1. The Night Sky and Celestial Objects

When we look at the sky on a clear night, we see many tiny shining points. These are mostly stars. We may also see the Moon and sometimes bright planets. All the natural objects that we see in the sky are called celestial objects.

Examples of celestial objects:

• Stars – like our Sun, the Pole Star, and other distant stars.
• Planets – Mercury, Venus, Earth, Mars, etc.
• Moon – Earth’s natural satellite.
• Comets, asteroids, meteors (shooting stars).
• Artificial satellites – man-made objects launched into space.

The study of these celestial objects and phenomena is called astronomy. In this chapter, we focus mainly on the Solar System and important objects related to it.

2. Stars and Constellations

(a) Stars

A star is a huge ball of hot glowing gases that emits light and heat on its own. Stars are extremely far away from us, so they appear like tiny points. Our Sun is also a star, but it appears large because it is much nearer to us than other stars.

Important points about stars:

• Stars are self-luminous (they have their own light).
• They are very large in size, but appear small due to large distance.
• Different stars have different colours and brightness.
• Stars appear to move from east to west due to the rotation of the Earth.

(b) Constellations

A constellation is a group of stars that appear to form a recognizable pattern or shape in the sky. These patterns often resemble animals, people or objects. Constellations helped ancient people in finding directions and in making calendars.

Some important constellations for Class 8:

• Ursa Major (Great Bear or Big Dipper): Looks like a big ladle or “question mark”. It is useful in locating the Pole Star.
• Orion (The Hunter): Looks like a hunter with a belt. It is visible in winter nights in the Indian sky.
• Cassiopeia: Appears like a distorted “W” or “M”. It is easily visible in the northern sky.

(c) Pole Star (Polaris)

The Pole Star is a special star which appears almost fixed in the sky. Other stars seem to revolve around it. It lies nearly along the direction of the Earth’s axis and is located in the northern sky.

Key points:

• It is used to find the north direction.
• It is part of the Little Bear (Ursa Minor) constellation.
• We can locate it by using the two “pointer stars” in Ursa Major.

3. The Solar System

The solar system consists of the Sun and all the celestial objects that revolve around it. These include eight planets, their moons (natural satellites), asteroids, comets, meteoroids and other small bodies.

(a) The Sun – The Head of the Solar System

The Sun is a star at the centre of the solar system. It is the main source of heat and light for all planets.

• The Sun is a huge sphere of hot gases (mainly hydrogen and helium).
• It holds the solar system together through its gravitational pull.
• Without the Sun, life on Earth would not be possible.

(b) Planets

A planet is a large spherical body that revolves around a star (like the Sun). Planets do not have their own light; they only reflect the light of the Sun.

Order of the eight planets from the Sun:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

Classification of planets

• Inner (terrestrial) planets: Mercury, Venus, Earth and Mars – close to the Sun, relatively smaller, rocky in nature.
• Outer (giant) planets: Jupiter, Saturn, Uranus and Neptune – far from the Sun, very large, mostly made of gases, have rings and many moons.

Short note on each planet (per NCERT level)

• Mercury: Nearest planet to the Sun, smallest, no atmosphere, extreme temperatures.
• Venus: Earth’s “twin” in size, brightest planet, sometimes called morning or evening star, thick atmosphere of carbon dioxide.
• Earth: Only known planet with life, has air, water, suitable temperature and protective atmosphere.
• Mars: Known as the red planet due to iron-rich soil; has thin atmosphere; explored by many space missions.
• Jupiter: Largest planet, mainly gaseous, has faint rings and many moons.
• Saturn: Second largest, famous for its broad beautiful rings, low density.
• Uranus: Rotates on its side, has rings and very cold atmosphere.
• Neptune: Farthest known planet, very cold and windy, bluish in colour.

4. The Earth and the Moon

(a) Earth – Our Home in the Solar System

The Earth is the third planet from the Sun and the only known planet to support life. It has liquid water on its surface, a suitable range of temperature and a protective atmosphere with oxygen.

Earth shows two main types of motion:

• Rotation: Spinning of Earth on its axis, taking about 24 hours (causes day and night).
• Revolution: Movement of Earth around the Sun in an almost circular orbit, taking about 365 days (causes seasons).

(b) Moon – The Earth’s Natural Satellite

The Moon is the natural satellite of the Earth. It revolves around the Earth and also rotates on its own axis.

Important facts about the Moon:

• It has no atmosphere, so there is no air and no weather (no clouds, no rain).
• Its surface has craters, mountains and plains (called maria or “seas”).
• The Moon has no light of its own; it shines because it reflects sunlight.
• It takes about 27.3 days to revolve once around the Earth and almost the same time to rotate once on its axis.

(c) Phases of the Moon

The phases of the Moon are the different shapes of the bright part of the Moon as seen from the Earth. These phases occur because of the changing positions of the Sun, Earth and Moon.

Major phases:

• New Moon: Moon is not visible; the side facing Earth is dark.
• Crescent Moon: Only a thin curved part is visible.
• First Quarter: Half of the Moon’s visible side is lit.
• Gibbous Moon: More than half but not full is visible.
• Full Moon: Whole circular face is bright and visible.
• The cycle from one New Moon to the next takes about 29 days (lunar month).

5. Other Members of the Solar System

(a) Asteroids

Asteroids are small rocky bodies that revolve around the Sun. Most of them are found in a region between Mars and Jupiter known as the asteroid belt.

Key points:

• They are smaller than planets.
• They may be the remains of an early planet that did not form completely.
• Some asteroids may cross Earth’s orbit and can be dangerous if they collide with Earth.

(b) Comets

A comet is a small body made of ice and dust which moves around the Sun in a long elliptical orbit. When a comet comes near the Sun, it heats up and develops a bright head and a glowing tail.

• The tail of a comet always points away from the Sun because of solar radiation and solar wind.
• Comets are seen rarely and appear for a short time.
• Example: Halley’s Comet appears approximately every 76 years.

(c) Meteors and Meteorites

Meteors are small pieces of rock or metal that enter the Earth’s atmosphere at high speed. Due to friction with the air, they get heated up and burn, producing a streak of light in the sky. They are often called “shooting stars”, although they are not real stars.

If a meteor is large and does not burn completely, its remaining part falls on the Earth’s surface. This is called a meteorite. Meteorites may cause craters where they hit the ground.

(d) Artificial Satellites

Artificial satellites are human-made objects placed in orbit around the Earth (or another planet) using rockets. India and many other countries have launched several satellites for useful purposes.

Uses of artificial satellites:

• Communication – TV, telephone, internet signals.
• Weather forecasting – observing clouds, storms and cyclones.
• Remote sensing – collecting information about Earth’s surface, forests, crops, water bodies, etc.
• Scientific research – studying space, Sun, other planets and galaxies.

6. Important Differences to Remember

(a) Star vs Planet

• Star: Has its own light and heat; very large; twinkles; e.g. Sun, Pole Star.
• Planet: Does not have its own light; reflects sunlight; does not twinkle; moves around a star.

(b) Planet vs Satellite

• Planet: Revolves around a star (Sun).
• Satellite: Revolves around a planet (e.g. Moon around Earth).

(c) Meteor vs Meteorite

• Meteor: Burns in the atmosphere; seen as a shooting star.
• Meteorite: Part of meteor that lands on Earth’s surface.

(d) Inner vs Outer Planets

• Inner planets: Mercury, Venus, Earth, Mars – rocky, smaller, closer to Sun.
• Outer planets: Jupiter, Saturn, Uranus, Neptune – huge, gaseous, far from Sun, have rings.

7. Exam Tips and Quick Revision Points

• Learn the names and order of planets from the Sun using a simple mnemonic.
• Be able to define terms like star, planet, satellite, asteroid, comet, meteor, meteorite, constellation.
• Memorise at least two examples of constellations and be able to describe their shape.
• Understand why the Moon has phases and why we always see the same side of the Moon.
• Remember that Pole Star helps to find north direction and appears fixed in the sky.
• Know at least three uses of artificial satellites – communication, weather, remote sensing.
• Practise labelled diagrams – Solar System (schematic order of planets) and phases of the Moon (conceptual).

Regular reading and self-questioning from these notes will help you write short and long answers in your own words and score well in CBSE Class 8 examinations.