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

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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