e., longitudes measured positively to the east) when the rotation is retrograde. In simpler terms, imagine a distant, non-orbiting observer viewing a planet as it rotates. Also suppose that this observer is within the plane of the planet's equator. A point on the Equator that passes directly in front of this observer later in time has a higher planetographic longitude than a point that did so earlier in time.^[10]

However,planetocentric longitude is always measured positively to the east, regardless of which way the planet rotates.East is defined as the counterclockwise direction around the planet, as seen from above its north pole, and the north pole is whichever pole more closely aligns with the Earth's north pole. Longitudes traditionally have been written using "E" or "W" instead of "+" or "−" to indicate this polarity. For example, −91°, 91°W, +269° and 269°E all mean the same thing.^[10]

The modern standard for maps of Mars (since about 2002) is to use planetocentric coordinates. Guided by the works of historical astronomers,Merton E. Davies established the meridian of Mars atAiry-0 crater.^[11][12] ForMercury, the only other planet with a solid surface visible from Earth, a thermocentric coordinate is used: the prime meridian runs through the point on the equator where the planet is hottest (due to the planet's rotation and orbit, the Sun brieflyretrogrades at noon at this point duringperihelion, giving it more sunlight). By convention, this meridian is defined as exactly twenty degrees of longitude east ofHun Kal.^[13][14][15]

Tidally-locked bodies have a natural reference longitude passing through the point nearest to their parent body: 0° the center of the primary-facing hemisphere, 90° the center of the leading hemisphere, 180° the center of the anti-primary hemisphere, and 270° the center of the trailing hemisphere.^[16] However,libration due to non-circular orbits or axial tilts causes this point to move around any fixed point on the celestial body like ananalemma.

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Planetographic latitude andplanetocentric latitude may be similarly defined.The zerolatitude plane (Equator) can be defined as orthogonal to the meanaxis of rotation (poles of astronomical bodies).The reference surfaces for some planets (such as Earth andMars) areellipsoids of revolution for which the equatorial radius is larger than the polar radius, such that they areoblate spheroids.

Vertical position can be expressed with respect to a givenvertical datum, by means of physical quantities analogous to thetopographicalgeocentric distance (compared to a constantnominal Earth radius or the varyinggeocentric radius of the reference ellipsoid surface) oraltitude/elevation (above and below thegeoid).^[17]

Theareoid (the geoid ofMars)^[18] has been measured using flight paths of satellite missions such asMariner 9 andViking. The main departures from the ellipsoid expected of an ideal fluid are from theTharsis volcanic plateau, a continent-size region of elevated terrain, and its antipodes.^[19]

Theselenoid (the geoid of theMoon) has been measuredgravimetrically by theGRAIL twin satellites.^[20]

Reference ellipsoids are also useful for defininggeodetic coordinates and mapping other planetary bodies including planets, their satellites, asteroids and comet nuclei. Some well observed bodies such as theMoon andMars now have quite precise reference ellipsoids.

For rigid-surface nearly-spherical bodies, which includes all the rocky planets and many moons, ellipsoids are defined in terms of the axis of rotation and the mean surface height excluding any atmosphere. Mars is actuallyegg shaped, where its north and south polar radii differ by approximately 6 km (4 miles), however this difference is small enough that the average polar radius is used to define its ellipsoid. The Earth's Moon is effectively spherical, having almost no bulge at its equator. Where possible, a fixed observable surface feature is used when defining a reference meridian.

For gaseous planets likeJupiter, an effective surface for an ellipsoid is chosen as the equal-pressure boundary of onebar. Since they have no permanent observable features, the choices of prime meridians are made according to mathematical rules.

For theWGS84 ellipsoid to modelEarth, thedefining values are^[21]

a (equatorial radius): 6 378 137.0 m

${\displaystyle \frac{1}{f}}$ (inverse flattening): 298.257 223 563

from which one derives

b (polar radius): 6 356 752.3142 m,

so that the difference of the major and minor semi-axes is 21.385 km (13 mi). This is only 0.335% of the major axis, so a representation of Earth on a computer screen would be sized as 300 pixels by 299 pixels. This is rather indistinguishable from a sphere shown as 300 pix by 300 pix. Thus illustrationstypically greatly exaggerate the flattening to highlight the concept of any planet's oblateness.

Otherf values in the Solar System are1⁄16 forJupiter,1⁄10 forSaturn, and1⁄900 for theMoon. The flattening of theSun is about9×10⁻⁶.

In 1687,Isaac Newton published thePrincipia in which he included a proof that a rotating self-gravitating fluid body in equilibrium takes the form of an oblateellipsoid of revolution (aspheroid).^[22] The amount of flattening depends on thedensity and the balance ofgravitational force andcentrifugal force.

Equatorial bulge of the Solar Systems major celestial bodies

Body	Diameter (km)		Equatorial bulge (km)	Flattening ratio	Rotation period (h)	Density (kg/m3)	${\displaystyle f}$	Deviation from${\displaystyle f}$
	Equatorial	Polar
Earth	012,756.2	012,713.6	00 042.6	1 : 299.4	23.936	5515	1 : 232	−23%
Mars	006,792.4	006,752.4	00 040	1 : 170	24.632	3933	1 : 175	0+3%
Ceres	000 964.3	000 891.8	00072.5	1 : 13.3	09.074	2162	1 : 13.1	0−2%
Jupiter	142,984	133,708	09,276	1 : 15.41	09.925	1326	1 : 9.59	−38%
Saturn	120,536	108,728	11,808	1 : 10.21	10.56	0687	1 : 5.62	−45%
Uranus	051,118	049,946	01,172	1 : 43.62	17.24	1270	1 : 27.71	−36%
Neptune	049,528	048,682	00 846	1 : 58.54	16.11	1638	1 : 31.22	−47%

Generally any celestial body that is rotating (and that is sufficiently massive to draw itself into spherical or near spherical shape) will have an equatorial bulge matching its rotation rate. With11808 kmSaturn is the planet with the largest equatorial bulge in theSolar System.

Equatorial bulges should not be confused withequatorial ridges. Equatorial ridges are a feature of at least four of Saturn's moons: the large moonIapetus and the tiny moonsAtlas,Pan, andDaphnis. These ridges closely follow the moons' equators. The ridges appear to be unique to the Saturnian system, but it is uncertain whether the occurrences are related or a coincidence. The first three were discovered by theCassini probe in 2005; the Daphnean ridge was discovered in 2017. The ridge on Iapetus is nearly 20 km wide, 13 km high and 1300 km long. The ridge on Atlas is proportionally even more remarkable given the moon's much smaller size, giving it a disk-like shape. Images of Pan show a structure similar to that of Atlas, while the one on Daphnis is less pronounced.

Small moons, asteroids, and comet nuclei frequently have irregular shapes. For some of these, such as Jupiter'sIo, a scalene (triaxial) ellipsoid is a better fit than the oblate spheroid. For highly irregular bodies, the concept of a reference ellipsoid may have no useful value, so sometimes a spherical reference is used instead and points identified by planetocentric latitude and longitude. Even that can be problematic fornon-convex bodies, such asEros, in that latitude and longitude don't always uniquely identify a single surface location.

Smaller bodies (Io,Mimas, etc.) tend to be better approximated bytriaxial ellipsoids; however, triaxial ellipsoids would render many computations more complicated, especially those related tomap projections. Many projections would lose their elegant and popular properties. For this reason spherical reference surfaces are frequently used in mapping programs.

• Apparent longitude
• Areography (geography of Mars)
• Astronomical coordinate systems
• List of tallest mountains in the Solar System
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• Topography of Mars
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