Chapter 2Solar Energy to

Earth and the Seasons

Robert W. ChristophersonCharlie Thomsen

Solar Energy to Earth and the Seasons

The Solar System, Sun, and Earth  

Solar Energy: From Sun to Earth  

The Seasons 

Milky Way Galaxy

Figure 2.1

Our Solar System

Figure 2.1

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 

Wavelength and Frequency

Figure 2.5

Longer wave length, lower frequency, and lower intensity

The Electromagnetic SpectrumSun radiates shortwave energy

Earth radiates longwave energy

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%)

Solar and Terrestrial Energy

Figure 2.7

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

Figure 2.9

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

The Seasons  Seasonality 

Reasons for seasons  

Annual march of the seasons  

SeasonalitySeasonal changes in

1. Sun’s altitude – angle above horizon

2. Declination – location of the subsolar point

3. Daylength

Reasons for Seasons 1. Revolution

2. Rotation

3. Tilt of Earth’s axis

4. Axial parallelism

Revolution and Rotation

Figure 2.13

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)

Axial Tilt and Parallelism

Figure 2.14

(Circle of illumination)

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 Seasons

Figure 2.15

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

11:30 P.M. in the Antarctic

Figure 2.16

Midnight Sun

Figure 2.17

Seasonal Observations

Figure 2.18

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

If the tilted angle is 90 degree to the plane of ecliptic?

Circles would be at 90ºN/S

Tropics would be at equator

No seasonal variation

No day length changes