Have you ever wondered why shadows change length throughout the day, or why the Sun seems to move across the sky? These common observations connect us to the vast movements of Earth, its moon, and the Sun. Building on previous knowledge that Earth orbits the Sun and the Moon orbits Earth, this chapter explores these celestial motions in detail and explains fascinating phenomena like day and night, seasons, and eclipses.

12.1 Rotation Of The Earth

A fundamental observation is that the Sun appears to rise in the east and set in the west. This apparent motion leads to the cycle of day and night. But is the Sun actually moving around Earth, or is something else happening?

Activity 12.1: Let Us Explore

Imagine riding on a merry-go-round that is turning. While you are moving, the stationary objects around you (like trees or buildings) appear to be spinning around you in the opposite direction of your movement.

This experience helps explain the apparent movement of the Sun. The Sun appears to move across the sky because we are observing it from the Earth, which is itself in motion.

Just like a spinning top or a rotating fan, the Earth is constantly spinning around an imaginary line that passes through its center. This spinning motion is called rotation, and the imaginary line is the Earth's axis of rotation. The Earth's axis goes through the geographic North Pole and the South Pole.

The Earth completes one full rotation on its axis in approximately 24 hours.

Activity 12.2: Let Us Explore

Using a globe (representing Earth) and a torchlight (representing the Sun) in a dark room, you can visualize the effects of rotation. Shining the torch on the globe shows that only one half receives light (daytime), while the other half is dark (nighttime).

When viewed from above the North Pole, the Earth rotates in an anti-clockwise direction, which means it spins from West to East. As the globe rotates from West to East:

• Locations on the eastern side move into the light first, experiencing sunrise.
• As a location continues to rotate, it experiences daytime, then moves into darkness, experiencing sunset and night.

This rotation of the Earth from West to East is the fundamental cause of the cycle of day and night.

The Sun's apparent movement across the sky from East to West is simply a result of our perspective from the Earth rotating in the opposite direction (West to East).

Fascinating Fact: In the 19th century, scientist Leon Foucault used a long pendulum (now called a Foucault pendulum) to provide a visible demonstration of the Earth's rotation. Such a pendulum is now part of the new Parliament building in New Delhi, symbolizing India's connection to the cosmos.

Activity 12.3: Let Us Explore

Just like the Sun and Moon, the stars also appear to move across the night sky due to Earth's rotation. If you observe a constellation like the Big Dipper (Saptarishi) over several hours on the same night, it will appear to have moved.

In the Northern Hemisphere, the Earth's axis of rotation points very close to the Pole Star (Dhruva Tara). This is why the Pole Star appears almost stationary in the sky, while all other stars seem to rotate around it.

Fascinating Fact: Long-exposure photographs of the night sky capture the apparent paths of stars due to Earth's rotation as circular or arc-shaped trails around the Pole Star, known as star trails.

Fascinating Fact: Ancient Indian astronomers, including Aryabhata (5th century CE), understood that the apparent daily motion of celestial objects was due to the Earth's rotation. Aryabhata accurately calculated the duration of Earth's rotation.

12.2 Revolution Of The Earth

Besides rotating on its axis, the Earth also travels around the Sun. This motion of one object orbiting another is called revolution.

The path that the Earth follows as it revolves around the Sun is called its orbit. The Earth's orbit is nearly circular (though sometimes depicted as elongated from a side view).

Earth takes approximately 365.25 days (one year) to complete one full revolution around the Sun.

12.2.1 Changing View Of Night Sky From The Earth

As the Earth revolves around the Sun, our viewing direction relative to distant stars changes over the course of a year. This causes the specific stars and constellations visible in the night sky after sunset to change gradually throughout the year.

Observing the same area of the sky at the same time of night but in different months will show that different stars are visible as the Earth moves in its orbit.

Fascinating Fact: Many indigenous communities, like the Bhil and Pawara tribes in India, historically used the appearance of specific star patterns to predict the arrival of the monsoon season, demonstrating a practical understanding of the changing night sky caused by Earth's revolution.

12.2.2 Seasons On The Earth

The cycle of seasons (summer, autumn, winter, spring) that we experience throughout the year is a direct result of the Earth's revolution around the Sun combined with a crucial factor: the tilt of the Earth's axis of rotation.

The Earth's axis is tilted at an angle (approximately $23.5^\circ$) relative to the plane of its orbit around the Sun. This tilt remains pointed in roughly the same direction in space as the Earth revolves. This means that at different points in the orbit, either the Northern Hemisphere or the Southern Hemisphere is tilted towards the Sun.

• When a hemisphere is tilted towards the Sun (e.g., Northern Hemisphere in June):
  • Sunlight hits this hemisphere more directly, meaning the same amount of solar energy is concentrated over a smaller area, making the sunlight more intense.
  • This hemisphere receives sunlight for a longer duration during each 24-hour rotation (longer days and shorter nights).
  • These conditions result in Summer.
• When a hemisphere is tilted away from the Sun (e.g., Northern Hemisphere in December):
  • Sunlight hits this hemisphere less directly, spreading the same amount of solar energy over a larger area, making the sunlight less intense.
  • This hemisphere receives sunlight for a shorter duration during each 24-hour rotation (shorter days and longer nights).
  • These conditions result in Winter.

In March and September, the Earth's axis is tilted neither towards nor away from the Sun. Both hemispheres receive roughly equal amounts of sunlight, resulting in the less extreme temperatures of Spring and Autumn, and approximately equal day and night hours (equinoxes).

The seasons are reversed in the Northern and Southern Hemispheres. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, and vice versa.

Common Misconceptions: Seasons are NOT caused by the Earth being closer or farther from the Sun due to an elliptical orbit, or by the tilted hemisphere being physically closer to the Sun. While Earth's orbit is slightly elliptical and its distance from the Sun does vary, these variations are too small to cause the significant temperature differences observed in seasons. In fact, Earth is closest to the Sun in January (during winter in the Northern Hemisphere).

Solstices and Equinoxes:

• Around June 21st (Northern Hemisphere Summer Solstice): Longest day, shortest night in the Northern Hemisphere.
• Around December 22nd (Northern Hemisphere Winter Solstice): Shortest day, longest night in the Northern Hemisphere.
• Around March 20th/21st (Spring Equinox) and September 22nd/23rd (Autumnal Equinox): Approximately equal day and night hours (12 hours each) in both hemispheres.

Polar Regions and the Equator:

• Due to the Earth's tilt, the polar regions experience extreme variations in daylight, including periods of continuous daylight (polar summer) and continuous darkness (polar winter) for months.
• Regions near the equator experience relatively consistent temperatures and approximately 12 hours of daylight and 12 hours of darkness throughout the year, as the angle of sunlight doesn't change drastically.

12.3 Eclipses

An eclipse occurs when one celestial body blocks the light from another celestial body from reaching an observer.

12.3.1 Solar Eclipse

A solar eclipse happens when the Moon passes directly between the Sun and the Earth, blocking sunlight from reaching certain areas on Earth. You might wonder how the small Moon can block the light from the giant Sun.

Activity 12.4: Let Us Explore

Hold your thumb out at arm's length and try to cover someone's head standing some distance away. You can easily cover their entire head with your thumb, even though your thumb is much smaller than their head. This is because the apparent size of an object depends on both its actual size and its distance from the observer.

Similarly, although the Moon is physically much smaller than the Sun, it is also much, much closer to Earth. Due to this difference in distance, the apparent sizes of the Moon and the Sun in the sky are remarkably similar when viewed from Earth. This allows the Moon to appear large enough to cover the entire disk of the Sun during a solar eclipse.

Planets like Mercury and Venus are much larger than the Moon, but they are too far from Earth to block the Sun completely. When they pass between Earth and the Sun (a "transit"), they appear only as tiny dots.

During a solar eclipse, the Moon casts a shadow on the Earth. The type of solar eclipse observed depends on which part of the Moon's shadow falls on the observer's location:

• Total Solar Eclipse: Occurs for observers located in the darkest, central part of the Moon's shadow (the umbra). From this area, the Sun is completely blocked, and it becomes dark during the day.
• Partial Solar Eclipse: Occurs for observers located in the lighter, outer part of the Moon's shadow (the penumbra). From this area, only a portion of the Sun is blocked, and it appears as if a bite has been taken out of the Sun.

Total solar eclipses are relatively brief (lasting only a few minutes in any one location) and are visible only from a narrow path on Earth as the Moon's shadow moves across the surface due to Earth's rotation and the Moon's orbit.

Safe Viewing of a Solar Eclipse: It is extremely dangerous to look directly at the Sun, even during a solar eclipse. The Sun's rays can cause permanent eye damage or blindness. Never look at the Sun directly, through sunglasses, binoculars, or telescopes during a solar eclipse. The safest way to observe a solar eclipse is through specialized solar viewers provided by experts or by using indirect viewing methods, such as projecting the Sun's image onto a surface.

Historically, solar eclipses were often feared. Now, with scientific understanding, they are viewed as fascinating natural events and valuable opportunities for scientific research.

Fascinating Fact: Ancient Indian texts like the Surya Siddhanta contained methods for accurately predicting eclipses, showcasing advanced astronomical knowledge.

12.3.2 Lunar Eclipse

A lunar eclipse occurs when the Earth passes directly between the Sun and the Moon, blocking sunlight from reaching the Moon. In this case, the Earth's shadow falls on the Moon.

When the entire Moon passes into the darkest part of Earth's shadow (umbra), it is a total lunar eclipse. The Moon doesn't disappear entirely; it often appears reddish-orange because some sunlight is scattered and bent through Earth's atmosphere onto the Moon's surface. If only a portion of the Moon is in Earth's shadow, it is a partial lunar eclipse.

Unlike solar eclipses, lunar eclipses are safe to view with the naked eye. They are visible from a much larger area of Earth (anywhere on the night side facing the Moon) and typically last longer than solar eclipses, as Earth's shadow is larger than the Moon's.

Fascinating Fact: The Kodaikanal Solar Observatory in Southern India is a historic institution that has studied the Sun for over a century, contributing valuable data to solar astronomy.

KNOW A SCIENTIST: M.K. Vainu Bappu, considered the father of modern Indian astronomy, was instrumental in setting up major astronomical facilities in India and made significant contributions to stellar research, including studying solar eclipses.

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In a Nutshell:

• The Earth rotates on its axis from West to East, completing one rotation in about 24 hours.
• Earth's rotation causes the cycle of day and night and the apparent movement of the Sun, Moon, and stars across the sky.
• The Earth revolves around the Sun in a nearly circular orbit, taking approximately 365.25 days to complete one revolution.
• The change in the visible stars in the night sky over the year is due to Earth's revolution.
• Seasons on Earth are caused by the combination of Earth's revolution around the Sun and the tilt of Earth's axis of rotation relative to its orbital plane. This tilt affects the intensity and duration of sunlight in different hemispheres throughout the year.
• A solar eclipse occurs when the Moon passes between the Sun and Earth, casting a shadow on Earth and blocking sunlight.
• A lunar eclipse occurs when the Earth passes between the Sun and Moon, casting a shadow on the Moon and blocking sunlight from reaching it.
• The Moon's ability to cover the Sun during a solar eclipse is due to its apparent size being similar to the Sun's apparent size, despite the Moon being much smaller but much closer.
• Never look directly at the Sun during a solar eclipse; use safe viewing methods. Lunar eclipses are safe to view directly.

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