Glossary of Terms used in the Solar System Pages

Bulk Parameters

Mass                            Mass of the planet
GM                              Newtonian gravitational constant, G, times the mass of the body
Equatorial radius      	        Radius of the body at the equator
Polar radius                    Radius of the body at the poles
Core radius                     Radius of the planet core
Flattening                      The degree of oblateness, measured as the ratio (equatorial radius - polar radius)/(equatorial radius); also called the ellipticity; dimensionless
Mean density                    Average density of the body (mass/volume)
Moment of inertia coefficient   The moment of inertia of the body (about its spin axis) divided by  M R^2, where M is the mass, and R is the equatorial radius.  A hollow shell has a coefficient of 2/3, a homogeneous sphere 2/5
J2                              Coefficient of the potential; dimensionless; given by (C - A)/(M R^2), where C is the moment of inertia about the spin axis, and A is the moment of inertia about a perpendicular axis
Surface gravity                 Gravitational acceleration at the surface of the body, or the 1 bar level for fluid planets
Escape velocity                 Initial velocity required to escape the body's gravitational pull
Topographic range               Difference in elevation between the highest and lowest points on the planet's surface

Orbital parameters

Semimajor axis                  Mean distance from the Sun
Perihelion                      The point in a body's orbit closest to the Sun
Aphelion                        The point in a body's orbit furthest from the Sun
Sidereal period                 The time it takes the body to make one revolution about the Sun relative to the fixed stars
Tropical period                 The time for the body to make one revolution about the Sun from equinox to equinox; for Earth, this equals exactly 1 year
Eccentricity                    A measure of the circularity of the orbit, equal to (perihelion - aphelion)/(2 x semi-major axis); for a circular orbit, eccentricity = 0; dimensionless
Inclination                     The inclination of the orbit to the ecliptic
Synodic period                  The time interval between similar configurations in the orbit (e.g. opposition) of the body and Earth
Mean orbital velocity           The average speed of the body in its orbit

Radiative properties

Visual 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
Bond albedo                     The fraction of incident solar radiation reflected back into space without absorption; dimensionless; also called planetary albedo
Visual magnitude V(1,0)         The visual magnitude of the body if it were one AU from the Earth at a phase angle of zero; dimensionless
Solar irradiance                Solar energy impinging on the body
Black-body temperature          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

Spin properties

Obliquity                       The tilt of the body's equator relative to the ecliptic
Argument of prime meridian
Sidereal rotation rate          The angular velocity of the planet about its spin axis
Sidereal rotation period        The time for one rotation of the body on its axis relative to the fixed stars

Atmospheric properties

Surface pressure                Atmospheric pressure at the surface
Surface density                 Atmospheric density at the surface
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
Diurnal temperature range       Temperature range over an average day
Wind speeds                     Near-surface wind speeds
Atmospheric composition         Relative composition by volume of gasses in the atmosphere
Mean molecular weight           Average molecular weight of the atmospheric constituents in grams/mole
Atmospheric composition         Relative volume of constituents in the atmosphere, by percentage or ppm (parts per million)

Related Definitions

Ecliptic                        An imaginary plane defined by the Earth's orbit
Epoch                           The moment in time at which given data are valid
Equinox                         The point in a body's orbit when the sub-solar point is exactly on the equator
Phase angle                     The angle between the Earth and Sun as seen from the body