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.