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
→ 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
→ 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
→ 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.