The Earth - Notes | Geography for UPSC 2024 (Pre & Mains) PDF Download

 Page 1


 
 
THE EARTH 
 
SHAPE OF THE EARTH 
Earth is not perfect sphere, it is flattened at both ends. Hence, it is called 
geoid. 
 
GRAVITATION  AT POLES AND EQUATOR: 
 
 
    
 
Earth's gravity is stronger at the poles than the equator for two 
reasons: 
 
1) At latitudes nearer the Equator, the outward centrifugal force 
produced by Earth's rotation is larger than at polar latitudes. 
This counteracts the Earth's gravity to a small degree – up to a 
maximum of 0.3% at the Equator – and reduces the apparent 
downward acceleration of falling objects. 
 
2) The second major reason for the difference in gravity at different 
latitudes is that the Earth's equatorial bulge (itself also caused by 
centrifugal force from rotation) causes objects at the Equator to be 
farther from the planet's centre than objects at the poles. Because the 
force due to gravitational attraction between two bodies (the 
Page 2


 
 
THE EARTH 
 
SHAPE OF THE EARTH 
Earth is not perfect sphere, it is flattened at both ends. Hence, it is called 
geoid. 
 
GRAVITATION  AT POLES AND EQUATOR: 
 
 
    
 
Earth's gravity is stronger at the poles than the equator for two 
reasons: 
 
1) At latitudes nearer the Equator, the outward centrifugal force 
produced by Earth's rotation is larger than at polar latitudes. 
This counteracts the Earth's gravity to a small degree – up to a 
maximum of 0.3% at the Equator – and reduces the apparent 
downward acceleration of falling objects. 
 
2) The second major reason for the difference in gravity at different 
latitudes is that the Earth's equatorial bulge (itself also caused by 
centrifugal force from rotation) causes objects at the Equator to be 
farther from the planet's centre than objects at the poles. Because the 
force due to gravitational attraction between two bodies (the 
 
 
Earth and the object being weighed) varies inversely with the 
square of the distance between them, an object at the Equator 
experiences a weaker gravitational pull than an object at the 
poles. 
 
PLEASE NOTE: 
 
You also weigh less at equator because of the same reason. (W= 
mg)  
 
 
ROTATION OF THE EARTH 
 
     
The earth rotates about an imaginary line that passes through the North and 
South Poles of the planet. This line is called the axis of rotation 
Hence, one portion of the earth’s surface comes towards the rays of the 
sun and experiences day light. The other portion is away from the sun’s 
rays and that will be darkness. 
Earth rotates eastward. Earth rotates once in about 24 hours with respect to 
the Sun , but once every 23 hours, 56 minutes, and 4 seconds with respect 
to other, distant, stars. Our rotation period with respect to the stars  is 
called a sidereal day. 
Page 3


 
 
THE EARTH 
 
SHAPE OF THE EARTH 
Earth is not perfect sphere, it is flattened at both ends. Hence, it is called 
geoid. 
 
GRAVITATION  AT POLES AND EQUATOR: 
 
 
    
 
Earth's gravity is stronger at the poles than the equator for two 
reasons: 
 
1) At latitudes nearer the Equator, the outward centrifugal force 
produced by Earth's rotation is larger than at polar latitudes. 
This counteracts the Earth's gravity to a small degree – up to a 
maximum of 0.3% at the Equator – and reduces the apparent 
downward acceleration of falling objects. 
 
2) The second major reason for the difference in gravity at different 
latitudes is that the Earth's equatorial bulge (itself also caused by 
centrifugal force from rotation) causes objects at the Equator to be 
farther from the planet's centre than objects at the poles. Because the 
force due to gravitational attraction between two bodies (the 
 
 
Earth and the object being weighed) varies inversely with the 
square of the distance between them, an object at the Equator 
experiences a weaker gravitational pull than an object at the 
poles. 
 
PLEASE NOTE: 
 
You also weigh less at equator because of the same reason. (W= 
mg)  
 
 
ROTATION OF THE EARTH 
 
     
The earth rotates about an imaginary line that passes through the North and 
South Poles of the planet. This line is called the axis of rotation 
Hence, one portion of the earth’s surface comes towards the rays of the 
sun and experiences day light. The other portion is away from the sun’s 
rays and that will be darkness. 
Earth rotates eastward. Earth rotates once in about 24 hours with respect to 
the Sun , but once every 23 hours, 56 minutes, and 4 seconds with respect 
to other, distant, stars. Our rotation period with respect to the stars  is 
called a sidereal day. 
 
 
 
WHY IS THE EARTH ROTATING? 
The earth was formed out of a nebula that collapsed. As the nebula 
collapsed, it began rotating. The Earth's rotation comes from the initial 
tendency to rotate that was imparted on it when it formed. 
If an object is moving, then a force is required to slow it down or stop it, 
not to keep it moving. (Hence, "Objects in motion tend to stay in motion. 
Objects at rest tend to stay at rest.") In our everyday experience, it's the 
force of friction that tends to stop Earth-bound objects from moving forever. 
But for the Earth rotating on its axis, there is no force working to 
counteract the rotation (except the tidal effect of the Moon, but that's 
working very slowly). 
The shortest answer is angular momentum. Angular momentum is 
simply the name we give for the fact that things tend to rotate. 
 
REVOLUTION OF THE EARTH 
 
Earth's revolution around the Sun takes much longer than its rotation on its 
axis. One complete revolution takes 365.25 days, or one year. The Earth 
revolves around the Sun because gravity keeps it in a roughly circular 
orbit around the Sun. The Earth's orbital path is not a perfect circle, but 
Page 4


 
 
THE EARTH 
 
SHAPE OF THE EARTH 
Earth is not perfect sphere, it is flattened at both ends. Hence, it is called 
geoid. 
 
GRAVITATION  AT POLES AND EQUATOR: 
 
 
    
 
Earth's gravity is stronger at the poles than the equator for two 
reasons: 
 
1) At latitudes nearer the Equator, the outward centrifugal force 
produced by Earth's rotation is larger than at polar latitudes. 
This counteracts the Earth's gravity to a small degree – up to a 
maximum of 0.3% at the Equator – and reduces the apparent 
downward acceleration of falling objects. 
 
2) The second major reason for the difference in gravity at different 
latitudes is that the Earth's equatorial bulge (itself also caused by 
centrifugal force from rotation) causes objects at the Equator to be 
farther from the planet's centre than objects at the poles. Because the 
force due to gravitational attraction between two bodies (the 
 
 
Earth and the object being weighed) varies inversely with the 
square of the distance between them, an object at the Equator 
experiences a weaker gravitational pull than an object at the 
poles. 
 
PLEASE NOTE: 
 
You also weigh less at equator because of the same reason. (W= 
mg)  
 
 
ROTATION OF THE EARTH 
 
     
The earth rotates about an imaginary line that passes through the North and 
South Poles of the planet. This line is called the axis of rotation 
Hence, one portion of the earth’s surface comes towards the rays of the 
sun and experiences day light. The other portion is away from the sun’s 
rays and that will be darkness. 
Earth rotates eastward. Earth rotates once in about 24 hours with respect to 
the Sun , but once every 23 hours, 56 minutes, and 4 seconds with respect 
to other, distant, stars. Our rotation period with respect to the stars  is 
called a sidereal day. 
 
 
 
WHY IS THE EARTH ROTATING? 
The earth was formed out of a nebula that collapsed. As the nebula 
collapsed, it began rotating. The Earth's rotation comes from the initial 
tendency to rotate that was imparted on it when it formed. 
If an object is moving, then a force is required to slow it down or stop it, 
not to keep it moving. (Hence, "Objects in motion tend to stay in motion. 
Objects at rest tend to stay at rest.") In our everyday experience, it's the 
force of friction that tends to stop Earth-bound objects from moving forever. 
But for the Earth rotating on its axis, there is no force working to 
counteract the rotation (except the tidal effect of the Moon, but that's 
working very slowly). 
The shortest answer is angular momentum. Angular momentum is 
simply the name we give for the fact that things tend to rotate. 
 
REVOLUTION OF THE EARTH 
 
Earth's revolution around the Sun takes much longer than its rotation on its 
axis. One complete revolution takes 365.25 days, or one year. The Earth 
revolves around the Sun because gravity keeps it in a roughly circular 
orbit around the Sun. The Earth's orbital path is not a perfect circle, but 
 
 
rather an ellipse, which means that it is like a slight oval in shape. This 
creates areas where the Earth is sometimes farther away from the Sun than at 
other times.  
NOTE: 
 
We are closer to the Sun at perihelion (147 million kilometers) on about 
January 3rd and a little further from the Sun (152 million kilometers) at 
aphelion on July 4th. 
 
EARTH’S AXIAL TILT/OBLIQUITY 
 
     
Today, the Earth's axis is tilted 23.5 degrees from the plane of its 
orbit around the sun. But this tilt changes. During a cycle that 
averages about 40,000 years, the tilt of the axis varies between 22.1 
Page 5


 
 
THE EARTH 
 
SHAPE OF THE EARTH 
Earth is not perfect sphere, it is flattened at both ends. Hence, it is called 
geoid. 
 
GRAVITATION  AT POLES AND EQUATOR: 
 
 
    
 
Earth's gravity is stronger at the poles than the equator for two 
reasons: 
 
1) At latitudes nearer the Equator, the outward centrifugal force 
produced by Earth's rotation is larger than at polar latitudes. 
This counteracts the Earth's gravity to a small degree – up to a 
maximum of 0.3% at the Equator – and reduces the apparent 
downward acceleration of falling objects. 
 
2) The second major reason for the difference in gravity at different 
latitudes is that the Earth's equatorial bulge (itself also caused by 
centrifugal force from rotation) causes objects at the Equator to be 
farther from the planet's centre than objects at the poles. Because the 
force due to gravitational attraction between two bodies (the 
 
 
Earth and the object being weighed) varies inversely with the 
square of the distance between them, an object at the Equator 
experiences a weaker gravitational pull than an object at the 
poles. 
 
PLEASE NOTE: 
 
You also weigh less at equator because of the same reason. (W= 
mg)  
 
 
ROTATION OF THE EARTH 
 
     
The earth rotates about an imaginary line that passes through the North and 
South Poles of the planet. This line is called the axis of rotation 
Hence, one portion of the earth’s surface comes towards the rays of the 
sun and experiences day light. The other portion is away from the sun’s 
rays and that will be darkness. 
Earth rotates eastward. Earth rotates once in about 24 hours with respect to 
the Sun , but once every 23 hours, 56 minutes, and 4 seconds with respect 
to other, distant, stars. Our rotation period with respect to the stars  is 
called a sidereal day. 
 
 
 
WHY IS THE EARTH ROTATING? 
The earth was formed out of a nebula that collapsed. As the nebula 
collapsed, it began rotating. The Earth's rotation comes from the initial 
tendency to rotate that was imparted on it when it formed. 
If an object is moving, then a force is required to slow it down or stop it, 
not to keep it moving. (Hence, "Objects in motion tend to stay in motion. 
Objects at rest tend to stay at rest.") In our everyday experience, it's the 
force of friction that tends to stop Earth-bound objects from moving forever. 
But for the Earth rotating on its axis, there is no force working to 
counteract the rotation (except the tidal effect of the Moon, but that's 
working very slowly). 
The shortest answer is angular momentum. Angular momentum is 
simply the name we give for the fact that things tend to rotate. 
 
REVOLUTION OF THE EARTH 
 
Earth's revolution around the Sun takes much longer than its rotation on its 
axis. One complete revolution takes 365.25 days, or one year. The Earth 
revolves around the Sun because gravity keeps it in a roughly circular 
orbit around the Sun. The Earth's orbital path is not a perfect circle, but 
 
 
rather an ellipse, which means that it is like a slight oval in shape. This 
creates areas where the Earth is sometimes farther away from the Sun than at 
other times.  
NOTE: 
 
We are closer to the Sun at perihelion (147 million kilometers) on about 
January 3rd and a little further from the Sun (152 million kilometers) at 
aphelion on July 4th. 
 
EARTH’S AXIAL TILT/OBLIQUITY 
 
     
Today, the Earth's axis is tilted 23.5 degrees from the plane of its 
orbit around the sun. But this tilt changes. During a cycle that 
averages about 40,000 years, the tilt of the axis varies between 22.1 
 
 
and 24.5 degrees. The reason is that the earth’s axis also wobbles 
(move or cause to move unsteadily from side to side.) around itself. 
The wobble motion is called axial precession. It is caused by 
gravitational force from the sun, the moon and other planets and 
uneven distribution of mass inside the earth. A complete wobble of 
earth’s axis takes around 26,000 years.  
 
It is said that  When an object, the size of mars crashed into newly 
formed planet earth around 4.5 billion years ago, it knocked our 
planet and left it tilted to an angle. 
For the past 4.5 billion years, earth is orbiting the sun at a slant. 
This is the axial tilt and is called obliquity. 
 
As the earth orbits the sun at an angle, the solar energy reaching 
different parts of our planet is not constant. It varies during the 
course of the year. This is the reason why we have different seasons. 
It is also the reason, why seasons are opposite in southern and 
northern hemisphere. 
NOTE: 
The seasons are caused by the tilt of the Earth's rotational axis 
away or toward the sun as it travels through its year-long path 
around the sun. 
SOLSTICE