Geography Earth Sun Relationship Unit 1

Number of planets in the solar system and Earth's position relative to the Sun

There are $8$ planets in total; Earth is the $3^\text{rd}$ planet away from the Sun.

Size diameter ratio of the Sun to Earth and Sun's share of solar system mass

The Sun's diameter is $108$ times larger than Earth's, and it accounts for 99.8\text{\null}\% of the total mass in the solar system.

Role of the Sun in Earth's natural environment and solar system dynamics

Controls planetary orbital motion via gravitational pull and shapes Earth's natural environment, including wind, ocean currents, vegetation, soils, air masses, weather, climate, and glaciers.

Three main manifestations of changing Earth-Sun relationships

1. Solar elevation (angle of incidence / Noon Solar Angle)

2. Day length (duration of sunlight)

3. Geographic location of sunrise and sunset

Day lengths at $42^\text{o}\,\text{N}$ on June 21 vs. December 21

June 21: $15\,\text{hours}$ December 21: $9\,\text{hours}$

Day lengths on June 21 at $0^\text{o}$, $90^\text{o}\,\text{N}$, and $90^\text{o}\,\text{S}$

• $0^\text{o}$ (Equator): $12\,\text{hours}$

• $90^\text{o}\,\text{N}$ (North Pole): $24\,\text{hours}$

• $90^\text{o}\,\text{S}$ (South Pole): $0\,\text{hours}$

Global variation pattern of sunrise and sunset locations

Sunrise and sunset locations vary seasonally everywhere around the world except at the Equator.

Three types of Sun rays striking Earth's curved surface

1. Perpendicular Rays: Strike directly overhead at a $90^\text{o}$ angle.

2. Oblique Rays: Strike at an angle, resulting in less intense energy.

3. Tangent Rays: Rays parallel to the surface tangent, located $90^\text{o}$ away from perpendicular rays.

Circle of Illumination

The boundary line that continuously divides Earth into a daylit hemisphere and a shadowed hemisphere as parallel solar rays strike Earth's spherical surface.

Key parameters of Earth's rotation

• Period: $1$ complete rotation every $24\,\text{hours}$

• Direction: Counter-clockwise when viewed looking down at the North Pole

• Speed at Equator: Approximately $1000\,\text{mph}$

• Perception: Constant rotational velocity prevents human perception of motion

Characteristics of Earth's revolution, Perihelion, and Aphelion

• Revolution Period: $365.25\,\text{days}$ in an elliptical orbit

• Perihelion (Jan 3): Earth is closest to the Sun at $91.5\,\text{million miles}$

• Aphelion (July 3): Earth is farthest from the Sun at $94.8\,\text{million miles}$

• Seasonal Impact: Orbital distance does NOT cause seasons

Plane of the Ecliptic and Earth's axial orientation

• Plane of the Ecliptic: The flat orbital plane defined by Earth's revolution around the Sun

• Axial Tilt: Earth's rotational axis is tilted $23.5^\text{o}$ from perpendicular to the ecliptic plane

• Parallelism: Earth's axis continuously points toward Polaris (the North Star)

Four key astronomical dates and their respective subsolar points

• December 21 (Winter Solstice): Subsolar point at $23.5^\text{o}\,\text{S}$

• June 21 (Summer Solstice): Subsolar point at $23.5^\text{o}\,\text{N}$

• September 21 (Autumnal Equinox): Subsolar point at $0^\text{o}$ (Equator)

• March 21 (Vernal Equinox): Subsolar point at $0^\text{o}$ (Equator)

Characteristics of the Equinoxes (March 21 and September 21)

• Axial orientation: Neither tilted toward nor away from the Sun

• Subsolar point: Equator ($0^\text{o}$)

• Circle of Illumination: Passes directly through both geographic poles

• Daylight distribution: Every point on Earth receives $12\,\text{hours}$ of light and $12\,\text{hours}$ of darkness

Characteristics of the June 21 Solstice

• Axial orientation: Northern Hemisphere tilted maximally toward the Sun

• Subsolar point: Tropic of Cancer ($23.5^\text{o}\,\text{N}$ / $23^\text{o} 30'\,\text{N}$)

• Daylight: Longest daylight period of the year in the Northern Hemisphere

• Polar illumination: Arctic Circle ($66.5^\text{o}\,\text{N}$) receives $24\,\text{hours}$ of light; Antarctic Circle ($66.5^\text{o}\,\text{S}$) receives $24\,\text{hours}$ of darkness

Characteristics of the December 21 Solstice

• Axial orientation: Northern Hemisphere tilted maximally away from the Sun

• Subsolar point: Tropic of Capricorn ($23.5^\text{o}\,\text{S}$ / $23^\text{o} 30'\,\text{S}$)

• Daylight: Longest daylight period of the year in the Southern Hemisphere

• Polar illumination: Arctic Circle ($66.5^\text{o}\,\text{N}$) receives $24\,\text{hours}$ of darkness; Antarctic Circle ($66.5^\text{o}\,\text{S}$) receives $24\,\text{hours}$ of light

Two primary factors responsible for Earth's seasons

1. The $23.5^\text{o}$ axial tilt of Earth and its consistent parallelism

2. Earth's revolution around the Sun altering relative position throughout the year

Definitions of Noon Solar Angle, Subsolar Point, and Zenith Point

• Noon Solar Angle: The angular height of the Sun above the horizon at solar noon

• Subsolar Point (Solar Declination): The geographic location where Sun rays hit at $90^\text{o}$ directly overhead; migrates across $47^\text{o}$ of latitude over the year

• Zenith Point: The point in the sky located directly overhead ($90^\text{o}$ above horizon)

Mathematical calculation of Zenith Angle and Noon Solar Angle

• Zenith Angle ($ZA$): Latitudinal difference between observer latitude and subsolar point (subtract if in same hemisphere, add if in opposite hemispheres)

• Noon Solar Angle ($NSA$): Calculated as $NSA = 90^\text{o} - ZA$

Seasonal daylight duration extremes in Cedar Falls

• June 21 (Summer Solstice): Approximately $15\,\text{hours}$

• December 21 (Winter Solstice): Approximately $9\,\text{hours}$

Seasonal sunrise and sunset directions in Cedar Falls

• June 21: Rises Northeast, sets Northwest

• March 21 & Sept 21: Rises due East, sets due West

• December 21: Rises Southeast, sets Southwest

Two primary natural environmental rhythms on Earth

1. Daily alternation of sunlight and darkness caused by rotation

2. Annual seasonal cycle (solstices and equinoxes) driven by revolution and axial tilt