Explanation of Terms

Orbital Parameters

Mass (1024 kg)                 Mass of the body in 10^24 kilograms
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                   The ratio (equatorial - polar radius)/(equatorial radius)
                              also called the flattening (dimensionless)
Mean density (kg/m3)          Average density of the body (mass/volume) 
                              in kilograms/(meter^3)
Surface gravity (m/s2)        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
GM (x 106 km3/s2)              Gravitational constant times the mass of the body
                              in 10^6 kilometers^3/seconds^2
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 
                              (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 degrees Kelvin.
Topographic range (km)        Difference in elevation between the highest and lowest 
                              points on the planet's surface, in kilometers.
Moment of inertia (I/MR2)     The moment of inertia of the body expressed as 
                              the rotational inertia divided by the body's (mass 
                              x radius^2).  A hollow spherical shell has a moment of inertia 
                              of 2/3, a homogeneous sphere 0.4
J2 (x 10-6)                    The ratio of the difference in the moments of inertia                                               
                              to the mass of the body times the radius^2, 
                              (C-A)/(M R^2), x 10^-6, dimensionless

Orbital parameters

Semimajor axis (106 km)          Mean distance from the Sun 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.
Tropical orbit period (days)     The time for the body to make one revolution about the 
                                 Sun from equinox to equinox in days.
                                 For Earth, this equals exactly 1 year.
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 orbit, 
                                 in kilometers/second.
Orbit inclination (deg)          The inclination of the orbit to the ecliptic, 
                                 in degrees
Orbit eccentricity               A measure of the circularity of the orbit, equal to 
                                 (perihelion - aphelion distance)/(2 x semi-major axis)
                                 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.
Obliquity to orbit (deg)         The tilt of the body's equator relative to the body's  
                                 orbital plane, in degrees.

Atmospheres

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 
                      degrees Kelvin.
Diurnal temperature range: Temperature range over an average day in degrees Kelvin.
Wind speeds:  Near surface wind speeds in meters/second
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 (by volume): Relative volume of constituents in the atmosphere, 
                                     by percentage or ppm (parts per million).

Related Definitions

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 - Relates gravitational force to mass, 
                         = 6.6726 x 10^-11 meters^3 kilograms^-1 seconds^-2 

Opposition - An orbital configuration in which two bodies are on exact opposite sides of 
             the Sun

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

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