Understanding Earth's Seasons and Solar Radiation

Earth's Orbit and Distance to the Sun

• Earth travels around the Sun in an elliptical orbit, leading to variation in distance between 3-4 million miles throughout the year.
• Key Points:
• January: Perihelion (closest to the sun)
• July: Aphelion (farthest from the sun)
• Seasons are influenced not by distance but by Earth's axial tilt.

Axial Tilt and Sun Exposure

• Earth's axis is tilted, affecting how sunlight hits different parts of the planet.
• Modeling Earth’s Tilt:
• Imagine the sun (a chair) sending out rays of light.
• Northern Hemisphere tilts towards the sun in summer, increasing direct sunlight exposure.
• As the year progresses, South Hemisphere receives more direct sunlight in its summer, illustrating seasonal changes.

Solar Radiation and Energy Distribution

• Solar energy spreads over the Earth's surface; how this affects temperature:
• Direct light (closer to the equator) = more concentrated energy.
• Less direct light (closer to poles) = spread out energy, leading to cooler temperatures.
• Energy surplus near equator; energy deficit near poles.
• Implications for Weather:
• Differential heating affects weather patterns, ocean currents, and climate.

Subsolar Point

• The subsolar point is where the sun's rays strike the Earth directly at noon, leading to concentrated solar radiation.
• Movement:
• The subsolar point shifts north and south with changing seasons (bouncing between Tropic of Cancer and Tropic of Capricorn).
• Effects on day length and the changing seasons:
  • Northern Spring: subsolar point moves north; days get longer.
  • Southern Spring: subsolar point moves south; opposite is true for the Northern Hemisphere.

Seasons and Solstices

• Four seasonal markers annually:

1. December Solstice (Winter)
   • Date: December 21/22
   • Northern Hemisphere is tilted away from the sun (shortest day).
   • Southern Hemisphere experiences summer (longest day).
2. March Equinox (Spring)
   • Date: March 20/21
   • Subsolar point at equator, equal day and night length.
   • Spring begins in Northern Hemisphere, autumn in Southern Hemisphere.
3. June Solstice (Summer)
   • Date: June 21/22
   • Northern Hemisphere tilted toward the sun (longest day).
   • Southern Hemisphere experiences winter (shortest day).
4. September Equinox (Fall)
   • Date: September 22/23
   • Subsolar point back at equator, equal day and night length.

Variability of Solar Radiation

• Geometric Angle:
• At noon, a sun directly overhead = subsolar point; high solar energy density.
• As the sun angle decreases, energy is spread out more thinly, reducing energy per unit area.
• Practical Example:
• Equator experiences more concentrated sunlight vs. poles where sunlight is spread out.
• Energy surplus near equator leads to warmer temperatures and energy deficit at poles leads to cooler temperatures.

Analemma Visualization

• The analemma shows the sun's position at different latitudes throughout the year.
• Latitude Effects:
• Areas within Tropics receive direct sunlight at noon on two days of the year; outside the Tropics, sun will never be directly overhead.

Conclusion