Class 6 Geography Notes Chapter 3 (Motions of the Earth) – The Earth Our Habitat

Dear Students, read the detailed notes for Chapter 3: Motions of the Earth, from the NCERT Class 6 Geography textbook 'The Earth: Our Habitat', tailored for government exam preparation.

Chapter 3: Motions of the Earth

The Earth exhibits two primary types of motions simultaneously:

1. Rotation
2. Revolution

1. Rotation

• Definition: The movement of the Earth on its own axis.
• Axis: An imaginary line passing through the North Pole and South Pole, around which the Earth spins.
• Tilt of the Axis: The Earth's axis is tilted at an angle of 23.5° with respect to the vertical line perpendicular to its orbital plane. Alternatively, it makes an angle of 66.5° with its orbital plane (the plane formed by the Earth's orbit around the Sun). This tilt is crucial and remains constant throughout the orbit.
• Direction: The Earth rotates from West to East.
• Duration: It takes approximately 24 hours to complete one rotation. This period is known as the Earthday.
• Effects of Rotation:
  • Formation of Day and Night: As the Earth rotates, only one half of it faces the Sun at any given time, experiencing day. The other half, away from the Sun, experiences night.
  • Circle of Illumination: The imaginary circle on the globe that divides the part experiencing day from the part experiencing night. Importantly, due to the axis tilt, the Circle of Illumination does not coincide with the axis.
• Significance: Rotation causes the daily cycle of day and night, influences wind directions, ocean currents, and the daily rhythm of life.
• What if the Earth did not rotate? One half would permanently face the Sun, experiencing continuous warmth and daylight, leading to extremely hot conditions. The other half would remain in permanent darkness and be freezing cold. Life would likely not be possible in such extreme conditions.

2. Revolution

• Definition: The movement of the Earth around the Sun in a fixed, elliptical path called an orbit.
• Duration: It takes 365 days and 6 hours (approximately 365 1/4 days) for the Earth to complete one revolution around the Sun. We consider a year as consisting of 365 days for convenience.
• Leap Year:
  • The extra 6 hours saved every year are added up over four years (6 hours x 4 = 24 hours = 1 day).
  • This extra day is added to the month of February every fourth year.
  • Such a year with 366 days is called a Leap Year, and February has 29 days instead of the usual 28.
• Effects of Revolution:
  • Change of Seasons: The primary effect of revolution, combined with the constant tilt of the Earth's axis, is the cycle of seasons (Summer, Winter, Spring, Autumn).

3. Seasons

• Cause: Seasons change due to the change in the Earth's position as it revolves around the Sun, coupled with the unchanging tilt of its axis. The tilt causes different hemispheres to receive more direct or oblique sunlight at different times of the year.

• Key Positions and Dates:

  • Summer Solstice (Around June 21st):

    • The Northern Hemisphere is tilted towards the Sun.
    • The Sun's rays fall directly on the Tropic of Cancer (23.5° N).
    • Areas north of the Equator receive more heat and light.
    • Results in Summer in the Northern Hemisphere.
    • Days are longest, and nights are shortest in the Northern Hemisphere.
    • Places beyond the Arctic Circle (66.5° N) experience continuous daylight for about six months.
    • The Southern Hemisphere is tilted away from the Sun, experiences Winter, with shorter days and longer nights.

  • Winter Solstice (Around December 22nd):

    • The Southern Hemisphere is tilted towards the Sun.
    • The Sun's rays fall directly on the Tropic of Capricorn (23.5° S).
    • Areas south of the Equator receive more heat and light.
    • Results in Summer in the Southern Hemisphere (longest days, shortest nights).
    • Places beyond the Antarctic Circle (66.5° S) experience continuous daylight for about six months.
    • The Northern Hemisphere is tilted away from the Sun, experiences Winter, with shorter days and longer nights.

  • Equinox (Around March 21st and September 23rd):

    • On these two dates, neither pole is tilted towards the Sun.
    • The Sun's rays fall directly on the Equator (0°).
    • The Circle of Illumination passes through both poles.
    • The entire Earth experiences equal days and equal nights (approximately 12 hours each).
    • March 21st: Known as the Spring Equinox (or Vernal Equinox) in the Northern Hemisphere and Autumn Equinox in the Southern Hemisphere.
    • September 23rd: Known as the Autumn Equinox in the Northern Hemisphere and Spring Equinox (or Vernal Equinox) in the Southern Hemisphere.

Summary Table for Seasons:

These notes cover the core concepts of Earth's motions and their consequences as detailed in NCERT Class 6 Geography Chapter 3, focusing on points relevant for competitive exams. Remember to visualize these concepts using diagrams of the Earth's tilt and orbit.