1. Earth Sun Geometry

2. Define/describe aphelion, perihelion, North Star, plane of the ecliptic
Trace the path of the perpendicular rays of the sun over the surface of the Earth over a year
Explain how tilt generates the march of the seasons
Explain how tilt shapes sun angle and the length of daylight
Explain how sun angle controls the intensity and amount of insolation
Describe how insolation patterns vary for the tropics, the midlatitudes, and the poles over a year for both the Northern and Southern hemispheres
Sketch a conceptual model for the march of seasons
Define the components of Milankovitch orbital cycles
Define/describe interglacial, glacial, Pleistocene, Holocene
Explain how the last glacial maximum and Holocene warming have shaped sea levels
Describe how weak seasonal contrasts create glacial conditions
Describe how strong seasonal contrasts create interglacial conditions

3. Earth-Sun Geometry
Driving variable of environmental processes on Earth
Geometry determines the amount and intensity of incoming solar radiation (insolation) reaching particular earth
Geometry (and how it changes) determines:
Seasonality (1 yr)
Glacial (cold) and interglacial (warm) periods (1000’s of years) 3

4. Which determines march of the seasons?
Aphelion and perihelion?
Tilt?

5. Tilt is more relevant for the march of the seasons.
Titled toward the Sun in summer in the NH, tilted away from the sun in the NH winter.
Tilt is more relevant for the march of the seasons.

6. North Star Southern Cross
Axis points toward North Star in Northern Hemisphere and Southern Cross in Southern Hemisphere.
Southern Cross

7. Can tilt change on smaller time scales?
Yes, but very, very little
The Earth tilted about an inch (2.5 centimeters) on its axis, and day length decreased by 3 millionths of a second from the 2004 Indian Ocean earthquake (9.1 RS)
Natural daily wobble may range within a circle of 33 feet (like an unbalanced tire).
Above: Tsunami

8. In conjunction with the curvature of the Earth, tilt determines the concentration and distribution of insolation striking the earth
Flashlight effect
Headlights effect

9. Flashlight effect TROPICS High sun angle: Larger concentration
Same amount of insolation is spread over a larger area—more diffuse. Flashlight effect
TROPICS
High sun angle:
Larger concentration
of insolation per area
MIDLATITUDES
HIGH LATITUDES
Intermediate sun angle
Low sun angle:
Smaller concentration of
insolation per area.

10. Headlights effect

11. As the Earth revolves around the Sun, tilt creates variation in day and night lengths over the duration of a calendar year
Circle of illumination
What day of the year is this?

13. At Solstices, sun’s declination places it directly above a tropic.

14. But what more directly makes the weather change?
…the energy imbalance
between tropics and poles drives circulation of
atmosphere and ocean 14

15. Solar declination: latitude that receives direct overhead (90 degrees) insolation. Declination, migrates annually from Tropic of Cancer (+23.5° N) to Tropic of Capricorn (-23.5°S).

16. But what more directly makes the weather change?
…..and this heat imbalance shifts back and forth over a year in response to the Earth revolving around the Sun 16

17. How you experience the changing of the seasons in terms of Earth-Sun geometry
Sun’s altitude: angle between horizon and sun is different throughout the year. Sun is not always directly overhead at a 90° angle at noon. Max sun angles in summer, lower in winter
Day/night length: Maximum contrast at solstices to uniformity (12 hrs day/12 hours night) across the globe on the equinoxes.
Declinations are used for other things as well—magnetic declination, for example.

18. Long-term variations in Earth-Sun geometry
Contribute to alternating climates:
Interglacial (warm)
Glacial (cold)
Have profound effects on sea-level
Pleistocene: approximately 2 million year period of glacials and interglacials

19. Pleistocene was not an “Ice Age”, but a two million year period of
oscillating warm and cold climates.

20. Interglacials and glacial period20

23. Bering Straits—human migration; land mass of eastern seaboard, Bahamas, Gulf, California.

25. Important dates for natural climate change
Wisconsin glaciation (20,000 ybp)
Boreal forests extended as far south as Atlanta and Birmingham, Alabama.

26. Important dates for natural climate change
Holocene interglacial (10,000 ybp)
Warming marked the start of the Holocene. 26

28. Milankovitch Cycles
Name of the geometric changes that influence Earth-Sun geometry over long temporal scales
Three components:
Orbital eccentricity
Obliquity
Precession

29. Milankovitch cycles shape interglacials and glacial periods

30. 1. Orbital eccentricity
Distance between Earth & Sun changes over scale of ~100,000 years
This changes length of seasons

31. 2. Obliquity
Tilt varies between degrees over a time scale of ~40,000 years.
Present tilt of 23.5 degrees can be considered unchanging from your scale of observation

32. 3. Precession Earth wobbles on its axis Physics of a spinning object
Point in orbit where aphelion and perihelion varies slightly
Occurs over times scales of ~20,000 years 32

33. Components of
Milankovitch Cycles

34. Mechanism?
Milankovitch cycles create weaker or stronger seasonal contrasts.
Weaker seasonal contrasts►glacial climates dominate
Stronger seasonal contrasts►interglacial climates dominate

35. Interglacial (warm) epoch
Strong seasonal contrasts
Highly elliptic orbit and large tilt
Cold winters: less evaporation: less snow: less glacial accumulation
Hot summers: more glacial melting
Net loss of glacial extent and warmer temperatures
Sea levels rise

36. Glacial (cold) epoch Weak seasonal contrasts
Less elliptic orbit and small tilt
Warm winters: more evaporation: more snow: more glacial accumulation
Cool summers: less glacial melting
Net gain of glacial extent and cooler temperatures
Sea levels drop

37. Visualization of impacts of
Milankovitch cycles on
total solar radiation over 15,000 year period
More color equals greater seasonality
Note weak contrasts in seasonality roughly 19,000 years ago (last pulse of glacial expansion during the Wisconsin glaciation) and strong contrasts in seasonality 10,000 years ago (beginning of warming that marked start of Holocene)