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Chapter 2
Solar Energy to
Earth and the
Seasons
Robert W. Christopherson
Charlie Thomsen
© 2012 Pearson Education, Inc.
Solar Energy to Earth and
the Seasons
Topics in this chapter:
The Solar System, Sun, and Earth
Solar Energy: From Sun to Earth
The Seasons
Causes of the seasons
The Solar System, Sun,
and Earth
Our solar system is located in the milky way
galaxy, a flattened disk shaped mass estimated to
contain more than 400 billion stars.
Our solar system is more than halfway out from
the galatic centre in one of the milky way’s spiral
arms – the Orion arm.
The sun and the solar system are estimated at
having formed more than 4.6 billion years ago.
Milky Way Galaxy
Figure 2.1
Milky Way Galaxy
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Figure 2.1
Dimensions and Distances
Distances in space are given in light years
because it is so incredibly vast.
Light year: the distance that light travels in
one year. At a speed of 299 792 (300 000)
kmps that is a distance of about 9.5 trillion
kms per year.
Dimensions and Distances
Therefore in light years the:
Milky Way Galaxy 100,000 ly across
Our Solar System 11 light-hours across
Moon is 1.28 light-seconds away
Known universe that is observable from earth is
approx. 12 billion light-years in all directions.
Earth’s Orbit
Earth’s orbit around the sun is eliptical – a closed
oval path. The orbit varies it’s distance from the
sun.
Earth’s average distance from the sun is approx.
150 million kms.
Light from the sun reaches the earth in 8 mins 20
seconds.
Earth is closest to the sun during the perihelion
(SH summer – 3 Jan) and furtherest from the sun
during the aphelion (SH winter – 4 July).
Our Solar System
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Figure 2.1
The Sun
The sun is an average star in the galaxy.
Sun emits:
Radiant energy
Solar wind
It provides all the energy for the earth and biosphere
systems.
Due to the tremendous pressure and temperature of the sun
it’s hydrogen atoms pair together in a process of fusion.
During this reaction the hydrogen forms helium and
liberates enormous quantities of energy.
Temperature: 6000 K (6273°C)
Solar Wind
Consists of clouds of electrically charged
particles released by the sun, containing –
hydrogen and free electrons.
The solar wind approaches the earth and is
deflected by the earth’s magnetic field
towards the magnetic poles.
When this wind enters the earth’s
atmosphere at the poles it results in the
auroras.
Aurora Borealis
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Figure 2.4
Figure 2.2
The Sun and Sunspots
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Figure 2.2
The Electromagnetic Spectrum
Sun radiates electromagnetic energy of
various wavelengths. This travels at the
speed of light to earth.
This radiant energy is made up of different
wavelengths.
The sun radiates short wave radiation.
The earth absorbs this and reradiates long
wave radiation.
Wavelength and Frequency
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Figure 2.5
The Electromagnetic
Spectrum
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Figure 2.6
Earth’s Energy Budget
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Figure 2.8
Incoming Energy - Insolation
The outermost point of the earth’s
atmosphere is known as the thermopause.
The earth intercepts only one two-billionth
of the sun’s total energy output but this
supports all the earth’s systems.
Insolation: the energy that enters the
earth’s atmosphere.
Insolation at the top of the atmosphere is
referred to as the solar constant.
Insolation
Insolation is not evenly distributed over the earth’s surface.
It differs due to it’s angle and the amount of interception
that occurs as it passes through the earth’s atmosphere.
Subsolar point: the point on the earth receiving the most
insolation which is the point at which the sun’s rays are
perpendicular (90°) to the earth. This point shifts between
the tropics due to seasonal change.
The thermopause above the equatorial region receives 2.5
times more insolation annually than the thermopause above
the poles. Due to the effect of the earth’s curvature on the
angle of the sun’s rays.
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Figure 2.9
Insolation at Top of Atmosphere
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Figure 2.10
The Seasons
Seasonality
Refers to the seasonal variation of the sun’s position in
relation to the earth and the changing length of day and
night throughout the year.
This results from a change in the altitude which the
sun’s rays reach above the horizon, the declination of
the sun’s rays from the subsolar point and the length of
day and night.
At any distance from the subsolar point the radiation
received is more diffuse.
Daily Net Radiation
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Figure 2.11
The Seasons - Causes
Seasons result from the sun’s altitude above
the horizon, it’s declination and the
daylength. The variations in these are
caused by various physical factors:
Earth’s revolution around the sun
Earth’s rotation around it’s axis
Tilt of the earth’s axis
Axial parallelism
Sphericity
Reasons for Seasons
Revolution
Earth revolves around the Sun
Voyage takes one year
Earth’s speed is 107,280 kmph
Rotation
Earth rotates on its axis once every 24 hours
Rotational velocity at equator is 1674 kmph
Revolution and Rotation
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Figure 2.13
Reasons for Seasons
Tilt of Earth’s axis
Axis is tilted 23.5° from the perpendicular to
the plane of the ecliptic.
Axial parallelism
Axis maintains alignment during orbit around
the Sun
North pole points toward the North Star
(Polaris)
Sphericity
Earth’s shape is known as a geoid (equatorial
bulge and polar flattening).
Axial Tilt and Parallelism
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Figure 2.14
Annual March of the Seasons
Summer solstice – December 21 or 22
Subsolar point Tropic of Capricorn
Autumn equinox – March 20 or 21
Subsolar point Equator
Winter solstice – June 20 or 21
Subsolar point Tropic of Cancer
Spring equinox – September 22 or 23
Subsolar point Equator
Annual March of the Seasons
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Figure 2.15
The Four Seasons
Winter and Summer Solstice
Equinox
11:30 P.M. in the Antarctic
Figure 2.16
Midnight Sun
Figure 2.17
End of Chapter 2
Geosystems 8e
An Introduction to Physical Geography
© 2012 Pearson Education, Inc.
Robert W. Christopherson
Charlie Thomsen