Motions of the Earth
- Earth is an oblate spheroid. This means it is spherical in shape, but not perfectly round. It is slightly bulged at the equator and is flattened at the poles.
- The Earth has two main movements: rotation and revolution.
What is the Rotation of Earth?
- When the Earth spins or rotates around its axis, that movement of spinning is called Rotation of Earth.
- The axis is an imaginary line that goes through the North and South Poles.
- This rotation takes 24 hours, creating day and night.
- Equatorial Plane
- The imaginary plane that extends out from the Earth's equator.
- During Rotation: This plane is perpendicular to the Earth's axis and divides the Earth into the Northern and Southern Hemispheres.
What is the Revolution of Earth?
- The movement of the earth around the sun in a fixed path or orbit is called Revolution.
- It is an oval shaped path.
- This revolution takes 365.25 days, which is one year.
- Orbital plane
- The flat, imaginary surface on which the Earth’s orbit lies.
- The axis of the earth makes an angle of 66½° with its orbital plane.
The earth receives light from the sun and due to its spherical shape, only half of it gets light from the sun at a time.
The portion facing the sun experiences day while the other half, away from the sun, experiences night.
Session1: video
What happens if the earth did not rotate?
If the Earth did not rotate, one side would always face the Sun and the other side would always be in darkness. This would make the side facing the Sun extremely hot and the side in darkness very cold. There would be no regular day and night cycle. The weather patterns and ocean tides we are used to would also change drastically, creating extreme conditions.
No Day and Night: If the Earth stopped rotating, one side would always be daytime (always sunny), and the other side would always be night-time (always dark).
Extreme Temperatures: The side with constant day would become very hot, while the side with constant night would get very cold.
Strange Weather: Without the Earth's rotation, winds and ocean currents would change, causing strange and possibly dangerous weather, like super strong storms.
Changing Oceans: The oceans might move towards the North and South Poles, causing flooding in some places and making other places dry.
No Magnetic Shield: The Earth's rotation helps protect us from harmful solar rays. If it stopped, this protection might weaken, making it unsafe for living things.
Harm to Plants and Animals: Many plants and animals rely on the day-night cycle. If this changes, they could struggle to survive, leading to many species disappearing.
Notes:
- The revolution of the earth around the sun takes approximately 365 days and 6 hours.
- We ignore the 6 hours for our convenience.
- Once every four years, these 6 hours are added to make one day.
- This extra day is added to the month of February which has the fewest number of days.
Seasons
- Seasons change due to:
- The revolution of the earth around the sun
- Inclination of the earth's axis in a fixed direction.
- Due to the change in the position of Earth, a year is divided into summer, winter, spring, and autumn.
Summer Solstice
Summer Solstice begins on 21st June in the Northern Hemisphere.
On this day, the Northern Hemisphere is tilted towards the Sun.
Sunrays fall on the Tropic of Cancer.
The North Pole is inclined towards the Sun, and areas beyond the Arctic Circle experience continuous daylight for about six months.
The longest day and shortest night occur on 21st June, the Summer Solstice.
The conditions in the Southern Hemisphere are reversed, and it is winter season there.
Winter Solstice
The Winter Solstice occurs on the 22nd of December in the Northern Hemisphere.
The Southern Hemisphere at this time is tilted towards the Sun.
The Sunrays fall on the Tropic of Capricorn in the Southern Hemisphere.
The Southern Hemisphere has longer days during this period and shorter nights.
The conditions are reversed in the Northern Hemisphere.
Equinox
The direct rays of the Sun fall on the equator on 21st March and 23rd September.
The whole Earth experiences equal days and equal nights because neither of the poles is tilted towards the Sun.
On 23rd September, it is autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
It is the opposite on 21st March when it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
1. Answer the following questions briefly.
What is the angle of inclination of the earth's axis with its orbital plane?
Ans: The angle of inclination of the earth's axis with its orbital plane is 66 ½ °.
Define rotation and revolution.
Ans:
Rotation: It is the movement of the Earth on its axis. It takes 24 hours for Earth to complete one rotation. This movement of the planet Earth on its axis is considered responsible for day and night on Earth.
Revolution: This is the 360° movement of Earth in its orbit around the sun. It takes 365 ¼ days to complete one revolution. This causes seasons on Earth.
What is a leap year?
Ans: Each revolution of the earth takes 365 ¼ days i.e. 365 days and 6 hours. These 6 hours sum up to 24 hours i.e. one day in 4 years. This one extra day is added to February every fourth year and it has 29 days instead of 28 days. So, this year has 366 days and is called a leap year.
Differentiate between the Summer and Winter Solstice.
Ans:
Summer Solstice: On 21st June, the sun's rays directly fall on the Tropic of Cancer, and the Northern Hemisphere is tilted towards the sun. The areas near the pole receive minimal heat because of the slanting rays. The North Pole of the Earth is said to be inclined towards the sun and the places beyond the Arctic Circle experience continuous daylight for about six months. All these conditions are reversed in the Southern Hemisphere and it is winter season there. The nights are longer than the days. This is called the Summer Solstice.
Winter Solstice: In this, the Tropic of Capricorn receives direct rays from the sun as the south pole tilts towards the sun. This takes place on 22nd December every year. It is the summer season in the Northern Hemisphere whereas it is exactly opposite in the Southern Hemisphere i.e. the presence of summer. The regions experiencing winters have longer nights than days. This is called the Winter Solstice.
What is an equinox?
Ans: An equinox is a phenomenon when the sun's rays directly fall on the equator. At this position, the whole of the earth experiences equal days and nights as neither of the poles is tilted towards the sun. This occurs twice a year on 21st March and 23rd September.
Why does the Southern Hemisphere experience winter and Summer Solstice at different times than that of the Northern Hemisphere?
Ans: The Southern Hemisphere experiences winter and Summer Solstice at different times than that of the Northern Hemisphere because the poles tilt towards the sun during different times.
Why do the poles experience about six months of day and six months of night?
Ans: The poles experience about six months of day and six months of night because each of the poles remains tilted towards the sun for six months straight. So, this continuous tilt causes the poles to experience about six months of day and when the other pole tilts towards the sun it causes six months of night.
2. Tick the correct answer.
(a) The movement of the earth around the sun is known as
(i) Rotation
(ii) Revolution
(iii) Inclination
Ans: (ii) Revolution
(b) Direct rays of the sun fall on the equator on
(i) 21 March
(ii) 21 June
(iii) 22 December
Ans: (i) 21 March
(c) Christmas is celebrated in summer in
(i) Japan
(ii) India
(iii) Australia
Ans: (iii) Australia
(d) Cycle of the seasons is caused due to
(i) Rotation
(ii) Revolution
(iii) Gravitation
Ans: (ii) Revolution
3. Fill in the blanks.
A leap year has _____ number of days.
Ans: 366 days
The daily motion of the earth is ____ .
Ans: rotation
The earth travels around the sun in _____ orbit.
Ans: elliptical
The sun's rays fall vertically on the Tropic of _____ on 21st June.
Ans: cancer
Days are shorter during _____ season.
Ans: Winter
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