Chapter 7
Astronomical Control of Solar Radiation
By: Jessica Juday, Lyudmila Koba, Luke Mros, Grant
Prehn and Vincent Xu
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”
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)
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
At Winter Solstice, no direct sunlight reaches past this point
Equinoxes
Equinoxes occur midway
between solstices.
• Direction of Earth’s tilt not
pointing towards/away
from the sun.
• Days/Nights become equal
in length
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”
Close pass: “Perihelion”
•
•
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
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…
Cycles and Modulation…
Changes like Earth’s orbit and amount of radiation
received occur in cycles.
We express these cycles as “Wavelengths”
•
Period
•
•
Frequency
•
•
A wavelength expressed in
units of time
# of cycles that occur in 1
Earth year
Amplitude
•
Measure of deviation from
long-term average
Modulation
• Amplitude of peaks and
valleys change in a cyclic
way
• Modulation of a cycle is not
in itself a cycle!
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
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
Decreasing Tilts
Decreased axis tilt diminishes the
difference in seasons and brings it closer
to that of the example below.
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.
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
The eccentricity of an ellipse is related to half
of the lengths of it’s longer and shorter axes
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.
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.
• What is it?
Precession
-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.
Earths Three Forms of Precession
1. Precession of the Axis
1. Precession of the Ellipse
1. Precession of the Solstices and the
Equinoxes
1. Precession of the Earths Axis
-One rotation every
25,000 years
-Caused by the
gravitational pull on the
earth's equatorial bulge
-Causes the Celestial northern star to vary
-Bright star at the bottom is the star Vega
Axial Precession Cycle
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
http://www.youtube.com/watch?v=82p-DYgGFjI&list=PL1Iewcbx3MoVFpQmNOgSy1cu8LeNZUegN
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.
What does it look like?
One full rotation
take between 19-23
Thousand years
Insolation Changes by Month and
Season
Long-term changes in
tilt
Long-term changes in
precession
June and December
insolation variations
Precession at low and
middle latitudes
effects of tilt evident
only at higher
latitudes
Phasing of insolation
maxima and minima
Difference between
North Pole and South
Pole
Insolation Changes by Caloric
Seasons
Family of monthly
precession curves
Caloric insolation
seasons
Caloric
season
insolatio
n
anomali
es
Complication from
overlapping cycles
Searching for Orbitalscale Changes in
Climatic Records
Time Series Analysis
•
•
● Power Spectrum
○ the result of a spectrum
o A method used to analyze
analysis
climate record data, in
○ Used to show data.
hopes to extract rhythmic
○ prone to interference from
cycles.
equipment and climate
o References patterns against
irregularities.
a time component.
● Filtering
Spectral Analysis
○ A method of honing in on a
o Referencing sine waves with
specific set of data to
climate data to measure
better analyze it without
their correlation.
referencing other data.
o A strong correlation
indicates a strong cycle.
Time Series Analysis
Effects of Undersampling
Climate Records
•
•
Time series analysis requires multiple cycle sightings to be accurate (>4).
Aliasing
o False trends found by way of undersampling.
o Combated with frequent readings and large sample sizes.
o False trends tend to only measure part of natural cycles. This causes
skewed results.
Tectonic-Scale Changes in
Earth’s Orbit
•
•
Earth’s characteristics are not set in stone and are prone to change
over time.
o 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.