Introduction to Earth and Earth-Sun Relations

Geography Definition: Derived from the Greek words "Geo" (earth) and "graphy" (writing); focuses on spatial distribution, landscape variations, and interrelationships across the planet.
Primary Branches:
- Physical Geography: Focuses on natural environmental processes (landforms, climate, vegetation, soils, water).
- Human Geography: Focuses on human culture, activities, population, settlements, and political systems.
Global Environmental Change: Involves both natural and human-caused processes altering Earth's landscapes, with special attention to accelerating human impacts.
Globalization: Refers to the increasing interconnectedness of global economic and environmental systems.

Origin and Age: Consensus indicates the Big Bang occurred approximately $13.7\,\text{billion years}$ ago; the Solar System formed between $4.5$ and $5\,\text{billion years}$ ago.
Earth's Shape: Properly classified as an oblate spheroid, bulging at the equator and flattening at the poles due to rotational forces.
Key Dimensions:
- Equatorial Circumference: $24,901\,\text{miles}$ .
- Equatorial Diameter: $7,926\,\text{miles}$ ( $12,756\,\text{km}$ ).
- Polar Diameter: $12,714\,\text{km}$ .
Geographic Grid System (Graticule):
- Latitude: Angular description of location measured north or south of the equator ( $0^\circ$ at the equator to $90^\circ\text{N}$ / $90^\circ\text{S}$ at the poles).
- Longitude: Angular description of location measured east or west of the Prime Meridian ( $0^\circ$ to $180^\circ$ ); meridians extend pole to pole, cross parallels at right angles, and converge at the poles.

Earth Motions:
- Rotation: West-to-east on its axis every $24\,\text{hours}$ (counterclockwise viewed relative to the North Pole).
- Revolution: Elliptical path around the Sun taking $365.25\,\text{days}$ (one tropical year).
Orbital Extremes:
- Perihelion: Point closest to the Sun ( $147,100,000\,\text{km}$ or $91,400,000\,\text{miles}$ ) occurring on January 3rd.
- Aphelion: Point farthest from the Sun ( $152,100,000\,\text{km}$ or $94,500,000\,\text{miles}$ ) occurring on July 4th.
- Distance variations do not cause changes in seasons.
Axial Tilt: Earth's rotational axis is tilted $23.5^\circ$ relative to the perpendicular of the plane of the ecliptic.
Annual March of the Seasons:
- June Solstice (June 21): Vertical rays strike the Tropic of Cancer ( $23.5^\circ\text{N}$ ); Arctic Circle ( $66.5^\circ\text{N}$ ) receives $24\,\text{hours}$ of daylight, while Antarctic Circle ( $66.5^\circ\text{S}$ ) experiences $24\,\text{hours}$ of darkness.
- December Solstice (December 21): Vertical rays strike the Tropic of Capricorn ( $23.5^\circ\text{S}$ ); Antarctic Circle receives $24\,\text{hours}$ of daylight, while Arctic Circle experiences $24\,\text{hours}$ of darkness.
- Equinoxes (March 20 & September 22): Vertical rays strike the Equator ( $0^\circ$ ); daylight and night hours are equal worldwide.

Eccentricity: Slow variation in the shape of Earth's elliptical orbit over a cycle lasting approximately $100,000\,\text{years}$ .
Obliquity: Fluctuations in the tilt angle of Earth's rotational axis between $22.1^\circ$ and $24.5^\circ$ (currently around $23.5^\circ$ ) over a $41,000\text{-year}$ cycle.
Precession: Axial wobbling that shifts the direction the axis points (between the North Star and Vega) in a $25,800\text{-year}$ cycle, altering the seasonal distribution of solar energy.