CBSE Class 6 Social Science Geography Chapter 3: Motions Of The Earth Notes

Key Definitions

Rotation

The movement of the Earth on its own axis. It causes day and night and takes approximately 24 hours to complete one rotation.

Axis

An imaginary line that passes through the centre of the Earth, around which it rotates. It is tilted at an angle of 23.5° from the perpendicular to the orbital plane (or 66.5° to the orbital plane).

Orbit

The fixed elliptical path along which the Earth revolves around the Sun.

Revolution

The movement of the Earth around the Sun in a fixed orbit. It takes approximately 365¼ days to complete one revolution, causing seasons.

Circle of Illumination

The imaginary circle that divides the Earth's surface into the day-lit half and the night-darkened half.

Solstice

The time of year when the Sun reaches its highest or lowest point in the sky at noon, marked by the longest and shortest days (Summer and Winter Solstice).

Equinox

The time or date (twice each year) at which the Sun crosses the celestial equator, when day and night are of approximately equal length (Spring/Vernal and Autumnal Equinox).

Leap Year

A year, occurring once every four years, which has 366 days instead of 365, with the extra day added to February (February 29th) to account for the quarter day from Earth's revolution.

Understanding Earth's Motions: Rotation and Revolution

The Earth is constantly in motion, performing two primary movements simultaneously: rotation and revolution. These motions are fundamental to our planet's climate, timekeeping, and life cycles. Understanding them is key to grasping basic geography.

Rotation is the spinning of the Earth on its imaginary axis. This axis is tilted at an angle of 23.5 degrees from the vertical (or 66.5 degrees to the orbital plane). One complete rotation takes approximately 24 hours, which is the duration of one day. The most significant effect of Earth's rotation is the occurrence of day and night. As the Earth spins, different parts of its surface face the Sun, experiencing daylight, while the side facing away experiences night. The Circle of Illumination is the imaginary line that separates the illuminated half from the dark half. Because the Earth is a sphere, only half of it gets light from the Sun at any given time.

Revolution is the movement of the Earth around the Sun in a fixed elliptical path called an orbit. One complete revolution takes about 365¼ days (or 365 days and 6 hours). This extra quarter day accumulates over four years to become one full day, which is added to the month of February, creating a leap year with 366 days. The Earth's revolution, combined with the tilt of its axis, is responsible for the change of seasons. As the Earth orbits the Sun, different hemispheres are tilted towards or away from the Sun at various times of the year, leading to variations in the amount of direct sunlight received and thus, temperature differences that define seasons. The tilt ensures that at certain times, the Northern Hemisphere leans towards the Sun (summer in the North) while at other times, it leans away (winter in the North), with opposite effects in the Southern Hemisphere.

Rotation vs. Revolution: Key Differences

Must Remember: Key Facts about Earth's Motions

• Earth's axis is tilted at an angle of 23.5° from its orbital plane's perpendicular, or 66.5° to its orbital plane itself.
• The Earth rotates from West to East, causing the Sun to appear to rise in the East and set in the West.
• The period of rotation is called an 'Earth day'.
• If the Earth's axis were not tilted, there would be no seasons; the length of day and night would be equal everywhere throughout the year.
• The revolution of the Earth causes changes in the position of the sun in the sky and thus, the changing duration of days and nights.
• Summer Solstice (June 21st): Northern Hemisphere tilts most towards the Sun, experiencing the longest day and shortest night. Southern Hemisphere has winter.
• Winter Solstice (December 22nd): Southern Hemisphere tilts most towards the Sun, experiencing the longest day and shortest night. Northern Hemisphere has winter.
• Equinox (March 21st & September 23rd): Neither hemisphere is tilted towards the Sun, resulting in equal day and night length across the globe.

Real-World Examples of Earth's Motions

• {"title":"Day and Night Cycle","description":"Every 24 hours, we experience a full cycle of daylight and darkness. This predictable pattern is a direct result of the Earth's rotation on its axis. As your city faces the sun, it's day; as it turns away, it's night."}
• {"title":"Seasonal Changes","description":"In India, we experience distinct seasons like summer, monsoon, and winter. These changes, such as hotter days in summer and colder days in winter, are caused by the Earth's revolution around the Sun combined with the tilt of its axis. For example, during June, the Northern Hemisphere (where India is located) is tilted towards the Sun, leading to longer, hotter days."}
• {"title":"Varying Day Length","description":"Notice how days are longer in summer and shorter in winter. This variation in the duration of daylight hours is another consequence of Earth's revolution and axial tilt. During summer solstice, areas closer to the poles experience very long days, even 24 hours of daylight in extreme northern/southern regions."}

Exam Tip: Differentiating Solstices and Equinoxes

Students often confuse solstices and equinoxes. Remember that 'solstice' means 'Sun stands still', referring to the Sun's apparent pause before reversing direction in the sky, marking the extreme points of daylight duration (longest/shortest day). 'Equinox' means 'equal night', indicating when day and night are roughly equal in length. Always relate these terms to Earth's tilt relative to the Sun: solstices occur when one hemisphere is maximally tilted towards or away from the Sun, while equinoxes occur when the Earth's axis is neither tilted towards nor away from the Sun, causing the Sun's rays to fall directly on the Equator.

Practice Questions with Solutions

• Q: What is the main cause of day and night? A: The main cause of day and night is the Earth's rotation on its axis.
• Q: How long does the Earth take to complete one revolution around the Sun? A: The Earth takes approximately 365¼ days to complete one revolution around the Sun.
• Q: What is a Leap Year and why do we have it? A: A Leap Year occurs every four years, adding an extra day (Feb 29th) to the calendar. It's needed because Earth's revolution is 365 days and 6 hours; these extra 6 hours accumulate to a full day every four years.
• Q: What is the significance of the Earth's tilted axis? A: The Earth's tilted axis, combined with its revolution around the Sun, is the primary reason for the occurrence of different seasons on Earth.

Frequently Asked Questions