Chapter 2
Solar Energy to
Earth and the
Seasons
Robert W. Christopherson
Charlie 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
Dimensions and Distances
Earth’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
Longer
wave
length,
lower
frequency,
and lower
intensity
Figure 2.5
The Electromagnetic Spectrum
Sun radiates shortwave energy
Earth radiates longwave energy
The Electromagnetic
Spectrum of the sun
Ultra Violet: <0.4μm (8%)
Visible light: 0.4-0.7μm (47%)
Infra-red: >0.7μmb(45%)
Figure 2.6
Solar and
Terrestrial
Energy
Figure 2.7
Earth’s Energy Budget
Insolation: intercepted solar radiation
Solar constant: average value of the insolation received at the top of the
atmosphere when earth is at its average distance from the sun
Figure 2.8
Distribution of Insolation
Tropics 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
Seasonality
Seasonal changes in
Sun’s altitude – angle above horizon
2. Declination – location of the subsolar point
3. Daylength
1.
Reasons for Seasons
1.
2.
3.
4.
Revolution
Rotation
Tilt of Earth’s axis
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 day
Earth rotates on its axis once every 24 hours
Rotational velocity at equator is 1674 kmph
(1041 mph)
Axial Tilt and Parallelism
(Circle of illumination)
Figure 2.14
Reasons for Seasons
Tilt of Earth’s axis
Axis is tilted 66.5° from plane of ecliptic
Axial parallelism
Axis 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 Seasons
Winter solstice – December 21 or 22
Subsolar point Tropic of Capricorn; shortest daylight
hours in NH.
Spring equinox – March 20 or 21
Subsolar point Equator; equal daylight and night hours
everywhere on the earth
Summer solstice – June 20 or 21
Subsolar point Tropic of Cancer; Longest daylight
hours in NH
Fall equinox – September 22 or 23
Subsolar 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