Solar Energy to Earth and the Seasons
The Solar System, Sun, and Earth
Solar Energy: From Sun to Earth
The Seasons
Dimensions and DistancesEarth’s orbit
Average distance from Earth to the Sun is 150,000,000 km (93,000,000 mi)
Perihelion – closest distance (at January 3)147,255,000 km (91,500,000 mi)
Aphelion – farthest distance (at July 4)152,083,000 km (94,500,000 mi)
Plane of Earth’s orbit is the plane of the ecliptic
Solar Energy: From Sun to Earth
Electromagnetic spectrum of radiant energy
Intercepted energy at the top of the atmosphere-insolation
The Electromagnetic Spectrum of the sun
Figure 2.6
Ultra Violet: <0.4μm (8%)Visible light: 0.4-0.7μm (47%)Infra-red: >0.7μmb(45%)
Earth’s Energy Budget
Figure 2.8
Insolation: intercepted solar radiationSolar constant: average value of the insolation received at the top of the atmosphere when earth is at its average distance from the sun
Distribution of InsolationTropics receive more concentrated insolation due to Earth’s curvature (higher solar angles)
Tropics receive 2.5times more than poles
The higher the solar angle, the stronger intensity of solar radiation
Direct rays: perpendicular to the earth’s surface. The highest solar angle.
Direct rays only occur between Tropic of Cancer (23.5N) and Tropic of Capricorn (23.5S).
Subsolar point: the only point (latitude) on the earth’s surface that receives direct rays
Sun’s inclination: latitude of the subsolar point
SeasonalitySeasonal changes in
1. Sun’s altitude – angle above horizon
2. Declination – location of the subsolar point
3. Daylength
Reasons for Seasons 1. Revolution-length of the year
Earth revolves around the Sun
Voyage takes one year
Earth’s speed is 107,280 kmph (66,660 mph)
2. Rotation-length of the dayEarth rotates on its axis once every 24 hours
Rotational velocity at equator is 1674 kmph (1041 mph)
Reasons for Seasons Tilt of Earth’s axis
Axis is tilted 66.5° from plane of ecliptic
Axial parallelismAxis maintains alignment during orbit around the Sun
North pole points toward the North Star (Polaris)
Annual March of the SeasonsWinter solstice – December 21 or 22
Subsolar point Tropic of Capricorn; shortest daylight hours in NH.
Spring equinox – March 20 or 21Subsolar point Equator; equal daylight and night hours everywhere on the earth
Summer solstice – June 20 or 21Subsolar point Tropic of Cancer; Longest daylight hours in NH
Fall equinox – September 22 or 23Subsolar point Equator; equal daylight and night hours
If the tilted angle is 60 degrees to the plane of ecliptic, where would be tropics
and circles?Arctic/antarctic circle=titled angle (60ºN/S)
Tropics = 90-titled angle (30ºN/S)
Increased tropics area and increased polar areas
Larger seasonal variations in most places
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