NCERT Class 8 Science Chapter 17: Stars And The Solar System

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NCERT Class 8 Science Chapter 17: Stars and the Solar System

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Brain Teasers!!

  • What is light year?

  • Why pole star appears to be stationary from earth?

  • Differentiate asteroids, comets and meteors.

  • Which is the brightest and hottest planet?

  • Why meteors are not seen during the day time?

  • Why we see same side of moon always?

  • We see same side of moon always because the period of rotation of the moon on its axis is equal to the period of its revolution round the earth.

Night Sky

night Sky

Night Sky

night Sky

  • Night sky is very clear in villages – no smoke, dust and air pollution, no bright lights.

  • Objects that do not twinkle are planets.

  • The moon is the brightest object in the night sky. The stars, the planets, the moon and many other objects in the sky are called celestial objects.

  • The study of celestial objects and associated phenomena is called astronomy.

  • Ancient India - Passage of the Sun, stars, moon and planets in the sky helped them to devise calendars and almanacs – for sowing and crop choice.

Phases of the Moon

Phases Of The Moon

Phases of the Moon

Phases Of The Moon

The day on which the whole disc of the moon is visible is known as the full moon day.

On the fifteenth day the moon is not visible. This day is known as the ‘new moon day’

  • Only a small portion of the moon appears in the sky and is called crescent moon.

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

  • Full moon day celebration – Diwali, Budh Poornima, Guru Nanak Jayanti.

  • Maha Shivratri is celebrated on thirteenth night of waning moon; Eidul- Fitr is observed on the day following the sighting of crescent moon.

  • The time period between one full moon to the next full moon is slightly longer than 29 days.

  • Moon change its shape because we see only that part of the moon from which the light of the sun is reflected towards us. The phases are seen because the moon does not emit its own.

  • light and reflects the light of sun.

Moon is Visible Due to Reflected Sunlight

Moon Is Reflected Sunlight

Moon is Reflected Sunlight

Moon Is Reflected Sunlight

  • Moon is visible due to the reflected light of the sunlight and does not have its own light.

  • Remember that the moon revolves around the Earth. The Earth along with the moon, revolves around the Sun.

The Moon Appears Different at Different Positions in Its Orbit

Diffrent Positions In its Orbit

Diffrent Positions in Its Orbit

Diffrent Positions In its Orbit

Child experiment - white portion of the ball always towards the Sun. If you are performing this activity in the morning then the white portion of the ball should be kept towards the east. If the activity is being performed in the afternoon then the white portion of the ball should be kept towards the west.

Positions of Moon in Its Orbit and Its Corresponding Phases

Positions Of Moon

Positions of Moon

Positions Of Moon

Direction of sunlight is important to understand phases of moon.

Earth Accompanied by Moon Revolving Around the Sun

Earth Accompanied

Earth Accompanied

Earth Accompanied

  • You would have noted that the point of sunrise changes continuously. From summer solstice (around 21 June), the point of sunrise gradually shifts towards the south. The Sun is then said to be in dakshinayan (moving south). It keeps doing so till winter solstice (around 22 December). Thereafter, the point of sunrise changes direction and starts moving towards north. The Sun is now said to be in uttarayan (moving north). From the equator, only on two days, on the days of the equinoxes (around 21 March and 23 September) the Sun rises in the east. On all other days, it rises either north of east or south of east.

  • The Pole Star, which defines North, is a much better indicator of directions.

  • About 500 years ago, a Polish priest and astronomer, named Nicolaus Copernicus (1473 – 1543), stated that the Sun was at the centre of the solar system and the planets revolved around it. It was revolutionary idea. Even Copernicus hesitated to publish his work. His work was published in the year of his death in 1543.

  • In 1609, Galileo designed his own telescope. Through this telescope Galileo observed moons of Jupiter, phases of Venus and rings of Saturn. He argued that all the planets must orbit the Sun and not the Earth.

An Astronaut on the Moon

An Astronaut On The Moon

An Astronaut on the Moon

An Astronaut On The Moon

  • Moon’s surface is dusty and barren. There are many craters of different sizes. It also has a large number of steep and high mountains.

  • The moon has no atmosphere. It has no water.

  • On July 21, 1969 (Indian time) American astronaut, Neil Armstrong, landed on the moon for the first time. He was followed by Edwin Aldrin.

  • Sound cannot travel when there is no medium. Then, how can we hear any sound on the moon? No.

Surface of the Moon

Surface Of The Moon

Surface of the Moon

Surface Of The Moon

The Pole Star Lies Close to the Axis of Rotation of the Earth

Pole Star Close to The Axiz Of Rotation Of The Earth

Pole Star Close to the Axiz of Rotation of the Earth

Pole Star Close to The Axiz Of Rotation Of The Earth

  • Sun is a star. Sun appears big as it is closeby. The stars are millions of times farther away than the Sun. Therefore, the stars appear to us like points.

  • The Sun is nearly 150,000,000 kilometres (150 million km) away from the Earth. The next nearest star is Alpha Centauri. It is at a distance of about 40,000,000,000,000 km from the Earth.

  • Such large distances are expressed in another unit known as light year. It is the distance travelled by light in one year. Remember that the speed of light is about 300,000 km per second. Thus, the distance of the Sun from the Earth may be said to be about 8 light minutes. The distance of Alpha Centauri is about 4.3 light years.

  • Stars are present in the sky during the day-time also. However, they are not visible then because of the bright sunlight.

  • Stars appear to move from east to west.

  • When she is in a moving train the nearby trees and buildings appear to move in the backward direction. If the stars appear to move from east to west, could it mean that the Earth, If light from stars takes rotates from west to east.

  • Sun appears to rise in the east and set in the west as the Earth rotates from west to east on its axis.

Pole Star Does Not Appear to Move

Pole Star Does Not Appear To Move

Pole Star Does Not Appear to Move

Pole Star Does Not Appear To Move

  • If there were a star located where the axis of rotation of the Earth meets the sky, could this star also be stationary?

  • There is actually a star, the pole star, which is situated in the direction of the earth’s axis. It does not appear to move.

Some Constellations in the Night Sky

Some constellations in the night sky

Some Constellations in the Night Sky

Some constellations in the night sky

  • The stars forming a group that has a recognisable shape is called a constellation.

  • Most famous constellation is Ursa Major or Big Dipper or Saptarshi or Great Bear – 7 stars with big ladle - 7 rishis.

  • Constellation appears to move in the sky from east to west.

  • Ursa Major moves around the Pole star.

  • Infact all stars move around pole star.

Dipper Used for Drinking Water in Ancient Times

Dipper used for drinking water in ancient times

Dipper Used for Drinking Water in Ancient Times

Dipper used for drinking water in ancient times

Ursa Major Moves Around the Pole Star &

Locating – Sirius

Ursa major

Ursa Major

Ursa major

  • About five times the distance between the two stars is location of Ursa Major’s star to Pole Star

  • Note that the Pole star is not visible from the southern hemisphere.

  • Orion is another well-known constellation that can be seen during winter in the late evenings. Called as Hunter. The three middle stars represent the belt of the hunter. The four bright stars appear to be arranged in the form of a quadrilateral.

  • The star Sirius, which is the brightest star in the sky, is located close to Orion. Straight line passing to east is Sirius.

  • Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night. It looks like a distorted letter W or M.

The Solar System (Not to Scale)

The Solar System

The Solar System

The Solar System

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

  • The Sun is the nearest star from us. It is continuously emitting huge amounts of heat and light.

  • The planets look like stars, but they do not have light of their own. They merely reflect the light of the sun.

  • Also 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.

  • Till 2006 there were nine planets in the solar system. Pluto was the farthest planet from the Sun. In 2006, the International Astronomical Union (IAU) adopted a new definition of a planet. Pluto does not fit this definition. It is no longer a planet of the solar system.

Planets Move in Their Own Orbits

Earths orbit

Earths Orbit

Earths orbit

  • The time taken by a planet to complete one rotation is called period of rotation.

  • The time taken by a planet to complete one revolution is called its period of revolution.

  • Some planets are known to have moons/satellites revolving round them. Any celestial body revolving around another celestial body is called its satellite.

  • Satellite for the bodies revolving around planets. Moon is a satellite of the Earth.

  • The practice of astronomy in ancient India is mentioned in Rig Veda which was composed about 4000 years ago. The work of Aryabhata on astronomy can be found in his writing. ‘Aryabhatiya’. He wrote it in 499 CE at the age of 23 years. The diameter of the Earth as stated by Aryabhata is close to its presently known value. Disregarding the popular view that Earth is ‘achala’ (immovable), Aryabhata stated that Earth is sphere and rotates on its own axis. His estimate about the sidereal period of Earth was 23 hours, 56 minutes and 4.1 seconds, which is very close to the presently known value. He also correctly stated that the moon and the planets shine due to reflected sunlight. He also gave a scientific explanation for solar and lunar eclipses. When the shadow of.

  • the Earth falls on the moon, it causes lunar eclipse. When the shadow of the moon falls on the Earth, it causes solar eclipse. Aryabhata also found the distance between the Earth and the.

  • moon, which is very close to the known value today.

The Earth (Prithvi)

Earth

Earth

Earth

Mercury (Budh)

Mercury

Mercury

Mercury

Venus (Shukra)

Venus

Venus

Venus

  • Mercury – nearest, smallest, closest to sun. However, it can be observed just before sunrise or just after sunset, near the horizon. So it is visible only at places where trees or buildings do not obstruct the view of the horizon. Mercury has no satellite of its own.

  • Venus – nearest to Earth, brightest, hottest, morning or evening star, rotates from east to west, no satellite.

  • Earth – planet with life, ozone layer, right distance, right temperature, blue green due to reflection of light from water and landmass, tilt is responsible for seasons, has one moon.

Phases of Venus

Phases of venus

Phases of Venus

Phases of venus

Jupiter (Brihaspati)

Jupiter

Jupiter

Jupiter

Mars (Mangal)

Mars

Mars

Mars

  • Mars – reddish, red planet 2 natural satellites

  • Indian Space Research Organisation (ISRO) launched India’s first Mars orbiter mission – Mangalyaan, on November 5, 2013. It was successfully placed into an orbit of Mars on September 24, 2014. With this India became the first country in the world to do so in its first attempt.

  • Jupiter – Largest, 1300 earth can be placed, mass is 318 times the earth mass, has many satellites, 4 large moons.

Earth Rotates on a Tilted Axis

Earth Rotates on A Titled Axis

Earth Rotates on a Titled Axis

Earth Rotates on A Titled Axis

Jupiter and Its Four Large Satellites

Jupiter and its four large satellites gif

Jupiter and Its Four Large Satellites Gif

Jupiter and its four large satellites gif

The plane of the equator is called the equatorial plane. The plane in which the Earth revolves round the Sun is called the orbital plane of the Earth. These two planes are inclined to each other at an angle of 23.5º. This means that the axis of the Earth is inclined to its orbital plane at an angle of 66.5º.

Uranus in Its Orbital Path

Uranus In Its Orbital Path

Uranus in Its Orbital Path

Uranus In Its Orbital Path

Saturn (Shani)

Saturn

Saturn

Saturn

Saturn – Yellow, rings but not visible by naked eyes, many satellites, least dense with density half of water

Uranus – Outermost and rotate east to west – tilted rotational axis

The first four planets, Mercury, Venus, Earth and Mars are much nearer the Sun than the other four planets. They are called the inner planets

The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus and Neptune are much farther off than the inner planets. They are called the outer planets. They have a ring system around

them. The outer planets have large number of moons.

The Asteroid Belt

The Asteroid Belt

The Asteroid Belt

The Asteroid Belt

Objects between Mars and Jupiter seen through large telescopes.

Different Position of a Comet

Different Position Of A Comet

Different Position of a Comet

Different Position Of A Comet

  • Comets revolve in elliptical path, revolution period is long with bright head and long tail. The length of the tail grows in size as it approaches the sun.

  • Tail is directed away from the sun.

  • Halley’s comet, which appears after nearly every 76 years. It was last seen in 1986.

  • Some people think that comets are messengers of disasters, such as wars, epidemics and floods. But these are all myths and superstitions. Appearance of a comet is a natural phenomenon.

Streak of a Meteor

Streak Of a Meteor

Streak of a Meteor

Streak Of a Meteor

  • Shooting stars, although they are not stars. They are called meteors. A meteor is usually a small object that occasionally enters the earth’s atmosphere. At that time it has a very high speed. The friction due to the atmosphere heats it up. It glows and evaporates quickly. That is why the bright steak lasts for a very short time.

  • Some meteors are large and so they can reach the Earth before they evaporate completely. The body that reaches the Earth is called a meteorite. Meteorites help scientists in

  • investigating the nature of the material from which the solar system was formed.

  • When the Earth crosses the tail of a comet, swarms of meteors are seen. These are known as meteor showers

  • The brightness of a meteor is extremely small compared to that of the sun, therefore, it is not seen during day time.

Some Indian Satellites

Some Indian Satelites

Some Indian Satelites

Some Indian Satelites

  • Artificial satellites are man-made. They are launched from the Earth. They revolve around the Earth much closer than earth’s natural satellite, the moon.

  • Aryabhata was the first Indian satellite.

  • There aim is forecasting weather, transmitting television and radio signals and are used for telecommunication and remote sensing

  • Kalpana Chawla is the first Indian woman astronaut. She was born on 17 March 1962 in Karnal, Haryana. She did her Bachelor of Science degree in Aeronautical Engineering from Punjab Engineering College, Chandigarh. In 1982, she moved to USA and obtained a Master of Science degree in Aerospace Engineering from the University of Texas and Ph.D. in Aerospace Engineering from University of Colorado. In 1988, she started working in NASA and was selected for her first flight in 1996. She was the first Indian-born woman and the second Indian person to fly in space. Unfortunately she was one of the seven astronauts who lost their lives in Space Shuttle Columbia disaster on 1 February 2003. She is a role model for many young women around the world.

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