Rotation and Revolution Chapter Notes | Geography for SSS 1 PDF Download

Introduction

The Earth undergoes two main types of movements: rotation and revolution. Rotation is the spinning of the Earth on its axis, which causes day and night, while revolution is the Earth's movement around the Sun, leading to seasonal changes. These movements, along with the inclination of the Earth's axis, influence various natural phenomena like the length of day and night, tides, and the Coriolis effect. Understanding these concepts is essential to grasp how Earth's motions shape our environment and daily life.

Rotation, Revolution & Inclination of Earth's Axis

Rotation

• Earth spins on its axis from west to east, completing one rotation in 24 hours.
• This spinning movement is called diurnal motion, causing the Sun to rise in the east and set in the west.
• At the equator, the Earth's rotational speed is about 1,670 km per hour, covering a distance of approximately 40,000 km in one rotation.
• Rotational speed decreases as you move north or south from the equator.
• At 60° north and south latitudes, the speed is half of that at the equator.
• At the poles, there is almost no movement due to rotation.

Effects of Rotation

• Causes the cycle of day and night.
• Creates time differences across different regions of the Earth.
• Makes the Sun appear to rise in the east and set in the west.
• Shapes the Earth into an oblate spheroid, with a bulge at the equator and flattened poles.
• Produces the Coriolis effect, which deflects winds and ocean currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Contributes to the daily occurrence of tides in oceans, influenced by the Moon’s gravitational pull, happening twice a day with a westward movement.

Illumination

• The Circle of Illumination separates the lit and dark parts of the Earth.

Revolution

• Earth moves around the Sun in an elliptical orbit, completing one revolution in 365¼ days.
• This movement is called annual motion and occurs from west to east.
• The elliptical orbit causes the distance between the Earth and Sun to vary slightly. The two positions are known as Perihelion (PERI = Close + HELIOS = Sun) and Aphelion (AP = Far + HELIOS = Sun).
• Perihelion is when Earth is closest to the Sun (147 million km) on January 3.
• Aphelion is when Earth is farthest from the Sun (152 million km) on July 4.

Effects of Revolution

• Leads to varying lengths of day and night, causing different seasons throughout the year.
• Results in changes in the altitude of the midday Sun.

Inclination of the Earth’s Axis

• The Earth's axis is tilted at 6½˚ to the orbital plane or 23½˚  from the vertical.
• This tilt causes opposite seasons in the Northern and Southern Hemispheres (summer in one, winter in the other).
• Without the axis tilt, all places on Earth would have equal day and night lengths, and there would be no seasons.

leap year

• A solar year is 365 days, with the extra 6 hours accumulated over four years to form an extra day, added to February in a leap year (366 days).

Seasonal Changes

• Earth’s revolution around the Sun in an elliptical orbit causes seasonal changes.
• The tilt of Earth's axis and its movement around the Sun lead to varying seasons and day-night lengths.
• Four main seasons are experienced: Summer, Autumn, Winter, and Spring.
• When the North Pole is tilted toward the Sun, the Northern Hemisphere has summer, and the Southern Hemisphere has winter.
• When the South Pole is tilted toward the Sun, the Southern Hemisphere has summer, and the Northern Hemisphere has winter.
• Seasons are linked to four key positions of Earth in its orbit: Summer Solstice, Autumn Equinox, Winter Solstice, and Spring Equinox.

Equinox

• Equinox means equal night, where day and night are of equal length (12 hours each) worldwide.
• Occurs when the Sun is directly over the Equator (0° latitude).
• Spring Equinox (March 21):
  • Sun’s rays fall directly on the Equator.
  • The Circle of Illumination passes through both poles, dividing all parallels equally.
  • Northern Hemisphere experiences spring, Southern Hemisphere experiences autumn.
• Autumn Equinox (September 23):
  • Sun’s rays fall directly on the Equator.
  • The Circle of Illumination passes through both poles, dividing all parallels equally.
  • Northern Hemisphere experiences autumn, Southern Hemisphere experiences spring.

Solstice

• Solstice means the Sun appears to stand still or reaches its highest point in the sky.
• Occurs when the Sun’s rays are directly over the Tropic of Cancer or Tropic of Capricorn.
• The Tropics mark the northern and southern limits of the Sun’s apparent movement.
• Sun’s rays are vertical between the Tropics; beyond them, rays are inclined.
• Summer Solstice (June 21):
  • Sun’s rays are directly over the Tropic of Cancer at noon.
  • Northern Hemisphere is tilted toward the Sun, experiencing longer days and shorter nights, receiving maximum heat.
  • Southern Hemisphere has shorter days and longer nights, receiving less heat.
  • The Circle of Illumination divides parallels unequally, causing unequal day and night lengths.
  • Northern Hemisphere experiences summer, Southern Hemisphere experiences winter.
  • North of the Arctic Circle (66½° N), continuous daylight occurs; south of the Antarctic Circle (66½° S), total darkness occurs for a few weeks.
• Winter Solstice (December 22):
  • Sun’s rays are directly over the Tropic of Capricorn at noon.
  • Southern Hemisphere is tilted toward the Sun, experiencing longer days and shorter nights, receiving maximum heat.
  • Northern Hemisphere has shorter days and longer nights, receiving less heat.
  • The Circle of Illumination divides parallels unequally, causing unequal day and night lengths.
  • Southern Hemisphere experiences summer, Northern Hemisphere experiences winter.
  • South of the Antarctic Circle (66½° S), continuous daylight occurs; north of the Arctic Circle (66½° N), total darkness occurs for a few weeks.

Twilight, Dawn, and Dusk

• Twilight is the period of diffused light in the sky due to the scattering or reflection of Sun’s rays when the Sun is up to 18° below the horizon.
• Dawn is the period of diffused light before sunrise.
• Dusk is the period of diffused light after sunset.

Key Words

• Rotation: Earth's spinning on its axis from west to east in 24 hours.
• Coriolis Effect: Deflection of winds and ocean currents due to Earth's rotation, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Circle of Illumination: Imaginary line separating the lit and dark parts of Earth’s surface, also a Great Circle.
• Revolution: Earth's movement around the Sun in 365¼ days from west to east.
• Elliptical Orbit: The oval-shaped path of Earth’s orbit around the Sun.
• Perihelion: Earth's closest position to the Sun in its orbit.
• Aphelion: Earth's farthest position from the Sun in its orbit.
• Axis of the Earth: Imaginary line through Earth on which it rotates from west to east.
• Orbital Plane: The plane in which Earth revolves around the Sun, also called the Plane of Elliptic.
• Leap Year: A year with 366 days, where February has 29 days, occurring every four years.
• Equinox: When the Sun’s rays are directly over the Equator, causing equal day and night lengths worldwide.
• Solstice: When the Sun’s rays are directly over the Tropics, causing unequal day and night lengths except at the Equator.
• Twilight: Period of diffused light in the sky due to scattering of Sun’s rays.
• Dawn: Period of diffused light before sunrise.
• Dusk: Period of diffused light after sunset.