Earth is always motion in two different ways. It spins around like a top, and it also travels around the Sun in a big circle. These movements of Earth cause many changes that we see in the sky every day. In this chapter, we will study how Earth moves and how this movement affects what we see in the sky.
Rotation of the Earth
What is Earth’s Rotation?
Earth spins around on its axis just like a spinning top. This spinning movement is called rotation. The axis is an imaginary line that passes through the North Pole and South Pole of Earth.
Imp Facts about Earth’s Rotation:
- Earth completes one full rotation in about 24 hours
- Earth rotates from West to East direction
- Because of this rotation, we get day and night
- The actual time for one rotation is 23 hours and 56 minutes
How Rotation Causes Day and Night
When Earth rotates, different parts of Earth face the Sun at different times. The part facing the Sun gets daylight, and the part facing away from Sun gets darkness (night).
What we see because of rotation:
- Sun appears to rise in the East every morning
- Sun appears to set in the West every evening
- Moon and stars also seem to move across the sky
- But actually, it’s Earth that is moving, not these objects
The Pole Star
There’s one star that doesn’t seem to move much in the sky – the Pole Star. This is because Pole Star is almost directly above Earth’s North Pole. As Earth rotates, all other stars appear to move in circles around the Pole Star.
Proof of Earth’s Rotation
Long time ago, people thought that Sun and stars move around Earth. But scientists proved that Earth rotates using a special device called Foucault pendulum. This pendulum keeps swinging in the same direction while Earth rotates underneath it.
In ancient India, a great astronomer named Aryabhata wrote about Earth’s rotation in his book “Aryabhatiya”. He explained that stars appear to move because Earth rotates.
Revolution of the Earth
What is Earth’s Revolution?
Apart from rotating, Earth also travels around the Sun in a big circular path called orbit. This movement around the Sun is called revolution.
Imp Facts about Earth’s Revolution:
- Earth takes about 365 days and 6 hours to complete one revolution
- The orbit is almost circular in shape
- This extra 6 hours is why we have leap year every 4 years
Changing View of Night Sky from the Earth
Because Earth revolves around the Sun, the stars we see at night keep changing throughout the year. Stars that are visible in January might not be visible in July because Earth is on the opposite side of its orbit.
How night sky changes:
- Stars seen after sunset shift gradually every night
- Different constellations appear in different seasons
- Same stars come back to same position after one year
Many tribal communities and farmers in India have been using star patterns to know about seasons and timing for farming activities.
Seasons on the Earth
Earth’s axis is tilted at an angle of about 23.5 degrees. This tilt, combined with Earth’s revolution around Sun, causes seasons.
How seasons happen:
- When Northern Hemisphere tilts toward Sun, it gets more direct sunlight
- This causes summer in Northern Hemisphere
- At the same time, Southern Hemisphere tilts away from Sun
- This causes winter in Southern Hemisphere
- After 6 months, the situation reverses
Imp dates for seasons:
- Summer Solstice (June 21): Longest day in Northern Hemisphere
- Winter Solstice (December 22): Shortest day in Northern Hemisphere
- Spring Equinox (March 21): Day and night are equal
- Autumn Equinox (September 23): Day and night are equal again
Special Cases
At the Poles:
- North Pole gets 24 hours of sunlight during summer (June)
- North Pole gets 24 hours of darkness during winter (December)
- South Pole has opposite pattern
At the Equator:
- Days and nights are almost equal throughout the year (12 hours each)
- Seasons are not very prominent
Common Mistake: Many people think seasons happen because Earth is sometimes closer to Sun and sometimes farther. This is wrong! Earth’s distance from Sun doesn’t change much. Seasons happen only because of Earth’s tilted axis.
Eclipses
Eclipse happens when one object in space blocks light from reaching another object. There are two types of eclipses we can see from Earth.
Solar Eclipse
Solar eclipse happens when Moon comes between Earth and Sun, blocking Sun’s light from reaching Earth.
Types of Solar Eclipse:
- Total Solar Eclipse: Moon completely blocks Sun’s light
- Partial Solar Eclipse: Moon blocks only part of Sun’s light
Why Solar Eclipse is Possible: Moon is much smaller than Sun, but it can still block Sun completely. This happens because Moon is much closer to Earth than Sun. From Earth, Moon and Sun appear to be almost the same size in the sky.
Imp Safety Rules:
- Never look at solar eclipse directly with eyes
- It can cause permanent damage to eyes
- Use special eclipse glasses or filters
- You can use a mirror to project Sun’s image on a wall
- Join astronomy clubs for safe viewing
Historical Note: Ancient Indian texts like “Surya Siddhanta” could predict eclipses very accurately. Scientists today study eclipses to learn more about Sun and space.
Lunar Eclipse
Lunar eclipse happens when Earth comes between Sun and Moon, blocking sunlight from reaching Moon.
What happens during Lunar Eclipse:
- Earth’s shadow falls on Moon
- Moon appears dark red or copper colored
- This happens because Earth’s atmosphere bends some red light toward Moon
Types of Lunar Eclipse:
- Total Lunar Eclipse: Earth’s shadow covers entire Moon
- Partial Lunar Eclipse: Earth’s shadow covers only part of Moon
Safety: Unlike solar eclipse, lunar eclipse is completely safe to watch with naked eyes.
Modern Tools: Apps like Stellarium can predict when eclipses will be visible from your location.
In a Nutshell
Earth’s Rotation:
- Earth rotates from West to East in 24 hours
- Rotation causes day and night
- Sun, Moon, and stars appear to move because of rotation
- Pole Star appears stationary
Earth’s Revolution:
- Earth revolves around Sun in about 365 days
- Revolution changes the view of night sky throughout year
- Same stars return to same position after one year
Seasons:
- Earth’s tilted axis and revolution cause seasons
- When Northern Hemisphere tilts toward Sun, it’s summer there
- When Southern Hemisphere tilts toward Sun, it’s winter in Northern Hemisphere
- Distance from Sun doesn’t cause seasons
Eclipses:
- Solar eclipse: Moon blocks Sun’s light
- Lunar eclipse: Earth blocks sunlight from reaching Moon
- Solar eclipse needs special safety measures
- Lunar eclipse is safe to watch directly
Questions and Answers
1. How many hours of sunlight do the North Pole and South Pole receive during one rotation of the Earth?
During one rotation (24 hours), the amount of sunlight North and South Poles get depends on the season:
In June (Summer Solstice):
- North Pole gets 24 hours of sunlight
- South Pole gets 0 hours of sunlight (complete darkness)
In December (Winter Solstice):
- North Pole gets 0 hours of sunlight (complete darkness)
- South Pole gets 24 hours of sunlight
This happens because Earth’s axis is tilted, so during summer, one pole tilts toward Sun and gets continuous sunlight while the other pole tilts away and stays in darkness.
Answer: North Pole: 24 hours in June, 0 in December; South Pole: 0 in June, 24 in December.
2. Fill in the blanks:
- (i) Stars rise in the _____ and set in the _____.
- (ii) Day and night are caused by the Earth’s _____.
- (iii) When the Moon fully covers the Sun from our view, it is called a _____ solar eclipse.
Answer: (i) East, West; (ii) rotation; (iii) total.
3. State whether True or False:
- (i) Lunar eclipse occurs when the Sun comes between the Earth and the Moon.
- (ii) Sunrise happens earlier in Gujarat than in Jharkhand.
- (iii) In Chennai, the longest day occurs on the summer solstice.
- (iv) We should watch the solar eclipse directly with our naked eye.
Answer: (i) False – Earth comes between Sun and Moon; (ii) False – Jharkhand is east, so sunrise is earlier there; (iii) True; (iv) False – it damages eyes.
4. If Earth’s axis were not tilted, what would be the effect on seasons?
If Earth’s axis was not tilted, there would be no seasons at all. Every place on Earth would get the same amount of sunlight throughout the year. Day and night would be equal (12 hours each) everywhere on Earth. The weather would be the same all year round at any given place.
Answer: No tilt eliminates seasons; sunlight and day length remain constant.
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