Earth, Moon, and the Sun
Class 7 Science (Curiosity) | Chapter Notes & Solved Study Material
1. Introduction
One morning in Kanniyakumari, 12-year-old Rashmika was cycling to school, excited for science class. The teacher asked the students to share interesting observations.
Rashmika noticed that the shadows of the coconut trees were long in the morning but shorter in the afternoon. She thought this happened because the Sun moved across the sky. She also remembered learning that the Earth moves around the Sun, so she wondered: Does the Sun move, or does the Earth move?
Let us explore how the Earth, Moon, and Sun work together to create day and night, seasons, and events such as eclipses.
2. Rotation of the Earth
The Earth's rotation is the spinning motion of the planet around an imaginary line called its axis, which passes through the North and South Poles. One complete rotation takes approximately 24 hours and produces the cycle of day and night.
Direction of Rotation
When viewed from above the North Pole, the Earth rotates counterclockwise, that is, from west to east.
Apparent Sun motion: This rotation makes the Sun appear to rise in the east, move across the sky, and set in the west. This apparent motion is caused by the Earth's rotation, not because the Sun moves around the Earth each day.
Daytime: The side of the Earth facing the Sun experiences daytime.
Nighttime: The side facing away from the Sun experiences nighttime.
Regional variation: For example, in India, the eastern regions see sunrise earlier than the western regions because they rotate into sunlight first.
General motion: The Earth's rotation makes the Sun, Moon, and stars appear to move across the sky from east to west during the day or night.
Pole Star (Dhruva Tara): Appears almost stationary because the Earth's axis points close to it.
Big Dipper (Saptarishi): Other stars, such as those in the Big Dipper, appear to circle around the Pole Star.
3. Revolution of the Earth
The Earth's revolution is its movement around the Sun along a nearly circular path called its orbit. One complete revolution takes about 365 days and 6 hours, which defines a year. The extra 6 hours are accounted for by adding one day approximately every four years. Such a year is called a leap year.
Changing View of the Night Sky
Directional shift: As the Earth revolves around the Sun, it faces different directions in space at different times of the year.
Sky changes: This causes the portion of the night sky visible after sunset to change gradually during the year.
Constellations: Different constellations and stars are visible during different seasons. Astronomers use this changing sky to study celestial motion.
Axial tilt: The Earth's axis is tilted by approximately 23.5° from a line perpendicular to its orbital plane.
Sunlight intensity: Because of this tilt and the Earth's spherical shape, different regions receive different amounts of sunlight at different times of the year. This produces the seasons.
June 21: About 21 June, the Northern Hemisphere tilts towards the Sun and receives more direct sunlight. This produces summer there, while the Southern Hemisphere experiences winter.
December 22: About 22 December, the situation reverses. The Northern Hemisphere tilts away from the Sun and has winter, while the Southern Hemisphere tilts towards the Sun and has summer.
Daylight length: Differences in the intensity of sunlight and the length of daylight during these periods cause changes in temperature and weather that define the seasons.
Summer Solstice: Around 21 June in the Northern Hemisphere, when that hemisphere has its longest day of the year.
Winter Solstice: Around 22 December in the Northern Hemisphere, when that hemisphere has its shortest day.
Equinoxes: Around 21 March and 23 September, day and night are approximately equal, with about 12 hours each, in most places on Earth.
Equator: At the equator, day and night remain almost equal throughout the year, so seasonal change is less pronounced there.
Distance myth: Seasons are not caused by the Earth being closer to or farther from the Sun. In fact, the Earth is closest to the Sun in January.
Actual cause: The tilt of the Earth's axis and its revolution around the Sun are the main reasons for seasons, not changes in the Earth's distance from the Sun.
The Moon is the Earth's natural satellite. It does not produce its own light; it reflects light from the Sun. The Moon revolves around the Earth, while the Earth and the Moon together revolve around the Sun.
Orbital period: The Moon takes about 27.3 days to complete one revolution around the Earth with respect to the distant stars.
Phases: As the Moon revolves around the Earth, we see different portions of its sunlit half. These changing appearances are called the phases of the Moon.
Alignment: Eclipses occur only when the Sun, Earth, and Moon become suitably aligned. Such alignment does not occur at every new moon or full moon because the Moon's orbit is slightly tilted compared with the Earth's orbital plane.
Eclipses occur when one celestial body casts a shadow on another by blocking its light. Solar and lunar eclipses result from the relative positions of the Sun, Earth, and Moon.
Solar Eclipse
Mechanism: A solar eclipse happens when the Moon comes between the Earth and the Sun, blocking sunlight from reaching parts of the Earth.
Apparent size: Although the Moon is far smaller than the Sun in actual size, it is much closer to the Earth. Therefore, their apparent sizes in the sky can be similar, making it possible for the Moon to fully or partially cover the Sun.
Total Solar Eclipse: The Moon completely blocks the Sun for observers inside a narrow path on the Earth. The sky becomes very dark, and totality lasts only for a few minutes.
Partial Solar Eclipse: The Moon blocks only part of the Sun, so observers see only a portion of the Sun obscured.
Safety Warning: Directly viewing the Sun is dangerous and can cause permanent eye damage, even during an eclipse.
Safe methods to observe a solar eclipse include:
Using specialised solar goggles designed for eclipse viewing.
Attending organised viewing events where experts ensure safe procedures.
Projecting the Sun's image using a pinhole projector, a simple telescope projection, or a mirror to view the eclipse indirectly.
Duration of a Solar Eclipse: Totality is visible only for a few minutes at any one location because of the relative motions of the Moon and the Earth and the narrowness of the Moon's central shadow. As the Moon moves in its orbit, the eclipse at a given place changes from total to partial, and daylight returns.
Historical Views on Solar Eclipses
Ancient fears: Ancient peoples often feared eclipses because they lacked a correct scientific explanation and thought that the Sun had been blocked by some cosmic force.
Superstitions: Many cultures developed superstitions about eclipses. For instance, some people refrained from cooking or going outdoors during an eclipse.
Modern scientific study: Modern science studies eclipses because they allow rare observations of the Sun's outer atmosphere, called the corona, and other phenomena that are usually hidden by the Sun's bright disc.
Inner planets: Mercury and Venus are closer to the Sun than the Earth, but they are still too small and far away to block the Sun significantly.
Transit of Venus: A rare event that occurs when Venus moves directly between the Earth and the Sun.
Visual appearance: During the Transit of Venus, Venus appears as a tiny black dot crossing the face of the Sun.
Rarity: This event is rare because Venus has to align precisely with the Sun and the Earth, which does not happen often.
A lunar eclipse occurs when the Earth comes between the Sun and the Moon, so the Earth's shadow falls on the Moon. A lunar eclipse can occur only at full moon, when the three bodies are approximately aligned.
Types of Lunar Eclipses
Total Lunar Eclipse: The entire Moon enters the Earth's shadow and often appears reddish, forming a 'blood moon'. This happens because the Earth's atmosphere scatters away much of the blue light and bends some of the remaining red light towards the Moon.
Partial Lunar Eclipse: Only part of the Moon enters the Earth's shadow, so a portion of the Moon darkens while the rest remains illuminated.
Eye safety: Unlike a solar eclipse, a lunar eclipse is safe to observe with the naked eye because the Moon does not produce intense light that can harm the eyes.
Visibility: Both total and partial lunar eclipses are visible from any location on Earth where the Moon is above the horizon at the time of the eclipse.
Contributions: M. K. Vainu Bappu is regarded as a leading figure in modern Indian astronomy. He helped establish important astronomical instruments and observatories in India, including those at Manora Peak in Uttarakhand and Kavalur in Tamil Nadu.
Legacy & Discoveries: The observatory at Kavalur has been named in his honour. Bappu made significant discoveries, including a comet, and studied stars and solar eclipses extensively.
Foucault Pendulum: Leon Foucault, a 19th-century scientist, used a long pendulum to demonstrate the Earth's rotation. A 22-metre Foucault pendulum is installed in the Constitution Hall of India's new Parliament building in New Delhi.
Aryabhata's Insight: Ancient Indian astronomer Aryabhata, in the 5th-century CE text Aryabhatiya, explained the apparent motion of the stars as resulting from the Earth's rotation. He gave an estimate of the rotation period close to the value known today, about 23 h 56 min 4.1 s.
Star Trails: Astrophotographers capture star trails using long exposures, showing the apparent motion of stars caused by the Earth's rotation.
Monsoon Prediction: Traditional communities such as the Bhil and Pawara used star patterns to predict monsoon rains, illustrating practical knowledge of celestial motions.
Polar Daylight Cycle: At the North Pole, the Sun rises near the spring equinox, about 21 March, and remains visible for about six months. The South Pole experiences the opposite cycle.
Ancient Astronomical Texts: The Surya Siddhanta and other ancient Indian texts used Sanskrit shlokas and mathematical rules to predict eclipses and other astronomical events.
Solar Research: The Kodaikanal Solar Observatory, established in 1899, has collected more than a century of solar observations and is run by the Indian Institute of Astrophysics, Bengaluru.
Rotation: The spinning motion of an object around its own axis. The Earth completes one rotation in about 24 hours.
Axis of rotation: An imaginary line passing through the North and South Poles about which the Earth rotates.
Revolution: The motion of an object around another object. The Earth's revolution around the Sun takes about 365 days and 6 hours.
Orbit: The path followed by an object as it revolves around another object, such as the Earth's orbit around the Sun.
Solstice: The time when the Sun reaches its greatest distance north or south of the equator, producing the longest or shortest day. The solstices occur around 21 June and 22 December.
Equinox: A time of the year when day and night are approximately equal in length. The equinoxes occur around 21 March and 23 September.
Solar eclipse: An event that occurs when the Moon passes between the Sun and the Earth and blocks sunlight from reaching parts of the Earth.
Lunar eclipse: An event that occurs when the Earth passes between the Sun and the Moon, causing the Earth's shadow to fall on the Moon.
Apparent size: How large an object looks from a particular point of view. It depends on the object's actual size and its distance from the observer.
Transit of Venus: A rare event when Venus passes directly between the Earth and the Sun and appears as a small dark dot moving across the Sun's face.
The Earth's rotation on its axis, from west to east, every 24 hours produces the day-night cycle and makes the Sun, Moon, and stars appear to move from east to west. The Earth's revolution around the Sun, which takes about 365 days, changes which stars and constellations are visible at night. Together with the tilt of the Earth's axis, this revolution produces seasonal variations. Solar eclipses occur when the Moon blocks sunlight from portions of the Earth, while lunar eclipses occur when the Earth's shadow falls on the Moon. Observations of these phenomena have been important to both ancient and modern astronomy and can be made safely with proper precautions.
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