Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 247

Vernal equinox
March 20

(a) THE ORBITAL CALENDAR (b) Precession of the Earth

axis

Summer
solstice
June 21

focii

1.47 . 10 8 km

Aphelion
July 4

Perihelion
Jan. 3

1.52 . 10 8 km

SUN

Winter
solstice
Dec. 21

center
of ellipse

*

The magnitude of axial tilt has also varied between
22 ° to 24.5 ° over the past 250 ky

Autumnal equinox
Sept. 22

(c) Precession of the equinoxes

March 20

Sept. 22

Dec. 21

TODAY 5.5 KA 11 KA

March 20

Dec. 21

June 21

SUN

Dec. 21 June 21

Sept. 22

Sept. 22

June 21 March 20

NOW

(d)

5

past future

eccentricity

3

+

1

precession

-

24.0

23.5

23.0
22.5

tilt

-250 -200 -150 -100 -50 0 +50 +100

ky BP ky AP

Fig. 6.12 (a), (b), and (c) are the various Milankovich mechanisms for long-term global climate change; (d) Computed changes in eccentricity,
tilt, and precession for the past 0.25 my and for the future 0.1 my.

circular, a fact known since the work of Kepler in the sev-
enteenth century. The very nature of an elliptical path,
with the radiating Sun at one focus, means that there are
seasonal variations in the amount of radiation received by

Earth at aphelion and perihelion. This is based on the
premise that the intensity of solar radiation is reduced with
distance from the Sun. At present, Earth is nearest the
Sun, by about 4.6 10 6 km, on 2-3 January and furthest