GEOG 203: Planet Earth - Solar Energy and the Seasons

The Solar System, Sun, and Earth

Speed of Light and Cosmic Scale
- Light travels at approximately $300,000 \text{ km/sec}$ ( $186,000 \text{ miles/sec}$ ).
- A light year is the distance light travels in one vacuum year, totaling roughly $9.5 \text{ trillion km}$ . This is a measure of distance, not time.
- Look-back Time: Observationally, because light takes time to travel, we see celestial objects as they were in the past.
- Moon: $384,400 \text{ km}$ away ( $1.28 \text{ light-seconds}$ ).
- Sun: $150,000,000 \text{ km}$ away ( $8 \text{ minutes and 20 light-seconds}$ ).
- Milky Way Galaxy: Approximately $100,000 \text{ light years}$ across, containing billions of stars.

Dimensions and Distances

Earth’s Orbit and the Plane of the Ecliptic
- The Plane of the Ecliptic is the imaginary flat surface defined by Earth's orbit around the Sun.
- The average distance (Earth-Sun) is approximately $150 \text{ million km}$ , defined as $1 \text{ Astronomical Unit (AU)}$ .
Orbital Variation: Perihelion and Aphelion
- Earth's orbit is an ellipse, not a perfect circle.
- Perihelion: Earth is closest to the Sun ( $147,000,000 \text{ km}$ ) on January 3rd. Interestingly, this occurs during the Northern Hemisphere winter, proving distance is not the primary cause of seasons.
- Aphelion: Earth is farthest from the Sun ( $152,000,000 \text{ km}$ ) on July 4th.

Solar Activity and Radiation

Sunspots and the Solar Cycle
- Sunspots are dark, cooler regions on the Sun caused by intense magnetic activity.
- They follow an approximately $11 \text{-year cycle}$ . High sunspot activity (Solar Maximum) correlates with increased solar radiation output and potential disruptions to satellite communications on Earth.
Solar Wind and the Magnetosphere
- The Sun emits a stream of charged particles called Solar Wind.
- Magnetosphere: Earth's magnetic field acts as a shield, deflecting these particles. Near the poles, particles interact with the atmosphere to create Auroras (Aurora Borealis in the North; Aurora Australis in the South).
Principles of Electromagnetic Radiation
- Energy is emitted in electromagnetic waves characterized by:
  - Wavelength ( $\lambda$ ): Distance between successive crests.
  - Frequency ( $f$ ): Number of waves passing a point per second.
- Wien’s Displacement Law: Hotter objects emit shorter wavelengths (Sun = high energy shortwave), while cooler objects emit longer wavelengths (Earth = lower energy longwave).

The Electromagnetic Spectrum and Energy Balance

Temperature Scales: Absolute zero is $0 \text{ K}$ ( $-273.15^\circ \text{C}$ ).
Solar Radiation (Shortwave): The Sun ( $6000 \text{ K}$ ) emits energy primarily in the visible light, ultraviolet, and near-infrared spectrums.
Terrestrial Radiation (Longwave): The Earth ( $288 \text{ K}$ ) emits thermal infrared energy.
Solar Constant: The average insolation at the top of the atmosphere is $1372 \text{ W/m}^2$ .
Albedo and Absorption:
- $45\%$ of incoming radiation is reflected or scattered (Albedo).
- $24\%$ is absorbed in the atmosphere.
- $47\%$ is absorbed at the surface and later re-radiated as longwave heat.

Insolation and Earth's Curvature

Insolation (Incoming Solar Radiation): The amount of solar energy intercepted by Earth.
The Sphericity Factor: Because Earth is a sphere, the Sun's rays strike the surface at different angles:
- Subsolar Point: The specific location where the Sun is directly overhead ( $90^\circ$ angle), receiving maximum energy per unit area.
- High latitudes receive rays at lower angles, causing the energy to spread over a larger area (Beam Spreading) and pass through more atmosphere (Path Length Tracking), leading to cooler temperatures.

Factors Affecting Seasons

Revolution: The $365.24 \text{-day}$ trip around the Sun determines the annual cycle.
Rotation: The $24 \text{-hour}$ turn on Earth's axis creates the day/night cycle and the Circle of Illumination (the line dividing day and night).
Tilt (Obliquity): Earth's axis is tilted at $23.5^\circ$ from a line perpendicular to the plane of the ecliptic.
Axial Parallelism: The axis remains pointed toward the North Star (Polaris) throughout the entire orbit.

Annual March of the Seasons

Summer Solstice (June 20-21):
- Subsolar point is at the Tropic of Cancer ( $23.5^\circ \text{N}$ ).
- 24 hours of daylight at the Arctic Circle ( $66.5^\circ \text{N}$ ); 24 hours of darkness at the Antarctic Circle ( $66.5^\circ \text{S}$ ).
Fall Equinox (September 22-23):
- Subsolar point is at the Equator ( $0^\circ$ ).
- All locations on Earth experience exactly $12 \text{ hours}$ of day and night.
Winter Solstice (December 21-22):
- Subsolar point is at the Tropic of Capricorn ( $23.5^\circ \text{S}$ ).
- Arctic Circle experiences 24 hours of darkness; Antarctic Circle experiences 24 hours of daylight (‘Midnight Sun’).
Spring Equinox (March 20-21):
- Subsolar point returns to the Equator ( $0^\circ$ ), signaling the start of the warming trend in the Northern Hemisphere.