Chapter 12 Earth, Moon, and the Sun Revision Notes Class 7 Science Curiosity
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NCERT Notes for Chapter 12 Earth, Moon, and the Sun Class 7 Science
The Earth’s rotation on its axis, from west to east every 24 hours, causes the day-night cycle and the apparent east-to-west motion of the Sun, Moon, and stars. Its revolution around the Sun, taking about 365 days, leads to changing night sky views and, due to the Earth’s tilted axis and spherical shape, seasonal variations. Solar eclipses occur when the Moon blocks sunlight, creating brief darkness, while lunar eclipses happen when the Earth’s shadow darkens the Moon. These phenomena, safely observed with proper precautions, highlight the dynamic interplay of Earth, Moon, and Sun, studied by ancient and modern scientists alike.
One morning in Kanniyakumari, 12-year-old Rashmika was cycling to school excited for science class. The teacher asked students to share interesting observations.
Rashmika noticed that the coconut tree shadows were long in the morning but shorter in the afternoon. She thought this happened because the Sun moved across the sky. But she also remembered learning that the Earth moves around the Sun, so she wondered, Does the Sun move, or does the Earth move?
Rotation of the Earth
The Earth's rotation is the spinning motion on its axis, which is an imaginary line is passing through the North and South Poles.
This rotation takes approximately 24 hours to complete one full turn, causing the cycle of day and night.
Direction of Rotation
- When viewed from above the North Pole, the Earth rotates counter clockwise, from west to east.
- This rotation makes the Sun appear to rise in the east, move across the sky, and set in the west, although the Sun itself remains stationary relative to the Earth.
Day and Night Cycle
- The Earth’s rotation causes one side to face the Sun, experiencing daytime.
- The opposite side faces away from the Sun, experiencing nighttime or darkness.
- For example, in India, the eastern regions see the sunrise first because they face the Sun earlier during the Earth’s rotation.
Apparent Motion of Celestial Objects
- The Earth’s rotation makes the Sun, Moon, and stars appear to move across the sky from east to west.
- The Pole Star (Dhruva Tara) appears almost stationary because the Earth’s axis points close to it.
- Other stars, such as those in the Big Dipper (Saptarishi), seem to circle around the Pole Star.
Foucault Pendulum demonstrates Earth’s rotation physically using a swinging pendulum that changes its plane of oscillation over time.
Revolution of the Earth
The Earth's revolution is its movement around the Sun along a nearly circular path called an orbit. It takes about 365 days and 6 hours to complete one revolution, which defines a year.
Changing View of the Night Sky
- As the Earth revolves around the Sun, it faces different directions in space throughout the year. This causes the portion of the night sky visible after sunset to change gradually.
- Different constellations and stars appear at different times during the year.
- This changing view of the night sky helps us observe various celestial patterns and seasons.
- It also allows astronomers to track the movement of stars and planets across the sky.
Astrophotographers use long-exposure photography, keeping the camera’s shutter open for an extended time. This technique captures the apparent motion of stars as curved arcs called star trails, showing how stars seem to move in circular paths due to Earth’s rotation.
Seasons on the Earth
- The Earth’s axis is tilted at an angle relative to its orbit around the Sun.
- This tilt, along with the Earth’s spherical shape, causes the different seasons we experience.
- Around June 21, the Northern Hemisphere tilts toward the Sun, receiving more direct and intense sunlight.
- During this time, the Sun stays above the horizon for more than 12 hours, resulting in summer in the Northern Hemisphere.
- At the same time, the Southern Hemisphere tilts away from the Sun, getting less direct sunlight and shorter daylight hours, causing winter there.
- Around December 22, the situation reverses: the Northern Hemisphere tilts away from the Sun, leading to winter, while the Southern Hemisphere tilts toward the Sun, enjoying summer.
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| (a) More intense sunlight in the Northern Hemisphere and less intense sunlight in the Southern Hemisphere in June (b) The opposite situation happens in December |
- The difference in the Sun’s intensity and the length of the day during these times causes the changes in temperature and weather that define the seasons.
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| In the Northern Hemisphere (a) Longer daytime in June(b) Shorter daytime in December |
Solstices and Equinoxes
- In the Northern Hemisphere, the longest day of the year, called the summer solstice, occurs around June 21.
- The shortest day, known as the winter solstice, happens around December 22.
- On March 21 (the spring equinox) and September 23 (the autumn equinox), day and night are equal, each lasting about 12 hours.
- At the equator, days and nights remain almost equal (12 hours each) throughout the year, with very little seasonal change.
The polar regions experience extreme daylight conditions — either six months of continuous daylight (known as the Midnight Sun) or six months of darkness.
At the North Pole, the Sun rises on March 21 and stays above the horizon for six months, setting on September 22. The South Pole experiences the opposite—six months of darkness followed by six months of daylight.
Misconceptions about Seasons
- Seasons are not caused by the Earth being closer to the Sun when tilted toward it or by an oval orbit with significant distance variations.
- The Earth is actually closest to the Sun in January, but the tilt and spherical shape are the primary reasons for seasonal changes.
Eclipses
Eclipses occur when one celestial body blocks light from another, casting a shadow. The Moon’s position relative to the Earth and Sun causes solar and lunar eclipses.
There are two types of Eclipses:
- Solar Eclipse
- Lunar Eclipse
Solar Eclipse
- A solar eclipse occurs when the Moon comes between the Earth and the Sun, blocking sunlight from reaching certain parts of the Earth.
- Although the Moon is much smaller than the Sun, their apparent sizes in the sky are similar because the Moon is much closer to Earth.
- This alignment allows the Moon to fully or partially cover the Sun.
Types of Solar Eclipses
There are two types of Solar Eclipse:
- Total Solar Eclipse
- Partial Solar Eclipse
- The Moon completely blocks the Sun, casting a shadow on a small area of the Earth.
- Observers in this area experience total darkness during the day. The sky becomes dark, and the Sun is completely obscured for a few minutes.
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| Total solar eclipse |
A ‘diamond ring’ seen after a total solar eclipse, just when the Moon starts to move away
2. Partial Solar Eclipse:- The Moon only partially covers the Sun, and observers in the affected area see a part of the Sun obscured by the Moon.
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| Partial Solar Eclipse |
Safe Viewing of Solar Eclipses
Direct viewing of a solar eclipse is dangerous because the Sun’s intense light can cause permanent blindness, even when the Sun is only partially covered by the Moon.- Using specialized solar goggles designed for eclipse viewing.
- Attending organized viewing events where safety measures are in place.
- Projecting the Sun’s image using a mirror to watch the eclipse safely.
Duration of the Solar Eclipse
The total solar eclipse is visible only for a few minutes because of the Earth's rotation and the Moon's motion in its orbit.- As the Moon moves away from the Sun, the eclipse shifts into a partial solar eclipse, and daylight begins to return.
Historical Views on Solar Eclipses
Ancient civilizations feared eclipses because they did not understand the phenomenon. They believed that the Sun, a major source of heat and light, had been momentarily blocked by some cosmic force.- Many superstitions surrounded solar eclipses, with people refraining from activities like eating, cooking, or leaving the house.
- Today, scientists study solar eclipses because they offer a rare opportunity to observe the Sun's atmosphere and other phenomena that are not visible during normal circumstances.
The Sanskrit word for eclipse is "grahan." Ancient Indian astronomical texts, such as the Surya Siddhanta, provided calculations to predict eclipses long before modern astronomy developed. These texts use poetic shlokas to describe the phenomenon.
Why Planets Can't Block the Sun?
- Mercury and Venus are closer to the Sun than Earth, but they are still too small and far away to block the Sun significantly.
- Transit of Venus is a rare event that occurs when Venus moves directly between Earth and the Sun.
- During the Transit of Venus, Venus appears as a tiny black dot crossing the face of the Sun.
- This event is rare because Venus has to align perfectly with the Sun and Earth, which doesn’t happen often.
- Although Venus is much smaller than the Sun, it’s visible as a small dot when it crosses in front of the Sun.
Lunar Eclipse
A lunar eclipse occurs when the Earth comes between the Sun and the Moon, blocking the sunlight from reaching the Moon. This results in the Earth's shadow falling on the Moon. The event is only visible during a full moon, when the Earth is directly aligned between the Sun and the Moon.
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| Lunar Eclipse on Earth |
Types of Lunar Eclipses
There are two types of Lunar Eclipse:
- Total Lunar Eclipse
- Partial Lunar Eclipse
- In a total lunar eclipse, the Moon is completely covered by the Earth's shadow.
- During this event, the Moon takes on a dark red color, which is often referred to as a "blood moon." The reddish hue is caused by the Earth's atmosphere scattering sunlight, which then reaches the Moon.
- The Moon remains red until it moves out of the Earth's shadow.
- In a partial lunar eclipse, only part of the Moon enters the Earth's shadow. The rest of the Moon remains visible and appears unaffected.
- This creates a distinctive shape where part of the Moon looks darker, and the rest remains bright.
Viewing a Lunar Eclipse
- Unlike a solar eclipse, a lunar eclipse is safe to watch with the naked eye. Since the Moon is not as bright as the Sun, the Earth's shadow provides a dimmer, more diffuse light.
- It is also possible to observe both total and partial lunar eclipses from the Earth without special protection, making them easier for amateur astronomers and enthusiasts to view.
The Kodaikanal Solar Observatory in southern India, established in 1899, has been providing valuable data about the Sun for over 100 years. It is operated by the Indian Institute of Astrophysics (IIA), Bengaluru. The observatory has contributed significantly to solar studies.
M.K. Vainu Bappu
- M.K. Vainu Bappu is regarded as the father of modern Indian astronomy. He led efforts to set up many astronomical instruments and telescopes in India, such as those at Manora Peak (Uttarakhand) and Kavalur (Tamil Nadu).
- The observatory at Kavalur has been named after him in recognition of his contributions.
- Bappu made significant discoveries, including a comet, and was known for his studies on stars and solar eclipses. He travelled worldwide to study solar eclipses, contributing greatly to modern astronomical research in India.
Points to Remember
- In the 19th century, scientist Leon Foucault used a long pendulum, known as the Foucault pendulum, to demonstrate the Earth’s rotation. A 22-meter Foucault pendulum is installed in the Constitution Hall of India’s new Parliament building in New Delhi, symbolizing India’s connection to the cosmos.
- Ancient Indian astronomer Aryabhata, in his 5th-century text Aryabhatiya, explained the apparent motion of stars due to Earth’s rotation, comparing it to a person on a moving boat seeing stationary objects move backward. He estimated the Earth’s rotation period as 23 hours, 56 minutes, and 4.1 seconds, remarkably close to the modern value.
- Astrophotographers capture star trails—arcs of star movement in long-exposure photographs—showing the apparent motion of stars due to Earth’s rotation.
- The Bhil and Pawara communities in India’s Tapi Valley used the appearance of specific star patterns to predict monsoon rains, demonstrating traditional knowledge of celestial movements.
- At the North Pole, the Sun rises on March 21 (spring equinox) and remains visible for six months until setting on September 22. The South Pole experiences the opposite, with six months of darkness followed by six months of daylight.
- Ancient Indian texts like the Surya Siddhanta, written in Sanskrit shlokas, provided calculations to predict eclipses, reflecting advanced astronomical knowledge.
- The Kodaikanal Solar Observatory, established in 1899 in Tamil Nadu’s Palani hills, has collected over 100 years of solar data, operated by the Indian Institute of Astrophysics, Bengaluru.
Important Terms
- Rotation: The spinning motion of an object, like the Earth, around its own axis, completing one turn in about 24 hours.
- Axis of Rotation: An imaginary line passing through the North and South Poles, around which the Earth rotates.
- Revolution: The motion of an object, like the Earth, around another object, such as the Sun, taking about 365 days and 6 hours.
- Orbit: The nearly circular path the Earth follows while revolving around the Sun.
- Solstice: The longest (summer solstice, around June 21) or shortest (winter solstice, around December 22) day in the Northern Hemisphere, due to the Earth’s tilt.
- Equinox: Days when day and night are equal (12 hours each), occurring around March 21 (spring equinox) and September 23 (autumn equinox) in the Northern Hemisphere.
- Solar Eclipse: An event where the Moon blocks sunlight from reaching Earth, causing total or partial darkness in specific areas.
- Lunar Eclipse: An event where the Earth blocks sunlight from reaching the Moon, casting a shadow that makes the Moon appear dark red (total) or partially shadowed (partial).
- Apparent Size: The perceived size of an object in the sky, depending on its actual size and distance from the observer, allowing the Moon to appear as large as the Sun during a solar eclipse.
- Transit of Venus: A rare event where Venus appears as a small black dot crossing the Sun’s face, due to its small apparent size compared to the Sun.