1. The Sun’s Motion and The Four Seasons

2. Annual Motion of Sun The Ecliptic
Sun appears to move east to west each day due to Earth’s rotation.
Superimposed on this is a yearly motion towards the East that is a reflection of Earth’s orbital motion.
Signs of Zodiac.
This path is called the Ecliptic.

3. The Ecliptic
The Earth’s rotation axis is tilted 23.5 degrees to its orbital plane.
The rotation axis points to the same place in the sky (toward the star Polaris) no matter where the Earth is located in its orbit.

4. The Ecliptic
The tilt causes the Sun’s path in the sky to be at a 23.5 degree angle to the Earth’s equator.
This 23.5 degree angle is the cause of our seasons.

5. Thought Question TRUE OR FALSE? Earth is closer to the Sun in summer
and farther from the Sun in winter.

6. Why doesn’t distance matter?
Variation of Earth-Sun distance is small — about 3%; this small variation is overwhelmed by the effects of axis tilt.

7. Cause of the Seasons

8. Cause of the Seasons Direct and indirect lighting by the Sun.
Number of hours daylight to produce heating is changed.

9. Cause of the Seasons
These two causes occur because of the 23.5 degree angle and the Earth’s orbital motion.

10. What causes the seasons?
Seasons depend on how Earth’s axis affects the directness of sunlight

11. Direct light causes more heating.

12. Axis tilt changes directness of sunlight during the year.

13. We can recognize solstices and equinoxes by Sun’s path across sky:
Summer solstice: Highest path, rise and set at most extreme north of due east.
Winter solstice: Lowest path, rise and set at most extreme south of due east.
Equinoxes: Sun rises precisely due east and sets precisely due west.

14. Sun’s altitude also changes with seasons
Sun’s position at noon in summer: higher altitude means more direct sunlight.
Sun’s position at noon in winter: lower altitude means less direct sunlight.

15. Seasonal changes are more extreme at high latitudes
Path of the Sun on the summer solstice at the Arctic Circle

16. Summary: The Real Reason for Seasons
Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun.
Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct.
AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth.

17. First Day of Spring Spring or Vernal Equinox ~ March 21st
Sun directly overhead at Earth’s Equator
12 hours of daylight everyone on the Earth.

18. First Day of Summer Summer Solstice ~ June 21st
Sun is directly overhead at 23.5 degrees north, the Tropic of Cancer.
North Pole has 24 hours of sunlight, “Land of the Midnight Sun.”
Antarctica has 24 hours of darkness.

19. First Day of Autumn Autumnal Equinox ~ September 21st
Sun directly overhead at Earth’s Equator.
12 hours of daylight for everyone

20. First Day of Winter Winter Solstice ~ December 21st
Sun is directly overhead at 23.5 degrees south, at the Tropic of Capricorn.
South pole has 24 hours of daylight.
North pole has 24 hours of darkness.

21. The Seasons as Viewed From North Pole

22. Seasons and Weather In summer the Sun is overhead north of equator.
Intertropical conversion is north of equator.
Subtropical jet north of 30 degrees.
Polar jet north of 60 degrees.
In the summer the tropical jet stream drives our weather patterns.

23. Seasons and Weather In winter the Sun is overhead south of equator.
Intertropical conversion is south of equator.
Subtropical jet south of 30 degrees and near equator.
Polar jet south of 60 degrees and it may be south of Atlanta when the Siberian Express occurs.
In the winter the polar (can be split into north & south branches) and tropical jet streams drive our weather patterns.

24. Resources Ahrens, (1999). Meteorology Today (6th Ed.)
Allen, George (translation, 1964). The Flammarion Book of Astronomy
(Book VIII.)
Chaisson & McMillan, (2004). Astronomy Today (5th Ed.)
Bennett et al. (2004) The Cosmic Perspective (2nd Ed.)
Shipman, Wilson, and Todd, (2003). An Introduction to Physical Science
(10th Edition).