1. Unit 4: Sun-Earth Relationships
Revolution
Rotation
Declination (Tilt of Earth’s Axis)
Seasons
Time Zones
Insolation and its variation
Sunrise over the Earth. Source:

2. OBJECTIVES Examine the Earth’s motions relative to the Sun
Demonstrate the consequences of the Earth’s axis tilt for the annual march of the seasons
Introduce the time and spatial variations in solar radiation received at surface locations

3. Revolution of the Earth around the Sun
Perihelion-closest to Sun Jan. 3rd
Aphelion-farthest from Sun July 4th
Distance differences from Sun do not influence amount of solar energy received significantly
Earth revolves around the Sun in an elliptical path. Earth and Sun are not drawn to scale, and the orbit’s elongation is highly exaggerated for clarity.

4. Counterclockwise movements
Revolution-annual cycle
Rotation-daily cycle

5. Declination of the Earth’s Axis
Constant tilt of 23.5 degrees
Provides seasons
Video shows misconceptions about the cause of the seasons.

6. Midnight Sun: Within the Arctic and Antarctic Circle the Sun does not set at Summer Solstice

7. Solar altitude-Sun’s height above the horizon
Vertical (90o) Sun is at the equator on equinoxes
Vertical Sun is at the Tropic of Cancer on summer solstice
Vertical Sun is at the Tropic of Capricorn on winter solstice

8. March of the Seasons

9. World’s Time Zones Why are the boundaries not straight lines?
Source:
Why are the boundaries not straight lines?
Why are the Prime Meridian, International Dateline significant?

10. Daylight hours by dates and latitudes

11. Intensity of Sunlight
Source:
Reception of solar radiation at different latitudes showing direct, indirect rays

12. Relationship between solar noon altitude and daylight hours
Daylength (left axis) and noon solar altitude (right axis) at 45° north latitude. These factors work together in producing pronounced seasonality at middle and high latitudes. Notice also how the pace of change in both is much greater near the equinoxes than near the depths of summer and winter.

13. Values of seasonal differences of time and space

14. Spatial distribution of insolation at top of the atmosphere as percentage of global average
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Seasonal and spatial variation in solar radiation reaching the top of the atmosphere as a percentage of the global average. The latitude axis is scaled to acount for shrinking area at higher latitudes. Compare with patterns of the previous figures to see how solar position modulates the effect of daylength.