04_G1_Seasons

Seasons

What Causes Seasons?

  • The tilt of Earth’s axis causes two primary effects:
    • Changing daylight hours
    • Changing solar intensity
  • If Earth’s axis weren’t tilted, there would be no seasons.

Seasons: Four Factors

  • Seasons are influenced by four main factors:
    1. Earth’s revolution and the plane of the ecliptic
    2. Axial tilt
    • Axial tilt of Earth is approximately 23.5°.
    1. Parallel axis
    • Earth's axis points to Polaris, also known as the North Star.
    1. Subsolar point
    • The subsolar point is the singular point at which the Sun’s rays are perpendicular to Earth’s surface, usually around noon.
    • The latitude of the subsolar point varies between 23.5° N (Tropic of Cancer) and 23.5° S (Tropic of Capricorn).

1. The Plane of the Ecliptic

  • The plane of the ecliptic refers to the flat plane that the orbital paths of the planets trace.
  • Earth is closer to the Sun in January, a phenomenon known as perihelion, than it is in July, referred to as aphelion.

2. Axial Tilt

  • The axial tilt of Earth is crucial for the changing seasons.
    • Tilt angles include 23.5°, 0° (no tilt), and extreme angles such as 90°.

3. Parallel Axis

  • The concept of the parallel axis indicates the orientation of Earth’s axis in relation to the other celestial bodies, maintaining a consistent alignment toward Polaris.

4. The Subsolar Point

  • The subsolar point plays a significant role in solar energy distribution across the planetary surface.

The Circle of Illumination

  • The circle of illumination is the line separating night from day on Earth.
  • Sunrise and sunset events occur along this line.
  • The latitude of the subsolar point is always 90° away from the circle of illumination.

Migration of the Subsolar Point

  • The subsolar point migrates between the Tropics of Cancer and Capricorn.
  • The latitude of the subsolar point is termed solar declination.

Solstices and Equinoxes

  • Key dates and their associated subsolar points:
    • December Solstice (typically 12/21)
      • Subsolar point at the Tropic of Capricorn
      • Represents winter solstice in the Northern Hemisphere.
    • March Equinox (typically 3/20)
      • Subsolar point crosses the equator
      • Marks the spring equinox in the Northern Hemisphere.
    • June Solstice (typically 6/21)
      • Subsolar point at the Tropic of Cancer
      • Represents summer solstice in the Northern Hemisphere.
    • September Equinox (typically 9/22)
      • Subsolar point crosses the equator
      • Marks fall equinox in the Northern Hemisphere.

What if there were no axial tilt?

  • North Pole
  • South Pole
  • With no axial tilt, the subsolar point would remain fixed at the equator.

What if there were extreme axial tilt?

  • 90° axial tilt:
    • The subsolar point changes dramatically, affecting global climate and daylight patterns.
  • Scenarios include subsolar points on the South Pole during extreme tilts.

Solstices, Equinoxes, and Day Length

  • Equinoxes: All locations on Earth experience 12 hours of day and night.
  • June Solstice: Day length increases in the Northern Hemisphere and decreases in the Southern Hemisphere.
  • December Solstice: Day length decreases in the Northern Hemisphere and increases in the Southern Hemisphere.

Solar Altitude

  • Solar altitude is the angle of the Sun above the horizon.
    • At the horizon = 0 degrees
    • Directly overhead = 90 degrees
  • Long path length and short path length refer to how sunlight travels through the atmosphere based on solar altitude.

Hours of Daylight on the Winter Solstice

  • Distribution of hours of daylight varies greatly at different latitudes:
    • Some areas receive little to no daylight (under 3 hours), while others may receive continuous daylight (over 10.5 hours) near the equator.
  • Specific city examples include:
    • Los Angeles: Typically receives more sunlight during winter solstice.
    • Arctic Circle: May experience long periods of darkness.

Land of the Midnight Sun

  • At the poles, there are phenomena of “6-month day” and “6-month night.”
    • Both poles experience 6 months of continuous darkness followed by 6 months of continuous daylight.
  • At the North Pole:
    • Noon solar altitude ranges from 23.5° to the south during the June solstice to 0° on the horizon at the September equinox.
    • After the September equinox, the Sun moves below the horizon for 6 months and does not reappear until the March equinox.