Solar-forcing related terminology
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aphelion: the point in Earth's orbit that is at the greatest distance
        from the sun

apsidal line: connects perihelion (Earth's closest approach to the
        sun) to aphelion (the point when Earth is furthest from the sun)

perihelion: the point in Earth's orbit that is at the smallest
        distance from the sun

celestial equator: a circle whose center is the Earth's center, that is
        drawn on the plane extending the Earth's equatorial plane beyond
	the Earth itself.

solstice: either of the two times of the year when the sun is at its
	greatest distance from the celestial equator. Also, the
        shortest and longest day, depending on the hemisphere

summer solstice: the longest day of sunlight, when the Sun appears to reach
       its most northerly (northern hemisphere) or southerly (southern
       hemisphere) excursion. Occurs around June 21st (northern hemisphere)
       and December 21st (southern hemisphere).

winter solstice: the shortest day of sunlight, see summer solstice for more.

equinox: two times of the year when the Sun can be observed to be directly
        above the equator. Also, times when the length of the day equals
        the length of the night.

vernal equinox: northern hemisphere spring equinox, March 21.

autumnal equinox: northern hemisphere autumn equinox, Sept 21.

subsolar point: the point at the Earth's surface directly under the sun.
	I.e. the point at the surface which lies on the line between
	the Earth's center and the sun.

zenith angle: angle between the line from a point at the Earth's surface
        to the sun and the normal to the local surface.

solar azimuth: angle between the longitude line passing through a
        given point and the line between the same point and the
        subsolar point.

declination angle: latitude at which the sun is exactly overhead.  Changes
	during the day, as well as on other time scales.

hour angle: the angle between the longitude at a given point and the
        longitude of the subsolar point.  Equals to the time in hours
        from the time of the highest sun location to current local time,
        divided by 24, times 360.

solar longitude: the angle between the current location of Earth in its
       orbit, and the location of the spring equinox.

solar constant: 1367 Watts/m^2: short wave flux arriving from the sun
        to the top of the atmosphere

axial precession (Precession of the equinoxes): the movement of the rotational
           axis of an astronomical body, whereby the axis slowly traces out
	   a cone. Also is the rotation of the line connecting the equinoxes
	   in space. Period: 26,000 years

perihelion precession (or apsidal precession): Earth's orbit around the Sun
	   does not follow an identical ellipse each time, but traces out
	   a flower-petal shape because the major axis of Earth's
	   elliptical orbit precesses within its orbital plane.
	   Period: 112,000 years

Combined precession period relevant to climate: on average, 23000 years
	   for the ellipse to revolve once relative to the vernal equinox,
	   that is, for the perihelion to return to the same date.

obliquity: the angle between the rotation axis of the Earth, and the plane of
        rotation of the earth around the sun.  Currently, 23.5, varies
        quasi-periodically from 22.1 to 24.5 with a period of 41kyr.

eccentricity: measures how different the Earth's orbit around the sun
        is from being circular.  If the major and minor axes of the
        ellipse are a and b, and the distance from the center of the
        ellipse to one of the foci points is c, then the eccentricity
        is defined as e=c/a=sqrt(1-b^2/a^2) or as e=(a-b)/(a+b).  It
        is zero for a spherical orbit.

precession angle: the angle between the spring equinox (where the length
        of the day equals the length of the night) and the location where
	the Earth is closest to the sun. 90deg means that the Earth is
	closest to the sun during NH summer. 270deg means that the Earth
	is closest during NH winter.

climatic precession parameter: e*sin(omega), where omega is the precession
	 angle and e is eccentricity.