Study Material and Notes of Ch 17 Stars and the Solar System Class 8th Science

Topics in the chapter

• Introduction
• The Moon
→ Moon's Surface
• The Stars
• Constellations
• Solar System
→ Planets
• Inner Planets
• Outer Planets
• Other members of Solar System
→ Asteroids
→ Comets
→ Meteors and Meteorites
→ Artificial Satellite

Introduction

→ The moon is the brightest object in the night sky.

→ All natural objects like the stars, the planets, the moon and many other objects in the sky are celestial objects.

The Moon


→ The various shapes of the bright part of the moon as seen during a month are called phases of the moon.

• New Moon day → when moon is not visible
• Full Moon day → when full moon is visible

→ Gap between consecutive new moon day and full moon day is of 15 days.

→ Rotational period and revolution period of moon are the same (almost 29 days).

Moon’s surface


→ Because of lack of atmosphere, one cannot hear any sound on moon.


→ Moon is visible due to reflected sunlight

The Stars

→ All stars emit their own light. They appear small because of large distances from the earth.

→ The sun appears bigger because it is nearer than any other stars on the space.

→ In day time, stars are not visible because of bright sunlight.

→ Stars appear to move from east to west because of earth’s rotation from west to east.


→ Pole star does not appear to move because it is very nearly situated on earth’s rotational axis over the North pole.

Constellations


→ The stars forming a group that has a recognizable shape is called a constellation.


→ Ursa Major also known as the Big Dipper, the Great Bear or the Saptarshi is one of the most famous constellations during summer time.

→ Orion also called the Hunter is another well-known constellation that can be seen during winter in the late evenings.

Solar System

→ The Sun and the celestial bodies which revolve around it form the solar system.

→ Sun is the nearest star from the earth.

→ It consists of large number of bodies such as planets, comets, asteroids and meteors.

→ The gravitational attraction between the Sun and these objects keeps them revolving around it.

Planets

→ Stars twinkle in the night sky, but planets do not.

→ Planets revolve around the sun along definite paths, called orbits.

→ Time taken by a planet to complete one revolution of its orbit is called revolution period.

→ Time taken by a planet to rotate about its axis is called period of rotation.

→ Satellites revolve around planets.

Inner planets

• Mercury

→ Nearest planet to the sun

→ It is seen just before sunrise and just after sunset near horizon. It has no satellite.

• Venus

→ Nearest planet to the earth

→ Brightest planet in the night sky

→ Seen in the eastern sky before sunrise and in the western sky after sunset

→ Also known as morning or evening star

→ Has no satellite and rotates from east to west (sun rises in the west of Venus).

• Earth

→ From space, it appears blue because of 75% water content.

• Mars

→ It appears reddish and therefore, is known as red planet.

Outer planets

• Jupiter

→ Largest planet in the solar system

→ Rotates very fast about its axis and has large numbers of satellites

• Saturn

→ Has prominent ring system and large numbers of satellites

→ Its density is less than water and is the least among the planets

• Uranus and Neptune

→ Both have ring system.

→ Uranus has a tilted rotational axis and appears to roll on its side.

→ Uranus rotates from east to west similar to Venus.

Other members of Solar System

Asteroids

→ Small rocky objects found in large numbers between Mars and Jupiter

Comets


→ Highly elliptical objects

→ Have a bright head and long gaseous tail.

→ Tail is always directed away from the sun.

→ Halley’s comet appears after every 76 years.

Meteors & Meteorites

→ Objects that enter the earth’s atmosphere and burn because of friction with the atmosphere

→ Large meteors that reach earth’s surface are called meteorites.

Artificial satellite


→ Revolves around the earth

→ Used for weather forecasting, remote sensing, communication system, etc.
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