Solar Radiation by Rujan Timsina EEC.ppt

Energy Engineering

1. Chapter 7:
Astronomical Control of Solar
Radiation
By: Rujan Timsina
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2. Earth’s tilted axes of rotation and the
seasons
•Earth spins on it’s axes once per day.
–Appears as Sun rising and setting.
•Is it really rising?
–No
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3. •Earth is constantly tilted at 23.5 degrees
•Obliquity
•Spins the fastest at the equator
•Earth goes around the sun (about) once per year
•Summer = more concentrated radiation
•Winter = weaker concentration of radiation
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4. Solstices
•Summer and winter solstice mark the longest and
shortest days of the year.
– (daylight hours, still 24 hours in a day)
–Longest day in northern hemisphere is June 21st
–Shortest day in northern hemisphere is December
21st.
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5. •South Pole, photo by NASA
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6. •Why is the north pole in
darkness for part of the
year?
•MATH, that’s why.
90 degrees –23.5 (the tilt) = 66.5
No light poleward of 66.5 during winter months
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7. Earth’s eccentric orbit
•Earth has an elliptical orbit
–Result of planets, moons, and sun’s gravity
•Ellipses have two focal points
•Sun lies at one
•The other lies empty
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8. •Perihelion
•Greek meaning “near the sun”
•Aphelion
•Greek meaning “away from the sun”
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9. •Equinoxes –night and day become equal in
each hemisphere
•Describing earth’s orbit around the sun
–Two equinoxes
–Two solstices
•Four points/ four different times of year
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10. •What causes the seasons?
–How close the earth is to the sun!
•No (come on we JUST covered that)
–The tilt of the earth, OBLIQUITY
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11. •Without tilt we would have no seasons
•Tilt has varied through time
–22.2 –24.5 degrees
–Time between tilt changes is about 41,000
years
–Changes in tilt angle can drastically amplify
seasons
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12. 7-5 part 1: Procession of the solstices and the equinoxes
around Earths orbit
•The positions of the equinoxes and solstices have not
always been fixed at their current locations
–Slowly shifted through time
–The cause of these changes: Long term wobbling
(similar to a “top”)
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13. •Wobbling motion of the Earth is defined “axial
precession”
–Caused by the Earths gravitational pull of the sun
and moon
•(bulge in Earths diameter)
•KEYNOTE: Earth wobbles very slowly!
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14. •Precession of the ellipse
•Entire elliptically shaped orbit of the earth rotates in
space
•This motion is even slower than the wobbling motion of
“axial precession”
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15. •“Precession of the equinoxes”
•Causes equinoxes and solstices to move around Earths
orbit with one 360 degree rotation around the sun
•This is completed every 22,000 years
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16. The procession of the equinoxes involves
complicated motions in 3D space
two geometric characteristics of motions
The first characteristic has to do with the
angular form of earths motion around the
sun
-W Omega
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17. •The second aspect of Earths orbital motion is eccentricity
–NOTE: If Earths orbit were a perfect circle the
movement of the solstices and equinoxes would not
change the amount of sunlight we see on earth
•Sun would remain constant through time
•Due to Earths non-circular orbit the movements of the
solstices and equinoxes do cause long term changes in
the amount of solar radiation received on Earth
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18. 7-5 part 2
Astronomical Control of Solar Radiation
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19. Aphelion and Perihelion
•Our current position:
•June 21st Solstice-Aphelion
•December 21st Solstice-Perihelion
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20. •Cycles occur over periods of 23,000
years
•Best explained as a sine wave
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21. •23,000 years –Precession
•Earth’s wobble
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22. •Eccentricity-Affects the magnitude of
change
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23. •High eccentricity results in more
contrasting change.
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24. •Why is this important?
•Seasons are more or less extreme due to
the change in the tropics position.
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25. •This also combines with the elliptical
orbit.
•Right now we are closest to the sun
during winter
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26. Insolation Changes by Month and
Season
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27. What is Insolation Changes?
•The amount of insolation that an area receives
vary over the course of a day or over a year.
•The highest point of the path in the sky is the time
when the maximum amount (intensity) of
insolation for the day reaches a location.
•June and December insolation values have a
strong dominance of the lower and middle
latitudes during summer season.
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28. What causes this?
•Tilt Change In Phases.
•Summer insolation maximum in the N.
hemisphere occur at the same time in the
41,000 year cycle as summer insolation
maximum in the S. hemisphere
•N and S poles are exactly out of phase at a
fixed position in the orbit
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29. Summer Insolation
•Insolation changes as a function of latitude
•Strong 23,000 precession signal at low to
middle latitudes
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30. Winter Insolation
•Similar pattern as the summer insolation.
•Strong 23,000 precession signal at low to
middle latitudes
•41,000 cycle
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31. Obliquity
•41,000 Year cycle.
•Not evident in low latitudes
•Evident in high latitudes
•Summer season changes larger then winter
•Winter small because no insolation at high
latitudes
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32. Eccentricity change in Insolation
•Eccentricity produces small insolation changes
•Change in seasonal insolation due to tilt and
precession
•Vary by ~10% about a mean value
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33. Precession Changes
•Precession causes out-of-phase changes
between hemispheres for their summer and
winter seasons.
•Earth-Sun distance controls change in insolation
•Insolation maximum on June 21 is a summermaximum
in the N hemisphere
•But a winterinsolation maximum in the S hemisphere
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34. 7-7: Isolation Changes by
Caloric Seasons
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35. Terms from 7-7
•Perihelion: the point of orbit closest to the
sun
•Aphelion: the point of orbit farthest from the
sun
•Result: the orbit of earth is fastest near
perihelion and slowest at aphelion
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36. Solstices
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37. Impact of Orbit
•Slower orbit farthest from the sun results in longer
solstice
•Faster orbit near the sun results in quicker solstice
•Total energy change in orbit is calculated by total
energy received by the earth
•Term for these energy changes: (1 –e2)1/2
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38. •Tilt causes in-phase changes for polar regions of
both hemispheres in their summer and winter
seasons
•Precessioncauses out-of-phase changes between
hemispheres for their summer and winter seasons
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40. Orbital Tilt
•During summer: Earth has a max of insolation of
hemisphere facing closer to the sun (more energy)
•During Winter: Earth has min of insolation of
hemisphere facing closer to the sun (less energy)
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41. Orbital Precession
•When aphelionhappens: Earth experiences min for
both summer and winter
–Because sun is farthest from the earth
•When perihelion happens: Earth has max for both
summer and winter
–Because sun is closest to the earth
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42. Closest to the Sun
•Precessional-cycle max occurring on June 21 (or Dec.
21) will be simultaneous everywhere on earth
•Important to remember that the perihelion phase of
earth’s orbit produces higher insolation than normal
over earth’s entire surface
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43. Caloric Isolation Seasons
•Summer caloric half-year is defined as the 182 days
of intense insolation that exceeds the other 182 days
of the winter caloric half-year.
•Impact: Allows for plotting of insolation graphs to
show how orbit changes the insolation patterns of
earth
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45. 7-8: Time Series Analysis
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46. Time Series Analysis
Time series analysis is a group of
techniques used for extracting periodic
signals from complex signals, and
quantifying their strength, relative
timing, and correlation
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47. Spectral Analysis
Spectral analysis is a numerical technique used for
detecting and quantifying the distribution of periodic
behavior in a complex signal.
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48. Power Spectrum
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Power spectrum is a graphic display of the
distribution of power against the period of
each cycle present in a signal
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49. 7-9: Effects of Undersampling
Climate Records
Aliasing is the misleading signals that result from
sampling a record of climate change at too low a
resolution.
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50. 7-10: Tectonic Scale Changes in
Earths Orbit
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51. Coral!
Corals are made of banded layers of CaCO
3
caused by seasonal changes in sunlight and
water temperature
Like tree-rings:
each band = 1 year
Unlike tree-rings, corals have secondary banding
due to their sensitivity to tidal cycles
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52. Coral continued…
Corals from 440MYA show 11% more tidal
cycles per year than our present day corals!
This suggests Earth experienced 11% more days
than today.
If we do the math approximating 365 days/year:
365 + 365(.11) = ~405 days in a year!
Approximately 21 hours in a day
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