Earth-Sun Relations

Insolation

• Incoming Solar Radiation - Solar Radiation intercepted at the surface of the earth.
• Amount and intensity are determined by the sun angle, day length, path length, ground slope, and state of the atmosphere.

  Axial Tilt

• The earth's axis of rotation is inclined 23 1/2 degrees from perpendicular.
• Earth's axis is tilted 66 1/2 degrees from the plane of the ecliptic - the plane the earth orbits the sun in.

Parallelism

• Parallelism means that the earth's axis remains parallel to its former position as it revolves around the sun.
• Axis "always" points in the same direction.

Sun Angle

• Tilt of the earth's axis controls:

  • Sun Angle - angle a beam of light makes with the  surface of the earth.
  • Sun angle determines the area of illuminated and intensity of heating.

  Perpendicular rays (A) concentrate energy over the smallest area. As the sun angle decreases (B) the area illuminated increases (indicated by dashed line).

Subsolar point - location where the sun is directly overhead at noon. Similar to declination of the sun.

Axial Tilt, Earth Orientation and Day length

Circle of Illumination

The circle of illumination is the division between day and night over the earth. The circle of illumination bisects (cuts in half) all latitudes on the spring and autumnal equinoxes. At this time, all places have equal day length (12 hours). The circle of illumination always bisects the equator (0 degrees latitude).

Animation of day light period through the year (Warning: 3 MB file; long download)

Ground Slope

• Influences local sun angle. Slopes tipped into the sun have higher sun angles and hence more intense insolation than horizontal or slopes tipped away from the sun.

Path length

• As the distance over which incoming solar radiation increases, greater chance for diffusion and reflection of light.
• Earth - sun distance is greatest in July, closest in January.

Due to the curvature of the earth, path length of a beam of light is longer at high latitudes

State of the Atmosphere

• Clouds reflect sunlight off their top reducing insolation

• Clouds diffuse sunlight

• Particulates reflect and diffuse light

Earth Orbit and Seasons

Orientation of Earth to Sun

Earth Orbit

• Elliptical Orbit of the earth around the sun

• Furthest from the sun in July, closest in January (Do you know why it's warmer in the summer in the Northern Hemisphere even though the earth is further away?)

Go to Earth Orbit animation