II. Earth and Sun 

A. Basics

1. rotation vs. revolution
2. perihelion and aphelion

1. Earth rotates on its axis once every 24 hours
2. receives light when facing the sun (day)
3. circle of illumination

1. controls of global temperature variations - the hemisphere that is tilted toward the sun receives more solar energy
   • sun angle
     • concentration of solar rays (fig. 2-3)
     • amount of atmospheric filtering (fig 2-4; Table 2-1)
   • length of day
2. causes of variations (Fig. 2-5)
   • rotation axis of the Earth is tilted 23 ½ ° away from perpendicular to plane of ecliptic
   • axis is inclined but has constant orientation with respect to outer space (points to N star, Polaris)
   • one hemisphere is facing toward the sun while the other hemisphere is tilted away from the sun
   • as the Earth revolves around the sun, the N hemisphere faces toward the sun when the sun is between the Earth and the North Star, but faces away from the sun when the Earth is between the sun and the North Star.
3. calculate range of sun angles (fig. 2-6)

1. tilt of rotation axis and circle of illumination
2. long in summer, short in winter (solstice)
3. 12 hours in spring and autumn (equinox)
4. noon sun angle & analemma (Box 2-2)