Motions of the Earth Class 6 NCERT

Overview: Why the Earth’s Motions Matter

The Earth has two main motions: rotation and revolution. Rotation means the Earth spins on its own axis, like a top. One complete rotation takes about 24 hours, giving us day and night. Revolution means the Earth moves around the Sun in a fixed path called an orbit. One complete revolution takes about 365 days and 6 hours. These extra 6 hours are important because after four years they become 24 hours, so one extra day is added to February in a leap year. The Earth’s axis is tilted at 23½ degrees. Because of this tilt, different parts of the Earth receive different amounts of sunlight during the year. This causes seasons.

Key Terms You Must Know

Axis

An imaginary line passing through the North Pole and South Pole on which the Earth rotates.

Rotation

The spinning movement of the Earth on its axis. It causes day and night.

Revolution

The movement of the Earth around the Sun in its orbit. It takes about one year.

Orbit

The fixed path along which the Earth moves around the Sun.

Leap Year

A year with 366 days. It occurs because the Earth takes 365 days and 6 hours to complete one revolution.

Solstice

A day when one hemisphere has its longest day or longest night because it is tilted most towards or away from the Sun.

Equinox

A day when day and night are almost equal all over the Earth.

Rotation and Revolution: Clear Comparison

How Seasons Happen Step by Step

1. Step 1: Notice the tilted axis — The Earth’s axis is not straight up and down. It is tilted at an angle of 23½ degrees. This tilt stays in the same direction while the Earth revolves around the Sun.
2. Step 2: Sunlight falls unevenly — When one hemisphere is tilted towards the Sun, it receives more direct sunlight and longer days. That hemisphere has summer. The opposite hemisphere receives slanting sunlight and has winter.
3. Step 3: Earth keeps revolving — As the Earth moves in its orbit, the position of each hemisphere compared to the Sun changes. After about six months, the hemisphere that had summer gets winter.
4. Step 4: Equal days occur twice a year — On about 21 March and 23 September, neither hemisphere is tilted strongly towards the Sun. Day and night are nearly equal everywhere. These days are called equinoxes.

Worked Examples with Tutor Reasoning

• Example 1: Why is it daytime in India when another part of the world may have night? Step 1: The Earth is spherical and rotates from west to east. Step 2: Only the half facing the Sun receives light at a given time; the other half remains in darkness. Step 3: As the Earth rotates, different places move into sunlight and then away from it. Final answer: Day and night happen because of Earth’s rotation, and different places face the Sun at different times.
• Example 2: Why do we add one day in a leap year? Step 1: The Earth takes 365 days and 6 hours to complete one revolution. Step 2: In a normal calendar year, we count only 365 days. Step 3: The remaining 6 hours add up over 4 years: 6 × 4 = 24 hours. Step 4: 24 hours make one full day, so it is added to February. Final answer: A leap year has 366 days because four extra 6-hour parts together make one extra day.
• Example 3: Why is 21 June the longest day in the Northern Hemisphere? Step 1: Around 21 June, the Northern Hemisphere is tilted towards the Sun. Step 2: It receives sunlight for a longer duration and the Sun’s rays are more direct. Step 3: Therefore, days are longer and nights are shorter in the Northern Hemisphere. Final answer: 21 June is the summer solstice in the Northern Hemisphere and gives its longest day.

Exam Tip: Do Not Mix Up the Two Motions

A common mistake is writing that revolution causes day and night. Remember: rotation causes day and night; revolution plus the tilted axis causes seasons. Also, do not write that the Earth’s axis changes direction every season. The axis remains tilted in the same direction while the Earth moves around the Sun. In answers, use exact terms like axis, orbit, rotation, revolution, solstice and equinox. A small labelled sketch of the Earth’s tilted axis can make a 3-mark answer much stronger.

Important Points to Remember

• The Earth rotates from west to east, which is why the Sun appears to rise in the east and set in the west.
• The circle that divides day and night on the globe is called the circle of illumination.
• The Earth takes about 24 hours to complete one rotation and about 365 days and 6 hours to complete one revolution.
• Seasons do not occur because the Earth becomes much closer to or farther from the Sun. They occur mainly because of the tilted axis and revolution.
• On equinoxes, day and night are nearly equal, while solstices bring the longest day or longest night in a hemisphere.

Practice Questions with Solutions

• Q: What is rotation? Explain one effect of rotation. A: Step 1: Rotation is the spinning of the Earth on its axis. Step 2: One complete rotation takes about 24 hours. Step 3: During rotation, the part facing the Sun has day and the part away from the Sun has night. Final answer: Rotation is the Earth’s spin on its axis, and its main effect is day and night.
• Q: The Earth takes 365 days and 6 hours to revolve around the Sun. How does this create a leap year? A: Step 1: A normal year counts 365 days. Step 2: The extra 6 hours remain each year. Step 3: In 4 years, 6 + 6 + 6 + 6 = 24 hours. Step 4: 24 hours make 1 full day, added to February. Final answer: Every fourth year becomes a leap year with 366 days.
• Q: Why does the Southern Hemisphere have winter around 21 June? A: Step 1: Around 21 June, the Northern Hemisphere is tilted towards the Sun. Step 2: At the same time, the Southern Hemisphere is tilted away from the Sun. Step 3: It receives slanting rays and has shorter days. Final answer: The Southern Hemisphere has winter around 21 June because it is tilted away from the Sun.
• Q: Differentiate between solstice and equinox. A: Step 1: A solstice occurs when one hemisphere is tilted most towards or away from the Sun. Step 2: It gives the longest day or longest night in that hemisphere. Step 3: An equinox occurs when day and night are nearly equal all over the Earth. Step 4: Equinoxes occur around 21 March and 23 September. Final answer: A solstice gives unequal day and night extremes, while an equinox gives nearly equal day and night.

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