Stars & the Solar System Summary Class 8 NCERT Summary

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.