CBSE Class 8 Science • Chapter 17 • Detailed Master Notes
Chapter Overview
When you look up at the night sky, you see thousands of twinkling stars and a glowing moon. In this chapter, we explore celestial objects like the moon, stars, planets, and the structure of our solar system.
The stars, the planets, the moon and many other objects in the sky are called celestial objects.
The moon is the brightest object in the night sky. It changes its shape day by day. The various shapes of the bright part of the moon as seen during a month are called phases of the moon.
The moon does not produce its own light. We see the moon because the sunlight falling on it gets reflected towards us. We see only that part of the moon from which the light of the sun is reflected towards us. As the moon revolves around the earth, the illuminated portion visible to us changes.
The moon's surface is dusty and barren. It has many craters of different sizes and steep, high mountains. The moon has no atmosphere and no water, so life cannot exist there.
AI Image Prompt: A diagram showing the phases of the moon. Earth is in the center, with sunlight coming from one side. The moon is shown at different positions in its orbit around Earth, indicating how the New Moon, Crescent, Half, Gibbous, and Full Moon phases appear to an observer on Earth.
Stars are celestial bodies that emit light of their own. The sun is also a star, but it appears much larger than other stars because it is near to us. Other stars are millions of times farther away than the sun.
Light Year: The distances in space are so large that they are measured in a unit called light year. A light year is the distance travelled by light in one year. Light travels at a speed of about 300,000 km per second.
Example: The sun is about 8 light minutes away from Earth. The next nearest star, Alpha Centauri, is at a distance of about 4.3 light years.
The stars appear to move from east to west because the Earth rotates from west to east on its axis. However, there is one star, the Pole Star (Dhruv Tara), which appears stationary. This is because it is situated in the direction of the Earth's axis of rotation.
The stars forming a group that has a recognizable shape is called a constellation.
The sun and the celestial bodies which revolve around it form the solar system. It consists of large numbers of bodies such as planets, comets, asteroids and meteors.
The sun is the nearest star to us. It is continuously emitting huge amounts of heat and light. The sun is the source of almost all energy on Earth.
The planets look like stars, but they do not have light of their own. They merely reflect the sunlight that falls on them. The planets keep changing their positions with respect to the stars.
A planet has a definite path in which it revolves around the sun. This path is called an orbit. The time taken by a planet to complete one revolution is called its period of revolution, which increases as the distance from the sun increases. A planet also spins on its own axis, which is called a rotation.
AI Image Prompt: A realistic illustration of the solar system. The bright yellow Sun is on the left, followed by the planets in orbit: Mercury, Venus, Earth with its moon, Mars, Jupiter, Saturn with its rings, Uranus, and Neptune against a dark starry background. Clear and educational.
| Planet | Key Characteristics |
|---|---|
| Mercury (Budh) | Nearest to the sun. Smallest planet in the solar system. Has no satellite of its own. |
| Venus (Shukra) | Earth's nearest planetary neighbour. The brightest planet in the night sky. Also called morning or evening star. It rotates from east to west. |
| Earth (Prithvi) | The only planet where life exists due to the right temperature range, water, and suitable atmosphere (ozone layer). It has one moon. |
| Mars (Mangal) | Appears slightly reddish and is called the red planet. It has two small natural satellites. |
| Jupiter (Brihaspati) | The largest planet. It is so large that about 1300 earths can fit inside it. It rotates very rapidly and has faint rings and many satellites. |
| Saturn (Shani) | Appears yellowish. Famous for its beautiful rings. It is the least dense among all the planets (less dense than water). |
| Uranus and Neptune | The outermost planets. Uranus also rotates from east to west and has a highly tilted rotational axis, making it appear to roll on its side. |
Q1. Why does Venus appear as the brightest planet in the sky?
Ans: Venus has a thick atmosphere that reflects most of the sunlight falling on it, making it highly luminous compared to other planets.
Artificial satellites are man-made and revolve around the Earth much closer than the moon (Earth's natural satellite).
Aryabhata was the first Indian satellite. Other Indian satellites include INSAT, IRS, Kalpana-1, and EDUSAT. Artificial satellites are used for forecasting weather, transmitting television and radio signals, telecommunication, and remote sensing.