Transcript Solar Energy

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
The Solar System, Sun, and Earth
Solar System formation and structure
Gravity
Planetesimal hypothesis
Dimensions and distances
Speed of light
Earth’s orbit
Solar System Formation and Structure
Gravity
Mutual attracting force exerted by mass on all other objects:
M1  M 2
F G
r2
G ≈ 6.67428×10−11 m3/(kg·s2).
F: kg*m/s2 =1 Newton
g=9.8 m/s2
Solar System Formation and Structure
Planetesimal hypothesis
Sun condensed from nebular clouds 4.6 billion
years ago. Big Bang ~13 billion years ago.
Milky Way Galaxy
Figure 2.1
Dimensions and Distances
Speed of light
299,792 km/s(≈3.0×108m/s) (186,282 mi/s)
Milky Way Galaxy 100,000 ly across
Our Solar System 11 light-hours across
Moon is 1.28 light-seconds away
Dimensions and Distances
Earth’s orbit
Average distance from Earth to the Sun is
150,000,000 km (93,000,000 mi)
Perihelion – closest at January 3
147,255,000 km (91,500,000 mi)
Aphelion – farthest at July 4
152,083,000 km (94,500,000 mi)
Earth is 8 minutes 20 seconds from the Sun
Plane of Earth’s orbit is the plane of the
ecliptic
Our Solar System
Figure 2.1
Pluto not a Planet since 2006
New Definition by International Astronomical Union in 2006:
A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has
sufficient mass for its self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round) shape, and (c) has
cleared the neighbourhood around its orbit.
Pluto is now called a Dwarf Planet on account of its size and the fact that
it resides within a zone of other objects, known as the Kuiper Belt
A dwarf planet is an object in orbit around the Sun that is large enough
(massive enough) to have its own gravity pull itself into a round (or
nearly round) shape. Generally, a dwarf planet is smaller than Mercury. A
dwarf planet may also orbit in a zone that has many other objects in it.
For example, an orbit within the asteroid belt is in a zone with lots of
other objects.
Figure 2.1
Solar Activity and Solar Wind
Diameter 1,392,000km (~109 Earths diameter ), Contain 99.86% of
mass in solar system.
3/4 of Sun is hydrogen, the rest is helium (<2% of other elements, Fe,
O, C etc.)
Nuclear fusion of hydrogen nuclei into helium, surface temp 5780 oK.
The Sun orbit the galactic center once per 225~250 my.
Sunspots have activity cycle of 11 years. It is indicator of solar
activity.
Solar wind: streams of electrically charged particles (hydrogen nuclei
and free electrons) leaving the sun and can reach the Earth in 3 days.
Figure 2.2
The Electromagnetic Spectrum
Sun radiates shortwave energy
Planck’s Law: Blackbody spectral radiant emittance
E 
2hc 2
5 ( e
hc
KT
 1)
Where h: Planck Constant, 6.626E-34 ws2
c: speed of light in vacuum 3.0E+8 m/s
: wavelength in meters
T: temperature in degrees Kelvin
K: Boltzman constant, 1.38054E-23 ws/K
M: blackbody spectral exitance at T,
unit = watts per square meter area per meter wavelength
The Electromagnetic Spectrum
Stefan-Boltzmann’s Law:


0
0
E   E d   2hc 25 (e
hc
KT
 1)1 d  T 4
Where  is Stefan-Boltzmann constant (5.676E-8wm-2K-4)
The Electromagnetic Spectrum
Wien’s Law: The wavelength at which a blackbody radiates
most energy:
max
A

T
A=2.898E-3 mK
Wavelength and Frequency
Frequency: number of full waves passing through a point in unit time
freq=c/λ
Figure 2.5
Particle Property
Particle Properties: Radiation travels in bundles of energy
unit, called photons. The photons possess particle property,
most notably photons can be reflected.
Energy of Photons
EMR transfers in terms of energy packets or quanta in accordance with
quantum theory. The particle that carries energy is called a photon. The
amount of energy (Joules) carried by a single photon is
q  h 
hc

Unit: Joules
Where h=6.6256E-34 J/s. It is obvious, the shorter the wavelength, the
more the energy a photon carries.
The total energy in a ray beam is the summation of the energy carried by
all the photons that makeup the beam.
n
Q   qi   N i h i
i 1
Unit: Joules
Unit of Energy
Energy:
Unit: Joules=J
Adding time energy measure: Energy flux
Unit: Joules/second=Watts=W
Adding area to energy flux: energy flux density
Unit: Joules/second/m2=W/m2
The Electromagnetic
Spectrum
Figure 2.6
Solar Constant
The radiant flux density at the top of the atmosphere
perpendicular to the sun beam:
S=1367 w/m2
Plane measuring S
Top of Atmosphere
Earth
Solar and Terrestrial Energy
Figure 2.7
Earth’s Energy Budget
Shortwave
Reflected
Radiation Balance Equation:
R n  Qsun  Qrefl  Learth  Lair  Qsun (1   )  Lnet
Figure 2.8
Distribution of Solar Radiation on
Earth Surface
Tropics receive more concentrated
insolation due to Earth’s curvature
Tropics receive 2.5× more than poles
Figure 2.9
The Seasons
Seasonality
Reasons for seasons
Annual march of the seasons
Insolation at Top of Atmosphere
Figure 2.10
Seasonality
Seasonal changes
Sun’s altitude – angle above horizon
Declination – location of the subsolar point
Daylength
TOA Daily Net Radiation
Unit: w/m2
Measured by Satellite (Nimbus-7) in space.
Figure 2.11
Reasons for Seasons
Revolution
Rotation
Tilt of Earth’s axis
Axial parallelism
Sphericity
Reasons for Seasons
Revolution
Earth revolves around the Sun
Voyage takes one year (365.25 days more
accurately)
Earth’s speed is 107,280 kmph (66,660 mph)
Rotation
Earth rotates on its axis once every 24 hours
Rotational velocity at equator is 1674 kmph
(1041 mph)
Revolution and Rotation
Figure 2.13
Reasons for Seasons
Tilt of Earth’s axis
Axis is tilted 23.5° from plane of ecliptic
Axial parallelism
Axis maintains alignment during orbit around the Sun
North pole points toward the North Star (Polaris)
Sphericity: not perfect sphere because of rotation,
the equator is bulging out
Sphericity: f = 1-short/long =1/298
f=? for a perfect sphere.
Axial Tilt and Parallelism
Sun declination angle: -23.5~23.5o
Figure 2.14
Annual March of the Seasons
Winter solstice – December 21 or 22
Subsolar point Tropic of Capricorn
Spring equinox – March 20 or 21
Subsolar point Equator
Summer solstice – June 20 or 21
Subsolar point Tropic of Cancer
Fall equinox – September 22 or 23
Subsolar point Equator
DL 
24  arccos(  tan  tan  )

11:30 P.M. in the Antarctic
Figure 2.16
Seasonal Observations
40oN
Figure 2.18
End of Chapter 2
Geosystems 7e
An Introduction to Physical Geography
Robert W. Christopherson
Charlie Thomsen