1. Astronomical Control of Solar Radiation
Chapter 7
Astronomical Control of Solar Radiation
By: Jessica Juday, Lyudmila Koba, Luke Mros, Grant Prehn and Vincent Xu

2. Earth’s Tilted Axis of Rotation and the Seasons
Earth spins on its axis
Makes 1 complete revolution every 24hrs
Earth’s axis is tilted at 23.5°
Earth’s tilt referred to as it’s “obliquity”

3. Earths Rotation Around the Sun
1 revolution around the Sun = 365 days
Earth’s tilt combined with solar orbit gives us seasons
Seasons accumulate at Solstices
Summer Solstice - longest day
Winter Solstice - shortest day
Northern Hemisphere:
Summer Solstice Jun 21st
Winter Solstice Dec 21st
Southern Hemisphere:
Summer Solstice Dec 21st
Winter Solstice Jun 21st
(reverse from that of the north)

4. Earths Tilt and it’s Direction Are Constant Throughout Orbit
Reason why we have regular seasons
Earths tilt defines the Arctic Circles at 66.5°
During Winter Solstice, no direct sunlight reaches poleward of this latitude

5. At Winter Solstice, no direct sunlight reaches past this point

6. Equinoxes Equinoxes occur midway between solstices.
Direction of Earth’s tilt not pointing towards/away from the sun.
Days/Nights become equal in length

7. Earths Eccentric Orbit
Earths orbit is “Elliptical”
Orbital eccentricity is due to gravitational pull on Earth from other planets
Earth’s distance from the Sun varies due to position in elliptical orbit
Close pass: “Perihelion”
Distant pass: “Aphelion”

8. Close pass: “Perihelion” Distant pass: “Aphelion”
146 million km
Jan 3rd
Distant pass: “Aphelion”
152 million km
Jul 4th
7 days longer between equinoxes
3% variation in distance
Slight changes in radiation received
Small effect on seasons

10. Long-Term Changes in Earth’s Orbit
Orbit varies due to gravitational attractions between Earth and other Celestial bodies
Causes variation in Earth’s angle of tilt, eccentricity of orbit and positions of solstices/equinoxes in the orbit
These variations are Cyclical…

11. Cycles and Modulation…
Changes like Earth’s orbit and amount of radiation received occur in cycles.
We express these cycles as “Wavelengths”

12. Period Frequency Amplitude A wavelength expressed in units of time
# of cycles that occur in 1 Earth year
Amplitude
Measure of deviation from long-term average

13. Modulation Amplitude of peaks and valleys change in a cyclic way
Modulation of a cycle is not in itself a cycle!

14. Extremes of Tilt
If the Earth had a circular orbit with no tilt to the axis, we would have no seasons for there would be no change in solar radiation.
No Tilt

15. Extremes of Tilt
Alternately, if Earth’s axis had a tilt of 90 degrees, the poles would alternate between day-long darkness, and day-long direct overhead sunshine.
Equator
90 degree tilt

16. Decreasing Tilts
Decreased axis tilt diminishes the difference in seasons and brings it closer to that of the example below.

17. Effects on Polar Regions
Increased tilt of the axis results in more solar radiation at the summer season poles, and less to the winter season poles.

18. a and b = ½ the lengths of major and minor
The Shape of an Ellipse
This can be described by reference to the major (longer) and minor (shorter) axis.
The degree of departure from a circular orbit can be described by this equation.
E = Eccentricity
a and b = ½ the lengths of major and minor

19. The eccentricity of an ellipse is related to half of the lengths of it’s longer and shorter axes

20. Changes in Earth’s Orbit Through Time
The earth’s orbit used to be more elliptical or “eccentric” than it is today.
There is orbital variation at periods of 413,000 years, and 100,000 years.

21. Eccentricity Cycles
The longer cycle of 413,000 is not as noticeable because it appears in between the 100,000 year cycles between large and small peaks.
Larger amplitudes appear at 200,000 yrs, 600,000 yrs, and 1,000,000 yrs.
A third cycle happens at 2.1 million years, but it is weak in amplitude.

22. Precession What is it? -The motion of the axis of a
spinning body, such as the wobble of
a spinning top,when an external force
acting on the axis.

23. Earths Three Forms of Precession
Precession of the Axis
Precession of the Ellipse
Precession of the Solstices and the Equinoxes

24. 1. Precession of the Earths Axis
-One rotation every 25,000 years
-Caused by the gravitational pull on the earth's equatorial bulge

26. -Causes the Celestial northern star to vary
-Bright star at the bottom is the star Vega

27. Axial Precession Cycle

28. 2. Precession of the Ellipse
- The ellipse of the
earth’s orbit also has
a precession
-Rate of precession
is even slower than
that of the axial
precession
~22,000 years

29. 3. Precession of the Equinoxes
- The point where the equinoxes and solstices occur has a precession of it’s own.
Why is it important?
-the point at which the solstices and equinoxes occur determines the intensity of the seasons
What is it caused by?
-the combination of the precession of the axis and the ellipse.

30. One full rotation take between 19-23 Thousand years
What does it look like?
One full rotation take between Thousand years

32. Insolation Changes by Month and Season
Long-term changes in tilt
Long-term changes in precession

33. June and December insolation variations
Precession at low and middle latitudes
effects of tilt evident only at higher latitudes

34. Phasing of insolation maxima and minima
Difference between North Pole and South Pole

35. Insolation Changes by Caloric Seasons
Family of monthly precession curves
Caloric insolation seasons

36. Caloric season insolation anomalies

37. Complication from overlapping cycles
Searching for Orbital-scale Changes in Climatic Records

38. Time Series Analysis Power Spectrum Time Series Analysis
A method used to analyze climate record data, in hopes to extract rhythmic cycles.
References patterns against a time component.
Spectral Analysis
Referencing sine waves with climate data to measure their correlation.
A strong correlation indicates a strong cycle.
Power Spectrum
the result of a spectrum analysis
Used to show data.
prone to interference from equipment and climate irregularities.
Filtering
A method of honing in on a specific set of data to better analyze it without referencing other data.

40. Effects of Undersampling Climate Records
Time series analysis requires multiple cycle sightings to be accurate (>4).
Aliasing
False trends found by way of undersampling.
Combated with frequent readings and large sample sizes.
False trends tend to only measure part of natural cycles. This causes skewed results.

41. Tectonic-Scale Changes in Earth’s Orbit
Earth’s characteristics are not set in stone and are prone to change over time.
Coral studies from 440 million years ago.
11% more tidal cycles per year.
Earth spun 11% more on its rotational axis.
Not everything in earth is a cycle we can see.