Motions of the Earth
Earth
has two types of motions:
Revolution-
Revolution can be defined as the movement of the Earth on its orbit around the
Sun
This
rotation and revolution are responsible for causing day and night, and
change in seasons and climate on the Earth.
ROTATION
We
know that the Earth spins on its axis, an imaginary line running north to south
across its center. The Earth rotates from west to east and completes one
rotation in 24 hours. This rotation leads to day and night on the planet. We
all know that the Sun rises in the east and sets in the west. It appears to
move from east to west in the sky. But in reality, the Sun is stationary and it
is the Earth which is rotating from west to east. For example, when we travel
in a car, things outside the car like buildings, trees, etc., appear to be
moving backwards. But we know that they are actually stationary and it is we
who are moving forward in the car.
When
the Earth rotates on its axis, only one half of its surface faces the Sun at a
time. This half of the Earth receives the Sun's light and heat and experiences
day time. The part of the Earth that faces away from the Sun experiences
darkness and night time. The circle of illumination is an imaginary line that
divides the half of the Earth that experiences day from that which experiences
night.
As
the Earth rotates, different regions experience the phenomena of day and night.
Look
at the image given below.
Consider
a point on the Earth's surface, say X. We know that the Earth rotates from west
to east. This means, the point X will at some time move east to position R. At
this position, it receives diffused light from the Sun. This time of the day
when the Sun is not visible and yet there is some light is called dawn.
In
time X will reach position P and experience sunrise. This denotes the time when
the Sun appears on the eastern horizon. When X reaches N, it will directly face
the Sun. This time of the day when sunlight falls directly on X is called noon
or mid-day.
After N, X will begin to move away from the Sun. At E, the Sun will appear to go down on the western horizon. This is the time of sunset. At G, X will receive some diffused light for a while after which it will experience darkness. This time of the day is called dusk or twilight. At H, X will be in complete darkness. This time is called midnight. As the Earth spins on its axis, these positions will keep repeating every 24 hours.
If
the Earth did not rotate it would always be day in the part of the Earth facing
the Sun. This part would experience continuous warmth. It would always be night
in the half facing away from the Sun. This half would remain freezing cold all
the time. Life would not have been possible in such extreme conditions.
REVOLUTION
The
Earth revolves around the Sun on a fixed path or orbit. This orbit is not circular,
but elliptical in shape. The rotation as well as the revolution of the Earth
takes place in the same direction. The Earth completes one revolution around
the Sun in 365 days and 6 hours. Therefore, revolution is also called annual
motion of the Earth. This motion causes change of seasons on the Earth. As the
Earth's orbit is elliptical, the Earth is closer to the Sun at certain points
on the orbit and far from the Sun at certain points. The Earth is said to be in
perihelion when its distance from the Sun is minimum. When the distance between
the Earth and the Sun is maximum, it is said to be in aphelion.
Perihelion and aphelion
Think
and Answer
Does
the Earth experience any other motion apart from rotation and revolution? It is
believed that every point in the entire universe is in motion relative to every
other point, because the entire universe is rapidly expanding. Besides the Earth's
movement around the Sun, the Earth along with the entire Solar System is moving around the center of the
Milky Way Galaxy. It takes our Solar System 225 million years to make one trip around the galaxy.
LEAP YEAR
The
Earth completes one revolution in 365 days and 6 hours. For our convenience, we
consider one year to have 365 days and ignore 6 hours. Every 4th years, the 6
hours add up to become 24 hours i.e., one day. The extra day is added to the
month of February. Therefore, every four years, February has 29 days instead of
28 days. So every fourth year has 366 days. Such a year is known as a leap
year.
A
leap year is always a multiple of 4. The Romans first designated February 29 as
leap day. A person born on February 29 is called a "leapling" or a
'leaper".
INCLINATION OF THE EARTH'S AXIS
While
it revolves around the Sun, the Earth remains tilted to one side. We study the
tilt of the Earth through its axis. The axis is an imaginary line passing
through the centre of the Earth around which the Earth rotates.
The
axis makes an angle of 66½° with the orbital plane of the Earth. The orbital
plane can be defined as the plane formed by the Earth's orbit. The axis makes
an angle of 23/½° with the plane perpendicular to the orbital plane. The two
ends of the axis are called the North Pole and the South Pole.
EFFECTS OF TILTED AXIS AND REVOLUTION
(a) Unequal length of days and nights
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Days
are longer in places nearer to the North Pole than the South Pole. At the
Equator, the length of the illuminated and dark parts of the circle is equal. That
means any point on the Equator has almost equal length of day and night all
through the year. As we go north, the length of the illuminated part of the
parallel is more than the darkened part. It means that days become longer and
nights become shorter when we move from the Equator towards the North Pole.
In
fact, the North Pole experiences six months of daylight. When the South Pole is
tilted towards the Sun, the Southern Hemisphere receives more heat in
comparison to the Northern Hemisphere. Therefore, the Southern Hemisphere
experiences summer, while the Northern Hemisphere experiences winter. As seen
in the figure, the Northern Hemisphere has longer nights and shorter days.
Places near the North Pole have six months of darkness.
(b) Change in temperature
|
Think and Answer Why is the Equator hot and wet
throughout the year? The Earth bulges at the Equator, so this part is always
close to the Sun and therefore, extremely hot. Warm air can hold more water
than cold air. Warm air is also less dense than cold air. As warm air rises,
it cools and the water vapour condenses and falls as rain. At the Equator,
there is an overall upward movement of air. This causes more condensation and
hence more rain. Think and Answer What is the average length of day and
night at the Equator? At the Equator, every day and every
night is 12 hours long. |
(c) Changing seasons
The
term 'season' refers to a period of the year when general weather conditions
remain the same. As the season changes, the weather conditions also change
consecutively. The four major seasons experienced in most parts of the world
are summer, autumn, winter, and spring. The interval between the seasons is
about three months.
CHANGE IN SEASONS
The
change in the position of the Earth as it moves around the Sun bring and the
tilt in it's axis about a change in seasons. This phenomenon is best understood
by studying the four positions of the Earth during its revolution around the
Sun.
The position on June 21
On
this date, the North Pole is tilted towards the Sun. The rays of the Sun fall
vertically on the Tropic of Cancer (23/½°N). It is summer season in the Northern
Hemisphere and winter season in the Southern Hemisphere. The position of the Earth on June
21 is also called 'summer solstice' in the Northern Hemisphere, and the 'winter
solstice' in the Southern Hemisphere.
The
term solstice refers to that time when the Sun is directly overhead one of the tropics. Because a large
portion of the Northern Hemisphere receives sunlight, regions lying north of
the equator experience the longest day and the shortest night on June 21. The
nights are longer than the days in areas in the Southern Hemisphere on this
day.
The position on December 22
On
this date, the South Pole is tilted towards the Sun and the rays of the Sun
fall vertically on the Tropic of Capricorn (23/½°S). It is winter season in the
Northern Hemisphere and summer season in the Southern Hemisphere. The position
of the Earth on December 22 is called 'winter solstice' in the Northern
Hemisphere, which experiences the shortest day of the year. It is called
'summer solstice for the Southern Hemisphere, which experiences longer days and
shorter nights.
The positions on March 21 and September 23
On
these two dates, neither of the Poles is inclined towards the Sun. The rays of
the Sun fall vertically over the Equator. As a result, on these days, days and
nights are of equal duration throughout the world. This is called an equinox.
The weather conditions in both the hemispheres are almost similar. It is
neither hot nor cold in either hemisphere.
There
is spring season in one hemisphere and autumn in the other. March 21 is called
'spring equinox' in the Northern Hemisphere and 'autumn equinox' in the
Southern Hemisphere. September 23 is called 'autumn equinox' in the Northern
Hemisphere and 'spring equinox' in the Southern Hemisphere.
Revision
Rotation: Movement of the Earth on its axis.
Axis: An
imaginary line running north to south across the centre of the Earth.
Circle of illumination: An imaginary circle that divides the half
of the Earth that experiences day from that which experiences night.
Dawn:
Time of the day when the Sun is not visible and yet there is some light.
Sunrise: The
time when the Sun appears on the eastern horizon.
Noon: Also
known as mid-day; time of the day when the sun is directly overhead a place.
Sunset:
The time when the Sun appears to go down on the western horizon.
Dusk:
Also known as twilight; time of the day when a place gets some diffused light
from the Sun before it gets completely dark.
Rotation causes
day and night. Revolution and earth's titled axis causes unequal lengths of day
and night, change of seasons, and changes in temperatures.
Revolution: Movement of the Earth on its orbit around the Sun.
Perihelion: When the distance between the Earth and the Sun is
minimum.
Aphelion: When
the distance between the Earth and the Sun is maximum.
Orbital plane: An imaginary plane formed by the Earth's orbit.
Season:
Refers to a period of the year when general weather conditions remain the same.
Solstice:
Refers to that time when the Sun is directly overhead one of the tropics.
Equinox: Refers
to the time when days and nights are of equal duration throughout the world.
Regions
lying in the Northern Hemisphere experience the longest day on June 21 and the
shortest day on December 22.
On
March 21 and September 23, days and nights are of equal duration throughout the
world.
Exercise
1. What is the
cause of day and night on Earth?
a) The Earth's
revolution around the Sun
b) The Earth's rotation on its axis
c) The Sun moving from east to west
d) The Earth’s tilt
Answer: b)
The Earth's rotation on its axis
2. In which
direction does the Earth rotate on its axis?
a) East to west
b) West to east
c) North to south
d) South to north
Answer: b)
West to east
3. How long does
it take for the Earth to complete one full rotation?
a) 12 hours
b) 24 hours
c) 365 days
d) 48 hours
Answer: b)
24 hours
4. What imaginary
line divides the part of the Earth experiencing day from the part experiencing
night?
a) Axis of
rotation
b) Equator
c) Circle of illumination
d) Tropic of Cancer
Answer: c)
Circle of illumination
5. Why does the
Sun appear to move from east to west in the sky?
a) The Sun is
moving around the Earth
b) The Earth rotates from east to west
c) The Earth rotates from west to east
d) The Earth is stationary
Answer: c)
The Earth rotates from west to east
6. When it is
daytime on one half of the Earth, what is happening on the other half?
a) It is also day
b) It is night
c) It is sunset
d) It is dawn
Answer: b)
It is night
7. Which of the
following best describes the axis of the Earth?
a) A real line
from the North to the South Pole
b) An imaginary line from the North to the South Pole
c) A real line from east to west
d) An imaginary line running around the Equator
Answer: b)
An imaginary line from the North to the South Pole
8. In which
dynasty was Aryabhatta born?
a) Maurya Dynasty
b) Gupta Dynasty
c) Chola Dynasty
d) Mughal Dynasty
Answer: b)
Gupta Dynasty
9. What
significant discovery did Aryabhatta make about Earth's motion?
a) The Earth
revolves around the Sun
b) The Earth rotates about its axis daily
c) The stars move across the sky
d) The Sun revolves around the Earth
Answer: b)
The Earth rotates about its axis daily
10. Aryabhatta was
known for his contributions to which fields?
a) Medicine and
Biology
b) Architecture and Art
c) Mathematics, Astronomy, and Physics
d) Music and Literature
Answer: c)
Mathematics, Astronomy, and Physics
11. What was the
prevailing belief before Aryabhatta’s theory about Earth's rotation?
a) The Earth was
stationary, and the sky rotated
b) The Earth rotated around the Sun
c) The stars and planets did not move
d) The Sun rotated around the Earth
Answer: a)
The Earth was stationary, and the sky rotated
12. What would
happen if the Earth did not rotate?
a) There would be
no change in seasons
b) Half of the Earth would experience continuous day and warmth
c) Both halves of the Earth would experience continuous day
d) The Earth would remain at the same temperature throughout
Answer: b)
Half of the Earth would experience continuous day and warmth
13. What is the
shape of Earth's orbit around the Sun?
a) Circular
b) Elliptical
c) Square
d) Irregular
Answer: b)
Elliptical
14. How long does
it take for the Earth to complete one revolution around the Sun?
a) 24 hours
b) 365 days
c) 365 days and 6 hours
d) 366 days
Answer: c)
365 days and 6 hours
15. What is the
annual motion of the Earth?
a) The Earth’s
rotation on its axis
b) The Earth's revolution around the Sun
c) The movement of the Sun around the Earth
d) The revolution of the Moon around the Earth
Answer: b)
The Earth's revolution around the Sun
16. When the Earth
is closest to the Sun, what is this position called?
a) Aphelion
b) Perihelion
c) Solstice
d) Equinox
Answer: b)
Perihelion
17. When the Earth
is farthest from the Sun, it is said to be in which position?
a) Perihelion
b) Aphelion
c) Solstice
d) Equinox
Answer: b)
Aphelion
18. How long does
it take the Solar System to complete one trip around the Milky Way Galaxy?
a) 225 million
years
b) 1 million years
c) 365 days
d) 25,000 years
Answer: a)
225 million years
19. What causes
the change of seasons on Earth?
a) The Earth’s
rotation
b) The Earth’s revolution around the Sun
c) The Earth's distance from the Moon
d) The Earth's movement around the Milky Way
Answer: b)
The Earth’s revolution around the Sun
20. Why is an
extra day added to February every four years?
a) To make up for
6 hours added annually
b) To correct the Earth's orbit
c) To balance the months
d) To adjust for the leap year
Answer: a)
To make up for 6 hours added annually
21. What is a person
born on February 29 called?
a) Leaper
b) Leapling
c) Extra-born
d) Februaryite
Answer: b)
Leapling
22.At what angle
is the Earth's axis tilted with respect to the orbital plane?
a) 45°
b) 90°
c) 66½°
d) 23½°
Answer: c)
66½°
23. What is the
angle between the Earth's axis and a perpendicular to the orbital plane?
a) 23½°
b) 45°
c) 66½°
d) 90°
Answer: a)
23½°
24. What causes
the unequal length of days and nights on Earth?
a) The Earth's
elliptical orbit
b) The Earth's rotation
c) The tilt of the Earth's axis
d) The Earth’s proximity to the Sun
Answer: c)
The tilt of the Earth's axis
25.When the North
Pole is tilted towards the Sun, which hemisphere experiences summer?
a) Southern
Hemisphere
b) Northern Hemisphere
c) Equator
d) Both Hemispheres
Answer: b)
Northern Hemisphere
26. At the
Equator, how do the lengths of day and night compare throughout the year?
a) Day is always
longer
b) Night is always longer
c) Day and night are almost equal in length
d) Day is twice as long as night
Answer: c)
Day and night are almost equal in length
27. As you move
from the Equator towards the North Pole during summer, what happens to the
length of days?
a) Days become
shorter
b) Days become longer
c) Days remain the same length
d) Days and nights become equal
Answer: b)
Days become longer
28. Why are
summers hot and winters cold?
a) Because the
Earth’s orbit changes
b) Due to the angle of the Sun’s rays
c) The Sun emits more heat in summer
d) The Earth’s rotation speeds up in winter
Answer: b)
Due to the angle of the Sun’s rays
29. What happens
to the temperature when a hemisphere receives slanting rays of the Sun?
a) Temperature
increases
b) Temperature remains constant
c) Temperature decreases
d) Temperature fluctuates
Answer: c)
Temperature decreases
30. Why is the
Equator hot throughout the year?
a) The Equator is
always close to the Sun
b) The Sun emits more heat at the Equator
c) The Equator receives slanting rays of the Sun
d) The Equator moves towards the Sun during summer
Answer: a)
The Equator is always close to the Sun
31. Why does the
Equator experience more rain?
a) Warm air holds
less water than cold air
b) Warm air rises and cools, leading to condensation
c) The Sun heats up the air quickly
d) Cold air from the poles moves to the Equator
Answer: b)
Warm air rises and cools, leading to condensation
32. What is the
average length of day and night at the Equator?
a) 10 hours of day
and 14 hours of night
b) 12 hours of day and 12 hours of night
c) 14 hours of day and 10 hours of night
d) 16 hours of day and 8 hours of night
Answer: b)
12 hours of day and 12 hours of night
33. Why do places
near the North Pole experience six months of darkness?
a) The North Pole
is tilted away from the Sun for six months
b) The Sun does not shine on the North Pole
c) The Earth’s rotation stops during this time
d) The Sun moves to the Southern Hemisphere for six months
Answer: a)
The North Pole is tilted away from the Sun for six months
34. What is the
term 'season' used to describe?
a) A specific
month of the year
b) A period of the year with consistent weather conditions
c) The time when the Earth rotates on its axis
d) The day and night cycle
Answer: b)
A period of the year with consistent weather conditions
35. What causes
the change in seasons on Earth?
a) The Sun’s
distance from Earth
b) The tilt of Earth’s axis and its revolution around the Sun
c) The Earth's rotation
d) The movement of the Moon around the Earth
Answer: b)
The tilt of Earth’s axis and its revolution around the Sun
36. What is the
position of the Earth on June 21 called in the Northern Hemisphere?
a) Winter solstice
b) Spring equinox
c) Summer solstice
d) Autumn equinox
Answer: c)
Summer solstice
37. What happens
in the Northern Hemisphere on June 21?
a) The shortest
day of the year
b) Equal duration of day and night
c) The longest day and the shortest night of the year
d) Winter season
Answer: c)
The longest day and the shortest night of the year
38. On which date
does the Northern Hemisphere experience the winter solstice?
a) March 21
b) June 21
c) December 22
d) September 23
Answer: c)
December 22
39.What is the
position of the Earth on March 21 and September 23 known as?
a) Solstice
b) Equinox
c) Tilt day
d) Leap day
Answer: b)
Equinox
40. What happens
to the duration of day and night on March 21 and September 23?
a) Days are
shorter than nights
b) Days are longer than nights
c) Days and nights are of equal duration
d) Nights are longer than days
Answer: c)
Days and nights are of equal duration
41. What is March
21 called in the Northern Hemisphere?
a) Autumn equinox
b) Winter solstice
c) Summer solstice
d) Spring equinox
Answer: d)
Spring equinox
42. On December
22, where do the Sun's rays fall vertically?
a) Tropic of
Cancer
b) Tropic of Capricorn
c) Equator
d) North Pole
Answer: b)
Tropic of Capricorn
43. What season
does the Southern Hemisphere experience during the December 22 solstice?
a) Winter
b) Autumn
c) Summer
d) Spring
Answer: c)
Summer
44. What is
"Perihelion"?
a) When the Earth
is closest to the Sun
b) When the Earth is farthest from the Sun
c) The point where the Earth rotates fastest
d) When the Sun is directly overhead the Equator
Answer: a)
When the Earth is closest to the Sun
45. What is the
"Orbital plane"?
a) The path the
Earth takes while revolving around the Sun
b) The imaginary line passing through the Earth's center
c) The axis of the Earth
d) The imaginary plane formed by the Earth's orbit
Answer: d)
The imaginary plane formed by the Earth's orbit
46. What is meant
by "Season"?
a) A period of one
month with fluctuating weather
b) The time when Earth moves closest to the Sun
c) A period of the year when general weather conditions remain the same
d) A point in Earth’s orbit where days are longer than nights
Answer: c)
A period of the year when general weather conditions remain the same
47. What does
"Solstice" refer to?
a) The time when
days and nights are of equal duration
b) The time when the Sun is directly overhead one of the tropics
c) When the Earth is at its farthest point from the Sun
d) When the Earth’s rotation slows down
Answer: b)
The time when the Sun is directly overhead one of the tropics
48. What is an
"Equinox"?
a) When the Earth
is tilted towards the Sun
b) When the distance between Earth and Sun is equal
c) When days and nights are of equal duration throughout the world
d) The shortest day of the year in the Northern Hemisphere
Answer: c)
When days and nights are of equal duration throughout the world
49. When does the
Northern Hemisphere experience its longest day?
a) December 22
b) March 21
c) June 21
d) September 23
Answer: c)
June 21
50. When do days
and nights have equal duration worldwide?
a) June 21 and
December 22
b) March 21 and September 23
c) July 1 and January 1
d) April 15 and October 15
Answer: b)
March 21 and September 23
51. When does the
Northern Hemisphere experience its shortest day?
a) June 21
b) September 23
c) December 22
d) March 21
Answer: c)
December 22
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