Notes on the Fact Sheets

Most values listed in the factsheets are from the following sources:
  IAU/IAG Working Group Report, 2006, 2015
  Astronomical Almanac, 2000, 2001, 2020
  Global Earth Physics, American Geophysical Union, 1995
  Astrophysical Quantities, C.W. Allen, 1981, 2000
  JPL Ephemeris DE430/DE431
  IAU bulletins/announcements
  Recent journal articles and personal communications
Note that the values listed on the factsheets are not "official" values, 
there is no single set of agreed upon values.  They are based on ongoing 
research and as such are under study and subject to change at any time.  
Every effort has been made to present the most up-to-date information, 
but care should be exercised when using these values. 

Bulk Parameters

Mass (1024 kg)                Mass of the body in 10^24 kilograms
                              [GM(measured)/G(CODATA 2018 - see below)]
Volume (1010 km3)             Volume of the body in 10^10 km^3
Equatorial radius (km)	      Radius of the body at the equator in kilometers
Polar radius (km)             Radius of the body at the poles in kilometers
Volumetric mean radius (km)   Radius of a sphere with the same volume as the body
Core radius (km)              Radius of the planet core in kilometers
Ellipticity (Flattening)      The ratio (equatorial - polar radius)/(equatorial radius), 
Mean density (kg/m3)          Average density of the body (mass/volume) 
                              in kilograms/meter^3
Surface gravity (mean) (m/s2) Mean gravitational acceleration at the surface of the 
                              body or the 1 bar level, not including the effects of  
                              rotation, defined as GM over the square of the volumetric 
                              mean radius, in meters/second^2
Surface acceleration (eq.) (m/s2) Effective equatorial gravitational acceleration 
                              at the surface of the body or the 1 bar level, 
                              including the effects of rotation, in meters/second^2
Surface acceleration (pole) (m/s2) Effective polar gravitational acceleration  
                              at the surface of the body or the 1 bar level, 
                              in meters/second^2
Escape velocity (km/s)        Initial velocity required to escape the body's 
                              gravitational pull in kilometers/second (at equator)
GM (x 106 km3/s2)              Gravitational constant times the mass of the body
                              in 10^6 kilometers^3/seconds^2
Geometric albedo              The ratio of the body's brightness at a phase angle of 
                              zero to the brightness of a perfectly diffusing disk 
                              with the same position and apparent size, dimensionless.
                              V-band (0.549 micrometers). Earth is highly variable.
Bond albedo                   The fraction of incident solar radiation reflected back 
                              into space without absorption, dimensionless.
                              Also called planetary albedo.
V-band magnitude V(1,0)       The magnitude of the body in the V-band (0.549 micrometers) 
                              if it were one AU (1.496 x 10^8  kilometers) from the Earth 
                              at a phase angle of zero, dimensionless.
Solar irradiance (W/m2)       Solar energy on the body in Watts/(meter^2)
Black-body temperature (K)    Equivalent black body temperature is the surface 
                              temperature the body would have if it were in  
                              radiative equilibrium and had no atmosphere, 
                              but the same albedo, in Kelvin.
Topographic range (km)        Difference in elevation between the highest and lowest 
                              points on the planet's surface, in kilometers.
Moment of inertia (C/MR2)     The moment of inertia of the body expressed as 
                              the rotational inertia divided by the body's mass 
                              x radius^2, where radius^2 = {2(Requator^2) + Rpolar^2}/3. 
                              A hollow spherical shell has a moment of inertia 
                              of 2/3, a homogeneous sphere 0.4, dimensionless
J2 (x 10-6)                    The ratio of the difference in the moments of  						   
                              inertia (rotational vs polar) to the mass of  
                              the body times the radius^2,(C-A)/(M R^2), 
                              x 10^-6, dimensionless

Number of natural satellites  The number of moons orbiting the planet, as certified by the IAU

Absolute magnitude (Comets)   The magnitude of the comet at 1 AU for Y = 10, where Y is the 
                              photometric parameter giving the observed dependence of the 
                              magnitude on heliocentric distance, also designated H(10).

Orbital parameters

Instantaneous values (semimajor axis, eccentricity, perihelion, aphelion, inclination) 
for planets referenced to Julian Date 2459000.5 (11 June 2020). 
[Updated from Julian Date 2451800.5 (13 September 2000) on 22 Dec 2021]

Semimajor axis (106 km)          Approximate mean distance from the Sun (or other central   
                                 body in the case of satellites) from center to center in 
                                 10^6 kilometers. 
Sidereal orbit period (days)     The time it takes the body to make one revolution about
                                 the Sun relative to the fixed stars in days.
                                 For Pluto, it is the time from the last zero longitude 
                                 crossing to the next (24 July 1820 - 2 July 2068).
Tropical orbit period (days)     The average time for the body to make one revolution  
                                 about the Sun from one point in its seasonal orbit to 
                                 the equivalent point (e.g. equinox to equinox) in days.
                                 For Earth, this equals exactly 1 year. Not known for Pluto. 
Synodic period (days)            The time interval between similar configurations in the 
                                 orbit (e.g. opposition) of the body and Earth, in days.
Perihelion (106 km)              The point in a body's orbit closest to the 
                                 Sun, in 10^6 kilometers.
Aphelion (106 km)                The point in a body's orbit furthest from  
                                 the Sun, in 10^6 kilometers.
Mean orbital velocity (km/s)     The average speed of the body in elliptical orbit, 
                                 in kilometers/second.
Max. orbital velocity (km/s)     Maximum orbital velocity, at perihelion, 
                                 in kilometers/second.
Min. orbital velocity (km/s)     Minimum orbital velocity, at aphelion, 
                                 in kilometers/second.    
Orbit inclination (deg)          The inclination of the orbit to the ecliptic, in degrees.  
                                 For satellites, this is with respect to the planet's equator.  
Orbit eccentricity               A measure of the circularity of the orbit, equal to 
                                 (aphelion - perihelion distance)/(2 x semi-major axis).
                                 For the Galilean satellites, the forced eccentricity is given.  
                                 For a circular orbit eccentricity = 0. Dimensionless.
Sidereal rotation period (hrs)   The time for one rotation of the body on its axis  
                                 relative to the fixed stars, in hours.  A minus sign 
                                 indicates retrograde rotation.
Length of day (hrs)              The average time in hours for the Sun to move from the 
                                 noon position in the sky at a point on the equator back 
                                 to the same position, on Earth this defines a 24 hour day. 
Obliquity to orbit (deg)         The angle between the body's equator and the body's orbital  
                                 plane, with north defined by the right-hand rule. (J2000)
Inclination of equator (deg)     The angle between the equator and orbital plane with north 
                                 defined as pole axis above (north of) the plane of the 
                                 solar system, also denoted as axial tilt. (J2000)      

Observational Parameters

Approximate values for the minimum and maximum distances of the planets from Earth. Orbits 
fluctuate over time, these values are calculated from the mean orbital elements for J2000 
250 year fits and the current orbits referenced to Julian Date 2459000.5 (11 June 2020). 

The planetary diameter used is two times the volumetric mean radius. For Venus, Jupiter, 
Saturn, Uranus, and Neptune, diameter is approximately to the visible cloud tops. 

Mean orbital elements

250-year least squares fit elements referenced to J2000 (Global Earth Physics, p. 14)

Longitude                          The point in a body's orbit around the Sun, defined from 
                                   0 to 360 degrees.  The 0 point of longitude is defined as 
                                   the first point of Aries.  This is the position of the Sun 
                                   as seen from Earth at Earth's vernal equinox, so at the 
                                   vernal equinox the Earth is at a longitude of 180 degrees.
Longitude of ascending node (deg)  The longitude in a body's orbit at which it crosses 
                                   the ecliptic plane with increasing latitude (i.e. 
                                   crosses the ecliptic from south to north).
Longitude of perihelion (deg)      The longitude in a body's orbit at which it reaches  
                                   the point closest to the Sun.
Mean longitude (deg)               The longitude a body was at in its orbit at 12:00 
                                   Universal (Greenwich) Time on January 1, 2000, 
                                   also known as J2000 or Julian Day 2451545.0


Dipole field strength:  The strength of the dipole portion of the planetary magnetic field 
                        outside the planet in Gauss-R^3, where R is in units of planet radius.
                        (Dividing by the distance R^3 gives the field in Gauss) 
Dipole tilt to rotational axis:  Tilt of the dipole axis to the axis of rotation in degrees. 
Dipole Offset:  The offset distance of the dipole center to the planet center in units of 
                planet radius. 
Surface field strength:  Range of total field strength at planet radius in Gauss. 


Surface Pressure:  Atmospheric pressure at the surface, in bars, millibars  
                   (mb = 10^-3 bar), or picobars (10^-12 bar).
Surface Density:  Atmospheric density at the surface in kilograms/meters^3.
Scale height:  The height interval in which the atmospheric pressure changes by a 
               factor of e = 2.7183
Average temperature:  Mean temperature of the body over the entire surface in Kelvin.
Diurnal temperature range: Temperature range over an average day in Kelvin.
Wind speeds:  Near surface wind speeds in meters/second
Atmospheric composition:  Relative composition by volume of gasses in the atmosphere.
Mean molecular weight:  Mean relative molecular mass of the atmospheric constituents in 
                       dalton (= unified atomic mass units), equivalent to grams/mole 
Atmospheric composition (by volume): Relative volume of constituents in the atmosphere, 
                                     by percentage or ppm (parts per million).

Related Definitions

Astronomical Unit (AU) - The standard distance from the Sun to the Earth = 149,597,870.7 km.

Bar - A measure of pressure or stress.  1 bar = 10^5 Pascal (Pa) = 10^5 kg m^-1 s^-2

Ecliptic - An imaginary plane defined by the Earth's orbit.

Equinox - The point in a body's orbit when the sub-solar point is exactly on the equator.

Gravitational Constant (G) - Relates gravitational force to mass, 
                           = 6.67430 x 10^-11 meters^3 kilograms^-1 seconds^-2 (CODATA 2018)

Julian Day (JD) - The amount of time that has passed, measured in days on the Julian  
                  calendar, since noon UT on 1 January 4713 B.C. 
                  1200 UT on 1 January 2000 A.D. is JD 2451545, also referred to as J2000.

Opposition - An orbital configuration in which two bodies are on exact opposite sides of the 
             Sun or are on the same side of the Sun forming a line with the Sun
             (neglecting inclination)

Phase Angle - The angle between the Earth and Sun as seen from the body.

Sub- and Superscripts

In the explanation of units, we've included a description of the units in the form where, for example, 10^24 equals 10 to the 24 power.

 Directory to the Planetary Fact Sheets

Dr. David R. Williams,
NSSDCA, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771

NASA Official: Dave Williams,
Last Updated: 23 May 2023, DRW