Chapter 3: Motions of the Earth
Social Science - Geography (CBSE Class 6)
Chapter Summary
- The Earth has two primary motions:
- Rotation: The movement of the Earth on its own axis.
- Revolution: The movement of the Earth around the Sun in a fixed path or orbit.
- Axis of the Earth: An imaginary line that passes through the centre of the Earth, around which it rotates. It is tilted at an angle of 66½° to its orbital plane (the plane formed by the Earth's orbit).
- The orbital plane is the plane in which the Earth revolves around the Sun.
- Rotation and Day and Night:
- The Earth rotates from west to east.
- The period of one rotation is approximately 24 hours, which is known as an "Earth day".
- Rotation causes day and night. The portion of the Earth facing the Sun experiences day, while the other half experiences night.
- Circle of Illumination: The imaginary circle that divides the day from the night on the globe. This circle does not coincide with the axis.
- Revolution and Seasons:
- The Earth takes 365¼ days (one year) to complete one revolution around the Sun.
- For convenience, we consider a year as consisting of 365 days and ignore the quarter day.
- Leap Year: The extra quarter day (6 hours) accumulates over four years to make one full day (6 × 4 = 24 hours). This extra day is added to the month of February, making it 29 days instead of 28. Such a year with 366 days is called a leap year, occurring every four years.
- Revolution, along with the tilt of the Earth's axis, causes the change of seasons.
- Seasonal Variations:
- Summer Solstice (June 21): The Northern Hemisphere is tilted towards the Sun. The Sun's rays fall directly on the Tropic of Cancer (23½° N). This results in longer days and shorter nights in the Northern Hemisphere. The areas beyond the Arctic Circle (66½° N) experience continuous daylight for about six months. This is summer in the Northern Hemisphere and winter in the Southern Hemisphere.
- Winter Solstice (December 22): The Southern Hemisphere is tilted towards the Sun. The Sun's rays fall directly on the Tropic of Capricorn (23½° S). This results in longer days and shorter nights in the Southern Hemisphere. The areas beyond the Antarctic Circle (66½° S) experience continuous daylight for about six months. This is winter in the Northern Hemisphere and summer in the Southern Hemisphere (e.g., Christmas in Australia is celebrated in summer).
- Equinox (March 21 & September 23): On these dates, the Sun's direct rays fall on the Equator. Neither pole is tilted towards the Sun, so the entire Earth experiences equal days and equal nights.
- March 21: Spring or Vernal Equinox in the Northern Hemisphere, Autumn in the Southern Hemisphere.
- September 23: Autumnal Equinox in the Northern Hemisphere, Spring in the Southern Hemisphere.
- Due to the Earth's revolution and the tilt of its axis, the positions of sunrise and sunset, and the length of day and night, keep changing throughout the year, leading to different seasons.
Questions and Answers
1. Answer the following questions briefly.
a) What is the angle of inclination of the Earth's axis with its orbital plane?
Answer: The angle of inclination of the Earth's axis with its orbital plane is 66½°.
b) Define rotation and revolution.
Answer:
- Rotation: The movement of the Earth on its own axis is called rotation. The Earth completes one rotation in approximately 24 hours, causing day and night.
- Revolution: The movement of the Earth around the Sun in a fixed path or orbit is called revolution. The Earth takes 365¼ days to complete one revolution, which causes the change of seasons.
c) What is a leap year?
Answer: The Earth takes 365¼ days to revolve around the Sun. For convenience, we consider a year to have 365 days. The extra quarter day (6 hours) accumulated over four years makes one full day (24 hours). This extra day is added to the month of February, making it 29 days instead of 28. A year with 366 days is called a leap year, and it occurs every four years.
d) Differentiate between the Summer Solstice and Winter Solstice.
Answer:
- Summer Solstice: Occurs on June 21. The Northern Hemisphere is tilted towards the Sun, receiving direct rays on the Tropic of Cancer. This results in longer days and shorter nights in the Northern Hemisphere and is considered summer. The areas beyond the Arctic Circle experience continuous daylight for about six months.
- Winter Solstice: Occurs on December 22. The Southern Hemisphere is tilted towards the Sun, receiving direct rays on the Tropic of Capricorn. This results in longer days and shorter nights in the Southern Hemisphere and is considered summer there. The areas beyond the Antarctic Circle experience continuous daylight for about six months. The Northern Hemisphere experiences winter during this time.
e) What is an equinox?
Answer: An equinox is a time when the Sun's direct rays fall on the Equator. On these dates (March 21 and September 23), neither of the Earth's poles is tilted towards the Sun, resulting in equal days and equal nights all over the Earth.
f) Why does the Southern Hemisphere experience Winter and Summer Solstice in different times than that of the Northern Hemisphere?
Answer: The Southern Hemisphere experiences Winter and Summer Solstice at different times than the Northern Hemisphere due to the Earth's tilt on its axis and its revolution around the Sun. When the Northern Hemisphere is tilted towards the Sun (Summer Solstice on June 21), the Southern Hemisphere is tilted away from it, experiencing winter. Conversely, when the Southern Hemisphere is tilted towards the Sun (Winter Solstice on December 22, but summer for SH), the Northern Hemisphere is tilted away, experiencing winter.
g) Why do the poles experience approximately six months day and six months night?
Answer: The poles experience approximately six months of day and six months of night due to the Earth's axial tilt (66½° to its orbital plane) and its revolution around the Sun.
- During the Summer Solstice (June 21), the Northern Hemisphere is tilted towards the Sun, causing the Arctic Circle and areas beyond to experience continuous daylight for about six months.
- During the Winter Solstice (December 22), the Southern Hemisphere is tilted towards the Sun, causing the Antarctic Circle and areas beyond to experience continuous daylight for about six months.
- During these periods, the opposite pole experiences continuous night. For example, when the Arctic experiences continuous day, the Antarctic experiences continuous night, and vice-versa.
2. Tick the correct answer.
a) The movement of the Earth around the Sun is known as:
i) Rotation
ii) Revolution
iii) Inclination
Answer: ii) Revolution
b) Direct rays of the Sun fall on the Equator on:
i) 21 March
ii) 21 June
iii) 22 December
Answer: i) 21 March (and 23 September)
c) Christmas is celebrated in summer in:
i) Japan
ii) India
iii) Australia
Answer: iii) Australia
d) The cycle of the seasons is caused due to:
i) Rotation
ii) Revolution
iii) Gravitation
Answer: ii) Revolution
3. Fill in the blanks.
a) A leap year has _________ number of days.
Answer: 366
b) The daily motion of the Earth is ___________.
Answer: rotation
c) The Earth travels around the Sun in an _____________ orbit.
Answer: elliptical
d) The Sun’s rays fall vertically on the Tropic of _________ on 21st June.
Answer: Cancer
e) Days are shorter during ___________ season.
Answer: winter
Comments
Post a Comment