Motions of the Earth

Introduction

The Earth is never still. It moves in two important ways:

1. Rotation — Spinning on its axis (like a spinning top)
2. Revolution — Moving around the Sun in a fixed path (orbit)

These two motions cause day and night, the seasons, and the passage of a year. Let us understand each in detail.

Rotation — Spinning on the Axis

Rotation is the spinning of the Earth on its imaginary axis.

Key Facts About Rotation

Effects of Rotation

1. Day and Night — When one side of Earth faces the Sun, it experiences day. The other side experiences night.
2. Apparent movement of the Sun — The Sun appears to rise in the east and set in the west because Earth rotates from west to east.
3. Difference in time — Different longitudes have different local times.
4. Tides — Rotation affects ocean tides (along with the Moon's gravity).
5. Coriolis effect — Winds and ocean currents are deflected (to the right in the northern hemisphere, to the left in the southern hemisphere).

'The Earth is rotating so fast that at the equator, you are moving at about 1,670 km per hour without even realising it!'

Why Don't We Feel the Rotation?

We do not feel the Earth's rotation because:

• Everything on Earth (including air and water) moves with it.
• The motion is constant and smooth.
• There is no sudden change in speed.
• Like sitting in a smoothly moving car, you only feel motion when you look outside.

Revolution — Orbiting the Sun

Revolution is the movement of the Earth around the Sun in a fixed path called an orbit.

Key Facts About Revolution

The Extra Quarter Day

The Earth takes 365 days and about 6 hours (¼ day) to complete one revolution. To account for the extra 6 hours:

• Every four years, we add one extra day (24 hours = 6 hours × 4).
• That year is called a Leap Year with 366 days.
• The extra day is added to February — making it 29 days instead of 28.
• Leap years are divisible by 4 (e.g., 2024, 2028, 2032).
• Exception: Century years (like 1900) are leap years only if divisible by 400.

The year 2024 was a leap year. The next leap year after 2026 will be 2028.

Earth's Tilt — Why Seasons Occur

The Earth's axis is tilted at 23½° from the perpendicular (vertical). This tilt is the main reason for seasons.

• If the Earth's axis were not tilted, there would be no seasons.
• The tilt causes different parts of Earth to receive more or less direct sunlight at different times of the year.
• The tilt always points in the same direction (towards the North Star/Pole Star).

The 23½° tilt also causes the Sun's apparent path (the ecliptic) to vary throughout the year, which is why the Sun is higher in the sky during summer and lower in winter.

Solstices

A solstice is when the Sun reaches its farthest point north or south of the Equator. There are two solstices each year.

Summer Solstice (June 21)

• The Sun is directly overhead the Tropic of Cancer (23½°N).
• The Northern Hemisphere is tilted towards the Sun.
• The Northern Hemisphere experiences the longest day and shortest night of the year.
• The Southern Hemisphere experiences the shortest day and longest night.
• Summer begins in the Northern Hemisphere, winter in the Southern Hemisphere.
• In Andhra Pradesh, days are longer and nights are shorter.

Winter Solstice (December 22)

• The Sun is directly overhead the Tropic of Capricorn (23½°S).
• The Northern Hemisphere is tilted away from the Sun.
• The Northern Hemisphere experiences the shortest day and longest night of the year.
• The Southern Hemisphere experiences the longest day and shortest night.
• Winter begins in the Northern Hemisphere, summer in the Southern Hemisphere.
• In Andhra Pradesh, days are shorter and nights are longer.

The word 'solstice' comes from Latin 'sol' (sun) and 'sistere' (to stand still), because the Sun appears to stop moving north or south before changing direction.

Equinoxes

An equinox is when day and night are equal in length (12 hours each) all over the world. There are two equinoxes each year.

Spring Equinox (March 21)

• The Sun is directly overhead the Equator.
• Day and night are equal (12 hours each) everywhere on Earth.
• Spring begins in the Northern Hemisphere.
• Autumn (fall) begins in the Southern Hemisphere.

Autumn Equinox (September 23)

• The Sun is directly overhead the Equator.
• Day and night are equal everywhere on Earth.
• Autumn begins in the Northern Hemisphere.
• Spring begins in the Southern Hemisphere.

The word 'equinox' comes from Latin 'aequus' (equal) and 'nox' (night) — meaning 'equal night'.

Summary of Seasons

Aphelion and Perihelion

The Earth's orbit is elliptical (oval-shaped), not a perfect circle. So the distance between Earth and Sun changes during the year.

Interesting fact: Earth is closest to the Sun in January (perihelion), when the Northern Hemisphere is experiencing winter! This shows that Earth's distance from the Sun does NOT cause seasons — the tilt of the axis does.

Key Point

Remember: The Earth is closest to the Sun in January (during Northern Hemisphere winter) and farthest in July (during Northern Hemisphere summer). This proves that seasons are caused by Earth's tilt, not by distance from the Sun.

Effects of Motions of the Earth

Common Mistakes Students Make

• Thinking rotation and revolution are the same — rotation is spinning on axis; revolution is moving around the Sun.
• Believing seasons are caused by Earth being closer to or farther from the Sun — seasons are caused by Earth's tilt.
• Confusing solstices with equinoxes — solstices are the longest/shortest days; equinoxes are equal day and night.
• Forgetting the exact dates of solstices and equinoxes.
• Thinking the entire Earth experiences the same season at the same time — when it's summer in the north, it's winter in the south.

Quick Self-Test

1. What is the difference between rotation and revolution?
2. How long does the Earth take to complete one rotation?
3. What causes day and night?
4. What causes seasons on Earth?
5. What is a solstice? Name the two solstices.
6. What is an equinox? When does it occur?
7. What is a leap year and why do we have it?
8. What is the tilt of the Earth's axis?
9. What is perihelion and aphelion?
10. When is the day longest in Andhra Pradesh?

Exam Focus (5 Marks)

• Explain rotation and revolution of the Earth with their effects.
• How do solstices and equinoxes occur? Explain with dates.
• Why does the Earth experience seasons? Explain the role of the Earth's tilt.