Geoscience Reference
In-Depth Information
Figure 9.6. Variation of daily total solar irradiance (W-h/m 2 ) with day of the solar year
(December 21 ¼ day 1) for several northern latitudes.
high latitudes, solar input during summer is of primary concern because that is
when it mostly takes place.
Yearly average solar intensity was evaluated by Ward (1974). The result is:
1 = 2 ð
2
Þ½ 1 e 2
2
1 = 2 d
h S H i¼½ 1,367
2
½ 1 ð sin L cos
cos L sin
sin
'
where the integral is taken from
. For any latitude and obliquity one
must perform the integral numerically. Note that although h S H i depends signifi-
cantly on obliquity (
' ¼ 0to2
) it also depends very weakly on eccentricity (e). The reason
for this is that the Earth is sometimes closer to the Sun and sometimes farther
from the Sun during the course of a year. When it is closer to the Sun it is
moving faster in its orbit, and when it is farther from the Sun it is moving slower
in its orbit, so the effect of eccentricity is diminished.
9.4
IMPORTANCE OF EACH ORBITAL PARAMETER
Each of the orbital parameters has an effect on the distribution variability of solar
input to the Earth.
Eccentricity varies with a period of roughly 100,000 years but the amplitude of
these variations is highly variable. Eccentricities as high as 0.05, and as low as
0.01, have occurred over the past 800,000 years.
Eccentricity determines the degree of elongation of the Earth's elliptical orbit.
The more eccentric the orbit, the greater the variation of solar input to the Earth
during the course of a year.
 
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