SRTERMS.TXT   Dictionary of TERMS used in Solar Radiation   2008/06/09

Aphelion            Time and location when the Earth is furthest from
                    the Sun in its annual revolution about the Sun.

Autumnal Equinox    Time and location of the Earth when the Sun is
                    directly over the equator and is heading into the
                    southern hemisphere.

Axis                Axis of rotation of the Earth; line that passes
                    through the center of the Earth and each pole.

Climatic Precession Equal to eccentricity times sine of longitude of
  Parameter         perihelion.

Declination Angle   Latitude above which the Sun is directly over head.

Eccentric Anomaly   Spatial angle measured along the eccentric circle
                    of the Earth's orbit from perihelion to the Earth's
                    absisca (where the absisca is directed from the
                    center of the eccentric circle to perihelion).

Eccentric Circle    Circle that circumscribes an ellipse.

Eccentricity        Eccentricity of the Earth's orbital ellipse about
                    the Sun.  One of the three orbital parameters.
                    Its symbol is e = sqrt(a²-b²)/a.

Greenwich Meridion  Longitude line that passes through the observatory
                    at Greenwich, England.  It is the 0° longitude line.

Insolation          Energy per square meter of sun light incident at
                    top-of-atmosphere.  The square meter should be
                    perpendicular to a vertical ray from the Earth's
                    center.  Insolation = S cos(Z) a² / r² , where
                    S = Solar Constant (W/m²),
                    Z = Zenith angle,
                    a = semi-major axis of the Earth's orbital ellipse,
                    r = distance from Earth to Sun.
                    cos(Z) = sin(D) sin(J) + cos(D) cos(J) cos(T) ,
                    D = Declination angle,
                    J = latitude,
                    T = local time in temporal radians, T = 0 at noon.
                    Insolation used to mean sun light incident on the
                    surface should be called "surface insolation".

Longitude of        Spatial angle from moving vernal equinox to
  Perihelion        perihelion with Sun as angle vertex.  One of the
                    three orbital parameters.  Its symbol is a small
                    omega with a tilda above it.

Mean Anomaly        Temporal angle from perihelion to current time in
                    units of 2*pi per tropical year.

Obliquity           Dihedral angle between the Earth's equatorial plane
                    and its orbital plane.  One of the three orbital
                    parameters.  Its symbol is a small epsilon.

Orbital Ellipse     An inverse square force field, such as the Sun's
                    gravitational force field, allows stable elliptical
                    orbits of point masses or planets.  The center of
                    the force field, the Sun, is located at one of the
                    two foci of the ellipse.

Orbital Energy      Energy of the Earth due to its orbit about the Sun:
                    E = .5*Me*v² - G*Ms*Me/r = -.5*G*Ms*Me/a , where
                    G  is the universal gravitational constant,
                    Ms is the mass of the Sun,
                    Me is the mass of the Earth,
                    v  is the speed of the Earth in its orbit,
                    r  is the distance between the Earth and Sun,
                    a = semi-major axis of the Earth's orbital ellipse.
                    The zero reference level for geopotential energy of
                    the Earth with respect to the Sun is at infinity.

Perihelion          Time and location when the Earth is closest to the
                    Sun in its annual revolution about the Sun.

Precession          Short name for the climatic precession parameter.

Revolution          Cyclic movement of the Earth through its orbit about
                    the Sun.  One revolution takes one tropical year.

Rotation            Cyclic movement of the Earth around its axis.  One
                    rotation takes about 23 hours, 56 minutes.  Each
                    year, the Earth rotates about its axis once more
                    than the number of days in a year.

Semi Major Axis     One half of the major axis of the Earth's orbital
                    ellipse.  Its symbol is a.

Semi Minor Axis     One half of the minor axis of the Earth's orbital
                    ellipse.  Its symbol is b.

Solar Constant      Mean annual incident solar radiation the Earth
                    receives from the Sun divided by cross sectional
                    area of the Earth; apprxoximately 1367 W/m².

Spring Equinox      Same as vernal equinox.

Summer Solstice     Time and location of the Earth when the Sun is
                    directly overhead the most northerly latitude in
                    the annual cycle.

Tropical Year       Average time from one vernal equinox to the next;
                    approximately 365.2425 days.

True Anomaly        Spatial angle from perihelion to current location
                    with Sun as angle vertex.

Vernal Equinox      Time and location of the Earth when the Sun is
                    directly over the equator and is heading into the
                    northern hemisphere.

Winter Solstice     Time and location of the Earth when the Sun is
                    directly overhead the most southerly latitude in
                    the annual cycle.

Zenith Angle        Angle between an upward vertical ray from a point
                    on the surface of the Earth and a ray from that
                    point to the Sun.


Many of the terms used here come from "Basic Physics of the Solar
System", by V.M. Blanco and S.W. McCuskey, 1961, Addison-Wesley.