1. Stars and Constellations

The night sky is a vast expanse filled with celestial bodies. When we observe the sky from a dark, pollution-free location like Nubra in Ladakh, we can see thousands of stars and distinct patterns.

Understanding Stars

• Stars are spherical balls of hot gases that shine with their own light.
• They appear as tiny dots because they are extremely far from the Earth.
• The Sun is also a star, but it looks much larger because it is the closest star to us.

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Constellations and Patterns

An imaginary group of stars that forms a recognizable shape of an animal, object, or mythological character is called a constellation.

• The International Astronomical Union (IAU) has officially listed 88 constellations to divide the entire sky into regions.
• Orion (The Hunter): Recognizable by the three bright stars in its "belt."
  • In Indian astronomy, the star Betelgeuse is known as Ārdrā.
  • It is often seen with Canis Major (The Great Dog), which contains Sirius—the brightest star in the night sky.
• Taurus (The Bull): Contains the bright star Aldebaran, known in India as Rohiṇī.
• Ursa Major: Contains a prominent pattern known as the Big Dipper. In India, this group of seven stars is called Saptaṛiṣhi.
• Ursa Minor: Contains the Little Dipper. The last star in its handle is the Pole Star.

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Navigation and the Pole Star

Before modern technology and the magnetic compass, stars were the primary tools for navigation:

1. The Pole Star (Dhruva tārā) remains stationary in the North direction.
2. It helps travelers and sailors determine the North direction in the Northern hemisphere.
3. To locate it, imagine a straight line passing through the last two stars of the Big Dipper's cup; this line points directly toward the Pole Star.

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Tribal Stories of India

• Central Indian Tribes: View the rectangle of the Big Dipper as a "grandmother’s cot" being carried away by three thieves.
• Konkan Coast Fishermen: Imagine the same pattern as a boat, where the trailing stars represent the neck of the boat.

2. Our Solar System: The Sun and Planets

The Solar System is a family of celestial bodies consisting of the Sun, eight planets, their moons, and other smaller objects like asteroids and comets. All these objects are bound by the Sun’s gravity and move around it in a process called revolution.

The Sun: The Center of the System

• The Sun is the primary source of energy (heat and light) for all life on Earth.
• It is worshipped as Sūrya in India and is considered a deity in ancient civilizations.
• Size: It is about 100 times bigger than the Earth in diameter.
• Distance: Located about 150 million km away from the Earth.

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The Eight Planets

A planet is a nearly spherical object that revolves around a star. The planets in our system are divided into two main categories:

I. Inner Planets (Terrestrial)

• These include Budha (Mercury), Śhukra (Venus), Pṛithvī (Earth), and Mangala (Mars).
• They are smaller in size and have solid, rocky surfaces.

II. Outer Planets (Gas Giants)

• These include Bṛihaspati or Guru (Jupiter), Śhani (Saturn), Uranus, and Neptune.
• They are much larger than the inner planets and are mostly made of gases.
• They possess ring-like structures made of dust and ice.

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Comparison Table of Planets

Planet	Indian Name	Special Identification / Color
Mercury	Budha	Closest to the Sun; Very hot.
Venus	Śhukra	Hottest Planet; Called the Morning or Evening Star.
Earth	Pṛithvī	Blue Planet (due to water coverage).
Mars	Mangala	Red Planet (due to reddish soil).
Jupiter	Bṛihaspati / Guru	Largest planet in the Solar System.
Saturn	Śhani	Known for its beautiful visible rings.
Neptune	-	Farthest from the Sun; Extremely cold.

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Dwarf Planets and Pluto

• Pluto was once considered the ninth planet.
• In 2006, the IAU redefined the term "planet."
• Since Pluto is smaller than the Moon and shares its path with other objects, it is now classified as a dwarf planet.

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3. Satellites and the Moon

In astronomy, a satellite is defined as any celestial object that revolves around a much larger object. These are primarily classified into natural and human-made satellites. While the Earth is a satellite of the Sun, the Moon is the natural satellite of the Earth.

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The Moon: Earth’s Nearest Neighbour

The Moon is the most prominent object in our night sky. Its key physical characteristics include:

• Distance: It is located approximately $3,84,000 \text{ km}$ away from the Earth.
• Size: Its diameter is about one-quarter ($\frac{1}{4}$) the size of the Earth’s diameter.
• Revolution: It takes nearly $27 \text{ days}$ to complete one full revolution around the Earth.
• Atmosphere: Unlike Earth, the Moon has no atmosphere, water, or life.
• Surface: The surface is rugged and covered with craters (bowl-shaped depressions) caused by the impact of asteroids and space rocks.

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India’s Lunar Exploration (Chandrayaan Missions)

The Indian Space Research Organisation (ISRO) has achieved global recognition through its lunar programs:

1. Chandrayaan-1 ($2008$): Confirmed the presence of water molecules on the lunar surface.
2. Chandrayaan-2 ($2019$): Successfully sent an orbiter that continues to provide data.
3. Chandrayaan-3 ($2023$): On August 23, 2023, the Vikram lander and Pragyan rover made a historic soft landing near the South Pole of the Moon.

To honor this achievement, the Government of India has declared August 23 as National Space Day.

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Comparison of Moons in the Solar System

Planet	Number of Known Moons
Mercury	$0$
Venus	$0$
Earth	$1$ (The Moon)
Mars	$2$ (Phobos and Deimos)
Outer Planets	Large number (e.g., Jupiter and Saturn)

4. Other Celestial Objects: Asteroids and Comets

The space between and beyond the planets is not empty; it contains millions of smaller members of the Solar System family.

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Asteroids: The Rocky Remnants

• Asteroids are small, irregular, rocky objects that revolve around the Sun.
• Most of them are found in a vast gap between the orbits of Mars and Jupiter, known as the Asteroid Belt.
• Their sizes vary significantly, ranging from 10 metres to 500 kilometres.

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Comets: The Icy Visitors

Comets are icy-rocky bodies that originate from the outer, colder regions of the Solar System. In Indian languages, they are known as Dhūmaketu.

• Composition: They are made of dust, frozen gases, rocks, and ice.
• The Tail: As a comet approaches the Sun, its ice evaporates, forming a long, glowing tail that always points away from the Sun.
• Visibility: They appear as bright streaks with tails and become visible only when they are near the Sun.
• Halley’s Comet: This is a famous periodic comet that appears every $76 \text{ years}$. It was last seen in $1986$ and is expected to return in $2061$.

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Local Terminology for Comets in India

• In Sanskrit: Dhūmaketu.
• Tribal names: Pucchya-Taro (Star with a tail) or Zendya-Taro (Star like a flag).

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5. Beyond the Solar System: Galaxies and the Universe

Our Solar System is merely a speck in the vastness of the Universe. To understand our place in the cosmos, we must look at the larger structures of stars.

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The Milky Way Galaxy (Ākāśha Gangā)

A galaxy is a massive collection containing millions to billions of stars, gas, and dust held together by gravity.

• Our Solar System is a part of the Milky Way Galaxy.
• In India, it is traditionally called Ākāśha Gangā.
• From extremely dark locations like Ladakh, it appears as a faint, glowing band of light stretching across the sky.

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The Universe and Exoplanets

The Universe is the sum of all space, matter, and energy, containing billions of galaxies.

• Exoplanets: These are planets that revolve around stars other than our Sun.
• Search for Life: Scientists are currently studying these exoplanets to find any evidence of life beyond Earth.

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Practical Tips for Night Sky Watching

To enjoy the wonders of the universe, follow these guidelines:

1. Minimize Light Pollution: Move away from bright city lights, smoke, and dust to an open dark area.
2. Dark Adaptation: Allow your eyes about $30 \text{ minutes}$ to adjust to the darkness; this helps in seeing dimmer stars.
3. Distinguish Objects:
   • Stars: These are point sources of light that twinkle.
   • Planets: These reflect sunlight and usually do not twinkle. Venus is the brightest planet, visible near the horizon at dawn or dusk.

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Key Differences: Stars vs. Planets

Feature	Stars	Planets
Light Source	Emit their own light.	Reflect sunlight.
Appearance	Appear to twinkle.	Give steady light.
Movement	Appear fixed in patterns.	Revolve around the Sun.

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