SEASONS:  A VISUAL DISPLAY
Diagrams based on images from pages 64-67,
John F. Kolars and John D. Nystuen, drawings by Derwin Bell,
Physical Geography:  Environment and Man, McGraw-Hill, 1975.
Figures produced here with permission.

SUNLIGHT INTENSITY IS A FUNCTION OF LATITUDE
At higher latitudes, sunlight is spread out over a wider area and is thus less intense.
The sun is most intense when the direct ray is overhead or orthogonal to a tangent plane to the Earth.

ANNUAL REVOLUTION OF THE EARTH AROUND THE SUN
Note the constant tilt of the Earth's polar axis.
The circle of illumination, or terminator, marks the separation of day and night.
At the equinoxes (about March and September 21), the circle of illumination passes through both poles (the only time of the year at which it does so).  Thus, at those times, the circle of illumination bisects all parallels of latitude; half the day is on the dark side, half the day is on the light side for all latitudes.  Hence, equinox=equal night, all over the world.


ANIMATION SHOWING THE EARTH VIEWED FROM THE SUN AT SOLSTICES AND EQUINOXES


LATITUDE AND LONGITUDE MEASURE POSITION OF SUN'S RAYS
The direct ray of the sun is overhead at 23.5 north on June 21 (as far north as it ever will be);
at 23.5 south on December 21 (as far south as it ever will be);
and at 0 degrees on the equinoxes.
The ecliptic is the Sun's diametral plane.



Institute of Mathematical Geography.  Copyright, 2005, held by authors.
Spatial Synthesis:  Centrality and Hierarchy, Volume I, Book 1.
Sandra Lach Arlinghaus and William Charles Arlinghaus