Celestial objects are natural objects that we see in the sky outside the Earth’s atmosphere.

Examples: Sun, Moon, stars, planets, comets, asteroids (any four).

Stars are extremely far away from the Earth. Due to this huge distance, even though they are very large in size, they appear as very small points of light in the night sky.

During the day, the Sun’s bright light scatters in the atmosphere and makes the sky appear bright.

The faint light of distant stars is then not visible to us against this bright background, although the stars are present in the sky.

Astronomy is the branch of science that studies celestial objects, space and the universe.

A person who studies and observes celestial objects scientifically is called an astronomer.

The Earth rotates from west to east on its axis.

Due to this rotation, stars appear to move in the opposite direction, that is from east to west, although in reality they are almost fixed relative to us.

The nearest star to the Earth is the Sun.

It is the main source of light and heat for all planets in the solar system and is essential for life on Earth.

A star is a huge ball of hot glowing gases which emits its own light and heat.

A planet does not have its own light; it only reflects the light of a star (for us, the Sun).

A constellation is a group of stars that appear to form a recognizable pattern in the night sky.

Examples: Ursa Major (Great Bear) and Orion (The Hunter).

Ursa Major, also called the Great Bear or Big Dipper, appears like a large ladle or a question mark.

It consists of a group of seven bright stars that form the shape of a “big dipper”.

Orion is also called The Hunter. It looks like a hunter holding a shield and club.

It has three bright stars in a straight line called the “belt of Orion” and is visible in winter nights in India.

The Pole Star (Polaris) is a star that appears almost fixed in the northern sky while other stars seem to revolve around it.

It is important because it lies close to the direction of the Earth’s axis and is used to find the north direction.

First locate the two end stars of the “bowl” of Ursa Major. These are called pointer stars.

Imagine a straight line from these stars and extend it about five times their distance. The fairly bright star you reach is the Pole Star.

Constellations appear to move because the Earth is rotating on its axis.

As the Earth rotates, our view of the sky changes, and hence constellations seem to shift their position during the night.

Stars are very far away and act like tiny point sources of light. Their light is easily disturbed by the Earth’s atmosphere, so they appear to twinkle.

Planets are closer and appear as small discs; the effect of atmospheric disturbance averages out, so they usually shine steadily and do not twinkle much.

The solar system consists of the Sun and all the celestial bodies that revolve around it such as the eight planets, their moons, asteroids, comets and other small objects.

The Sun is a medium-sized star at the centre of the solar system.

It provides light and heat, and its strong gravitational pull keeps all the planets and other bodies bound in their orbits around it.

The Sun gives light for seeing and heat to keep the Earth warm.

It drives the water cycle, weather, and supports photosynthesis in plants, which ultimately provides food and oxygen for living organisms.

A planet is a large body which revolves around a star and does not have its own light.

Each planet moves around the Sun in a fixed elliptical path called its orbit.

Inner planets: Mercury, Venus, Earth, Mars – they are closer to the Sun, smaller in size and mainly rocky.

Outer planets: Jupiter, Saturn, Uranus, Neptune – they are farther from the Sun, very large and mainly gaseous with rings and many moons.

An orbit is the fixed path in which a planet moves around the Sun.

Orbits are almost circular or elliptical and are maintained due to the gravitational force of the Sun.

The eight planets in order from the Sun are:

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

Mercury is closest to the Sun, so its day side becomes very hot.

It has almost no atmosphere to retain heat, so the temperature drops sharply at night making it very cold.

Venus is called Earth’s twin because it is almost similar to Earth in size and mass.

It is called the morning or evening star because it appears very bright and can be seen near the horizon just before sunrise or just after sunset.

Earth is unique because it has liquid water, suitable temperature, and a protective atmosphere with oxygen.

These conditions support the existence of life, which has not been found on any other known planet.

Mars is known as the Red Planet because its surface contains iron-rich rocks and soil.

The iron gets oxidised (rusted) and gives a reddish colour to the planet when seen from space.

The largest planet is Jupiter.

The second largest planet is Saturn.

Saturn is famous for its beautiful rings made of ice and dust particles.

It is also very light; its average density is less than that of water, so it could float if there were a huge ocean.

The axis of Uranus is highly tilted and almost lies in the plane of its orbit.

Because of this, Uranus seems to roll on its side as it moves around the Sun, unlike other planets whose axes are nearly upright compared to their orbits.

Neptune is the farthest planet from the Sun.

It receives very little sunlight and is extremely cold. It is a windy, bluish planet made mainly of gases and ice.

(i) Stars have their own light and heat, while planets do not; they reflect star’s light.

(ii) Stars twinkle and are very far, while planets generally do not twinkle and are comparatively closer to us.

A satellite is a body that revolves around a planet.

Moon is a natural satellite of the Earth.

(i) Rotation: Spinning of the Earth on its axis, causing day and night.

(ii) Revolution: Movement of the Earth around the Sun, causing seasons in a year.

The surface of the Moon has craters, mountains and flat plains.

It has almost no atmosphere, so there is no air, no weather, and no protection from meteors or harmful rays.

The Moon lacks a significant atmosphere. Without air, there can be no winds, clouds, or rain.

Therefore, the Moon has no weather changes like on Earth.

The phases of the Moon are the different shapes of the bright part of the Moon seen from the Earth.

They occur because the relative positions of the Sun, Earth and Moon change as the Moon revolves around the Earth, so we see varying fractions of the lit side.

On New Moon day, the side of the Moon facing Earth is dark, so the Moon is not visible.

On Full Moon day, the whole face of the Moon facing Earth is fully illuminated by the Sun and appears as a complete bright disc.

The Moon takes about 27.3 days to complete one revolution around the Earth.

This period is called a lunar month or sidereal month (about 27.3 days).

The Moon takes almost the same time to rotate on its axis as it takes to revolve around the Earth.

Because of this, the same side of the Moon always faces the Earth and we never see the far side from Earth’s surface.

Asteroids are small rocky bodies that revolve around the Sun.

Most of them are found in the asteroid belt between the orbits of Mars and Jupiter.

A comet is an icy object that moves around the Sun in a very long elliptical orbit.

When it comes near the Sun, it heats up and develops a glowing head and a long bright tail that always points away from the Sun.

The tail of a comet is made of gases and dust that are pushed away from the Sun.

Solar radiation and solar wind blow the material away from the Sun, so the tail always points in the opposite direction of the Sun.

A meteor is a small piece of rock or dust that enters the Earth’s atmosphere at high speed.

Due to friction, it gets heated and glows for a short time, appearing as a bright streak of light in the sky, so it is popularly called a shooting star, although it is not a real star.

A meteorite is the part of a meteor that does not burn completely in the atmosphere and falls onto the Earth’s surface.

A meteor burns up in the atmosphere and usually does not reach the ground.

Sometimes a large number of meteors enter the Earth’s atmosphere in a short time and we see many shooting stars in the sky.

This event is called a meteor shower. It often occurs when the Earth passes through the dusty path of a comet.

An artificial satellite is a man-made object that is placed in orbit around the Earth or another planet using rockets.

It continuously revolves around the planet due to gravitational attraction and its own motion.

Uses of artificial satellites:

(i) Communication – TV, telephone, internet signals.

(ii) Weather forecasting – studying clouds, cyclones, storms.

(iii) Remote sensing – mapping forests, crops, water bodies, minerals.

(iv) Scientific research – studying space, planets and the Sun.

Remote sensing is the technique of collecting information about the Earth’s surface from a distance, usually from satellites.

It helps in studying land use, crop health, water resources, forests and natural disasters without touching the area directly.

Natural satellite: Formed by nature, revolves around a planet. Example – Moon (Earth’s natural satellite).

Artificial satellite: Man-made, launched by rockets to revolve around Earth (or another planet). Example – INSAT (Indian satellite).

Weather satellites take pictures of clouds, sea conditions and wind patterns from space.

By studying these images, scientists can detect the formation and movement of cyclones and storms early and issue warnings to save lives and property.

Artificial satellites support communication, weather forecasting, disaster management, agriculture planning, defence and scientific research.

They help in better planning and development, making them very important for a country’s progress and security.