Our Universe, Galaxy, Stars, Solar System and Planets
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The vast space containing our earth and all other heavenly bodies, also called celestial bodies, constitutes the universe. These celestial bodies that form the universe include galaxies, stars, and comets along with the other members of the solar system, v.i.z, planets, asteroids, meteors, and satellites.
In your observations, you will identify a particular pattern of their positions in the sky. It is, for this reason, that, in ancient times when there were no watches or clocks, stars were used in keeping time at night and in finding the directions. The knowledge of their positions was of great value to navigators or travellers at night time.
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When you look in the sky at night, you see the stars that are twinkling luminous bodies and other celestial bodies that do not twinkle but are just visible to you. These celestial bodies are planets, asteroids, satellites, meteors or shooting stars. Let us learn about these different celestial bodies.
Stars are the luminous celestial bodies that emit both light and heat like the sun. The sun is also a star
that appears bigger and brighter being the nearest star to the earth. Alpha Centauri which is the second nearest star to the earth is at a distance of 4.3 light years from the earth. A light year is a large multiple unit of measurement. It is equal to the distance travelled by light in 1 year at the speed of
3ｘ10⁸ m/s or 3ｘ10⁵ km/s.
To convert a light year into metres or kilometres, let us first convert a year into seconds.
1 year = 365ｘ24ｘ60ｘ60 s
= 365ｘ86400 s
= 31536000 s
You know, Speed = Distance / Time
or, Distance = SpeedｘTime
Therefore, Distance = 3ｘ10⁸ m/sｘ31536000 s
= 9.46ｘ10⁵ m or 9.46ｘ10¹² km
In the sky, stars appear stationary but they are not. There are moving away from each other based on the Big Bang Theory that you will learn in your higher classes. They simply look stationary to you because of the large distances they are from the earth. One needs years to understand their increasing distances.
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You realized that stars you had identified moved from east to west during the course of your observation. It happened because the earth's surface you were standing on was rotating from west to east on its rotational axis.
It is because of this that the sun, which you now know is a star, rises in the east and sets in the west. There is only one star named Pole star or Polaris or Dhruv Tara that does not appear to move from the earth's surface. It appears stationary because of its location on the earth's rotational axis in the northern hemisphere. It is a member of constellation Ursa Minor about which you will learn in the next section.
There are two groups of stars in the universe. These are:
1. Galaxies
2. Constellations
Galaxies: A galaxy is a building block of the universe. It is described as a vast system of billions of stars along with a large number of gas clouds, mainly hydrogen gas and dust. The universe is of about 100 billion galaxies and each galaxy is further constituted of the same number of stars, i.e. 100 billion stars, such that the whole universe is constituted of about 10²² stars. This vast number of galaxies in the universe has been classified into three types based on their shapes. These are:
(a) Spiral galaxy
(b) Elliptical galaxy
(c) Irregular galaxy
The spiral galaxies are spiral or oval shaped, while the irregular galaxies do not have any definite
shape. The spiral galaxies are the largest of the three types and the elliptical ones are the brightest. There is only a very small percentage of irregular galaxies that are known to us.
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The galaxy we live is called, the Milky Way or 'Akash Ganga' because in a clear night sky our galaxy appears as a white band of stars faintly whitening the sky from north to south like a stream or river of light running from one corner to another.
Constellations: A constellation is a group of a limited number of stars that appear in a closed group to form a recognizable pattern or shape. There are 88 constellations known, called 'Nakshatra' by our ancestors. Each constellation has been assigned a name based on an object, an animal or a human being. Some constellations are:
Ursa Major (Great Bear): In Hindi, it is called Saptarshi because of the seven stars, it consists of.
These seven stars are so arranged that they form a pattern resembling a bear. The three stars in the row represent the tail of the bear and the quadrangle of four stars represents the body of the bear. The constellation is easily visible in the northern sky in the months of July and August.
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Ursa Minor (Little Bear): In Hindi, it is called Laghu Saptarshi because it follows the same pattern as the Ursa Major but is smaller in size. This constellation is of importance because the pole star about which you have read earlier is one of its members, Hence, in Hindu mythology, it is also called 'Dhruv Matsya'. The pole star in the Ursa Minor can easily be identified either by identifying the Ursa Minor itself or with help of two pointer stars of Ursa Minor that always points to Pole star.
Orion (Hunter or Mriga or Vyadha): It resembles the structure of a hunter. It is easily visible
during the winter season. It consists of seven bright stars. The four out of these seven stars are arranged in the form of a quadrangle. One of the largest of stars, Betelgeuse, is situated in one corner while on the opposite corner is the star called Rigel and the other three appear to be on a straight line in the middle of it.
Taurus (Bull or Vrish): It resembles the shape of a bull. It is easily visible in the winter season. The pattern of arrangement shows the front half part of a bull with horns.
Libra (Tula or Tarazu): It resembles the shape of a simple physical balance. It is easily visible in the winter season. It comprises of four brightest are arranged in the form of a quadrangle, the upper two stars represent the arm of balance while the lower two represent the pan of the balance.
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Scorpio (Vrashchika): It resembles the shape of Scorpion. It is easily visible in the summer season. The pattern that consists of seven stars represents a scorpion with tail and pincers.
It is a system of eight planets revolving around the sun in specific orbits along with their satellites. It also includes asteroids, meteors, and comets that revolve around with the sun in the centre.
Sun: It is a medium-size star of average brightness about 1.5ｘ10⁸ kilometre from the earth's surface.
It has a group of eight planets revolving around it.
Planets: On a clear night when you look at the sky, some celestial objects do not twinkle. These celestial objects are called planets. The planets appear bright like stars only because they reflect the sunlight falling on them.
There are eight planets in our solar system. These eight planets have been group into two types as terrestrial planets and jovian planets.
Terrestrial planets: The first four planets, the nearest to the sun have been group as terrestrial planets. These planets have a rocky crust like earth, a thin atmosphere and have neither no or few numbers of satellites (moons).
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Jovian planets: This planet lack the rocky crust. They are gaseous with a ring and many satellites.
The sun compared to earth is very big with its diameter about 100 times the diameter of earth and mass about a million times the mass of earth. It is not a solid body like the earth but it is a sphere of hot gases, mainly hydrogen and helium. These gases are responsible for the emission of a tremendous amount of energy from the sun by the nuclear fusion of hydrogen into helium that generates a temperature of 6000º C at the surface of the sun. You will study about this in detail and your higher classes.
Almost all the celestial objects you see in space reflects the light of the sun falling on them. It is the bright sunlight that unables you to see stars during the day, despite their presence in space.
Planets are comparatively near to the earth's surface and revolve round the sun in specific orbits. Some planets possess natural satellites.
Let us study about the eight planets of the solar system.
Mercury: It is called Buddha in Hindi. It is the nearest planet to the sun. When viewed from the earth, despite not being a star, it appears as the morning star in the east, mostly in the months of September and October just before sunrise and as an evening star in the west, soon after sunset in the months of March and April. It is a very hot planet because of its closeness to the sun. It bears many similarities with the moon in size, mass, and lack of atmosphere and having a rocky and mountainous crust.
Venus: It is called Shukra in Hindi. It is the second planet in terms of distance from the sun, with almost the same size and mass of about 4/5 times of the mass of earth. When viewed from earth it appears as the brightest celestial body in the universe, at night time other than the moon. It appears bright because of its close to the earth and its ability to reflect almost 75% of the sunlight falling on the earth's surface. Venus is also called the early morning star or late evening start because of its clear visibility in the eastern sky before sunrise and in the western sky after sunset. Venus, like mercury, has no satellite, but unlike mercury has its own atmosphere which is rich in gases such as carbon dioxide. This gas, because of the greenhouse effect which you will study in your higher classes, results in Venus becoming the hottest planet of all the nine planets.
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Earth: It is called Prithvi in Hindi. It is the third planet in terms of distance from the sun with a single natural satellite called the moon. The earth, when viewed from the outer space, appears bluish-green because of its reflection of light from the water and the land surface. The earth is a peculiar planet because of its ability to allow the origin, evolution, and sustenance of life. The various factors that helped the earth to achieve this include:
1. Presence of all essential elements such as nitrogen, carbon dioxide, and oxygen in its atmosphere.
2. Presence of liquid water in the ocean, rivers, lakes, and ponds.
3. Presence of a protective layer of ozone that prevents harmful radiations such as ultraviolet radiation to enter in the earth's atmosphere.
4. Presence of appropriate temperature conditions required for the sustenance of life because of greenhouse gases such as CO₂ and CH₄ in the earth's atmosphere.
The earth, as a planet, shows two types of motion called rotational motion and revolutionary motion.
(a) Rotational motion: The earth rotates on its rotational axis that passes through the North pole and the South Pole despite the fact that it is tilted along its own axis. It completes one rotation in 23 hours 56 minutes and 41 seconds from west to east. It is the rotational motion of earth that results in day and night bringing one part of the earth in front of the sun and simultaneously, taking the other part behind to bring darkness or night, behind its own shadow.
(b) Revolution motion: It is the second motion shown by the earth. In this motion, the earth revolves round the sun and its specific orbit that it completes in 365.25 days. This time period is called a year. It is because of the fact that the earth completes its one revolution in 365.25 days, every fourth year is a leap year having 366 days.
The tilting of the earth along its rotational axis and its revolution round the sun bring changes in its position with respect to the sun to help bring seasonal changes you experience throughout the year and also the difference in the seasonal changes experienced in the northern and the southern hemisphere. When the southern hemisphere is tilted towards the sun, it is summer there but winter in the northern hemisphere and vice versa. The seasons of spring and autumn occur when the earth is in between the two extreme positions of its tilt towards the sun.
Important dates in a year caused because of the revolution of the earth round the sun: The earth revolves round the sun but being tilted at its own axis rotational axis, few days of the year have their own significance. These days are:
(i) 21st June: When the North pole is inclined towards the sun, days are longer than nights. The north pole experiences a day of 24 hours and the South pole experiences a night of 24 hours. The position of the earth is called summer solstice.
(ii) 22nd December: When it is winter in the Northern hemisphere and summer in the southern hemisphere, the day length is the longest in the southern hemisphere and the shortest in the northern hemisphere. The position of the earth is called winter solstice.
(iii) 21st March and 23rd September: These are days when the length of day and night is equal in both hemispheres such that the seasons are either autumn or spring.
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Mars: It is called Mangal in Hindi. It is the fourth and the last of terrestrial planets of the solar system with two natural satellites called Phobos and Deimos. It is slightly more than half of the earth in diameter but is one-tenth in mass to the mass of the earth. It is a cold planet with a very thin atmosphere containing traces of gases like carbon dioxide, nitrogen, hydrogen, and oxygen. The oxygen present is too little to support life. The planet Mars from distance appears reddish in colour, hence it is also called the Red planet. It is visible from the earth for the most part of the year but the best time to observe is when it is opposite to the sun's position with respect to the earth.
Recently, in the last decade, the astronomers have happened to notice some changes on Mars's surface that have made them think of the presence of water and life in some form on Mars. However, so for all the efforts have gone in vain although the investigations are still in progress.
Jupiter: It is called Brahspati in Hindi. It is the first of the jovian planets, fifth in distance from the sun and the largest of all the planets of the solar system. It is composed mainly of hydrogen and helium gases along with other gases such as methane and ammonia found in crystalline form.
In brightness, it comes next to mercury and venus. When observed with a telescope, it appears very colourful with the faint ring around it. The 'Red Spot' developed because of some long-lasting storm in its atmosphere, is its characteristic feature. The astronomers so for believe the Jupiter to have 28 satellites.
Saturn: It is called Shani in Hindi. It is almost similar to Jupiter in size, mass, and composition but is more cooler owing to increased distance from the sun to the sixth planet. The planet is characterized by the presence of three distinct rings along with many faint rings around it. It is known to have 30 natural satellites.
Uranus: William Herchel in 1781 discovered Uranus. It is different from all other planets of the solar system as it rotates from east to west on its rotational axis, unlike other planets which rotate west to east. It is a very cold planet with 21 natural satellites and an atmosphere consisting of hydrogen, helium, and methane.
Neptune: The last planet of the solar system was discovered by U.J. Leverier, who after the discovery of Uranus, on the basis of size calculations, predicted the presence of Neptune and materialized it. It is almost the same as Uranus in size and atmosphere, being very cold with a surface temperature of -190º C.
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Moon is the natural satellite of the Earth. It appears to be the brightest celestial body other than the sun. Moon, being a satellite, revolves round the earth as the earth revolves around the sun. It is the revolution of moon around the earth that results in phases of the moon. The earth revolves round the sun and the moon revolves round the earth, therefore, it is natural for the moon's shadow to form on earth or the earth's shadow to fall on the moon. The shadow of earth or moon results in lunar and solar eclipses, you have studied in the previous class. The days on the basis of the moon's position with respect to the sun and the earth have been classified as full moon day and new moon day.
Full moon day (Purnima): It is the day when the earth lies between the sun and the moon making the full face of the moon visible.
New moon day (Amavasya): It is the day when sunlight falls on that side of the moon which is invisible, so you cannot see the moon.
Between the full moon day and the new moon day, you always happened to see a part of the moon, small or big which is called the crescent of the moon. The different phases of the crescent moon have different shapes. From the new moon day until full moon day it increases and then, after full moon day, until next new moon day, it continuously reduces.
The solar system along with sun, planets and their natural satellites contains some other celestial bodies also. The celestial bodies are asteroids, meteors, and comets.
Asteroids: There is a large gap between the orbits of Mars and Jupiter. In 1801, Guiseppe Piazza discovered the presence of small planets like the bodies of rock and metal revolving round the sun between these orbits of Mars and Jupiter. It is these small planets like bodies that are called asteroids. It is believed that because of the reasons unknown till date, at the time of the origin of the solar system, these asteroids had failed to assemble into a planet. It is considered so because each asteroid irrespective of its size, has its own orbit to reverse the like a planet. The biggest of the asteroid to be observed is Ceres, about 700 km in diameter.
Meteors: They are innumerable is small rock-like structures that revolve round the sun but, sometimes, one of them under the earth's gravitational pull happens to enter the earth's atmosphere where it gets heated up by friction posed by air. The heat generated is so high that it makes them glow and evaporate in atmospheric air within a short time. This is why they appear as a streak of light in the atmosphere for a short time, hence, called shooting star, despite not being a star.
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However, sometimes it happens that a meteor, your after entering the earth's atmosphere, failed to evaporate completely being too big, i.e. it survives the atmospheric friction to land on the earth's surface in the form of small rocks or big rocks which create the craters. Such craters are clearly visible on the surface of the moon as it has no atmosphere to oppose the fall of meteors. These meteors that land in solid form on earth's surface are called meteorite.
Comet: The orbit of Neptune, the last planet of the solar system, is not the end of the solar system. There are small celestial bodies called comets which, with highly elliptical orbits, extend beyond Neptune's orbit to revolve round the sun. A comet, being small and at large distance, is mostly invisible. It becomes visible only when it approaches the sun in its orbit, in the form of a structure having head and tail. The sun rays make the gases in them glow and expand away from the sun will the solid part of the comet remains in the orbit. Comets, like planets, have a specific period of revolution round the sun, for example, Halley's Comet, named after Edmund Halley, last seen in 1986, has a revolution period of 76 years.
Satellites (Natural and Artificial): A heavenly body that revolves around a planet in its orbit is called a satellite, such as a moon is the natural satellite of the earth. The other planets in the solar system also have their own satellites with Saturn having the maximum number of satellites or moons.
In modern times when you talk of a satellite, you need to be cautious about which type of satellite you are talking about natural satellite or artificial satellite. The artificial satellites are the man-made satellites that are launched into space by rockets to carry out the desired functions for mankind. At present only six countries in the world have the technology for developing the artificial satellite. India has launched many satellites into space. These include INSAT, IRS and Rohini series of satellites. Aryabhatta was the first artificial satellite to be launched by India in 1975.
The artificial satellites, based on the type of orbit in which they revolve around the earth have further been classified into two types, polar and equatorial orbit satellite. The polar orbit which passes over the north and south poles of the earth, the equatorial orbit lies around the equator of the earth.
Advantage of Artificial Satellites: The use of artificial satellites has influenced the life of every individual, whether a scientist or layman, to an extent that the people have started calling the present age as space age with the tremendous improvement in the quality of life ranging from the weather forecast to the use of cellular phones. Some applications of artificial satellites include:
1. Satellite Communication: It is the communication that uses a satellite to make long-distance calls, television transmission, and radio broadcast, and sending of the telex and fax messages. This is achieved by satellites orbiting in equatorial orbit at the speed of earth's rotation and remain stationary at a point with respect to earth to receive and transmit the messages. These satellites are also called geostationary satellites.
Long-distance transmission of television programs or radio broadcast has become possible with the help of geostationary satellites. The picture or sound or both are converted into electrical signals with the help of a video camera. These electrical signals are then converted into a type of waves that are transmitted using transmitter antenna to an artificial satellite station in the equatorial orbit. These satellites are equipped with special instruments to receive these signals and re-transmit them after their amplification. The signals transmitted to far off places are received by antennas and dishes like the ones used by cable operators. Once received, the waves are transformed back into electrical signals to operate the television screen or radio or telephone. These devices themselves act as a converter of electrical signals into an image and sound.
Moon is the natural satellite of the earth but it cannot be used for long-distance communication because of limitation which includes its long distance from the earth and the difference in its period of revolution and rotation of the earth.
2. Weather Monitoring: It allows providing beforehand information on weather conditions of an area to the public. This information, provided by a satellite orbiting in polar orbits, helps the public prepare beforehand for a forthcoming storm or hurricane to minimize the loss of life and property.
3. Remote Sensing: It is the taking of photographs and collection of information about a place from a distance without coming in physical contact. These satellites are launched in polar orbit for groundwater survey, assessment of drought, crop yield, and survey of coal, oil or ore reserves.
4. Space Exploration: It is a recent step taken in the last decade or little more to explore the space to gain more knowledge and information about the members of the solar system and other celestial bodies.
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Universe: All the space consisting of galaxies and other celestial bodies.
Galaxy: A group of about 100 billion stars.
Stars: They are the luminous bodies found in the universe.
Solar system: The system of eight planets revolving around the sun with other celestial bodies such as comets, asteroids, and meteors.
Planets: The non-luminous celestial bodies that revolve around the sun.
Constellation: A small group of stars positioned with respect to each other to give some definite shape.
Comet: A small celestial body with an elliptical orbit around the sun.
Meteors: The celestial bodies that happened to enter the earth's atmosphere.
Meteorites: Meteors that happened to land on the earth's surface in the form of rocks.
Asteroids: The celestial bodies like small planet that revolve around the sun between the orbits of Mars and Jupiter.
Satellites: The celestial bodies that revolve around a planet.
Artificial satellite: The man-made satellite that revolves around the earth.
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Now, Answer time!
 |
| Our solar system |
In your observations, you will identify a particular pattern of their positions in the sky. It is, for this reason, that, in ancient times when there were no watches or clocks, stars were used in keeping time at night and in finding the directions. The knowledge of their positions was of great value to navigators or travellers at night time.
 |
| Our universe |
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| Activity |
|---|
| On a clear night, when the moon is not visible in the sky, go out in open ground or on to the roof of your house and look at the sky. There you see small twinkling luminous objects, called stars. Identify a particular group of stars, locate this position with respect to your position. Continue your observation for seven consecutive days. Record your observations. |
| Group Discussion |
|---|
| How will you estimate the time if you do not have any clock or watch at the daytime? Discuss with your friends and write a note on the advantages and limitations of the method discussed. |
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Night Sky
 |
| Night sky |
Stars
Stars are the luminous celestial bodies that emit both light and heat like the sun. The sun is also a star
 |
| Stars |
3ｘ10⁸ m/s or 3ｘ10⁵ km/s.
To convert a light year into metres or kilometres, let us first convert a year into seconds.
1 year = 365ｘ24ｘ60ｘ60 s
= 365ｘ86400 s
= 31536000 s
You know, Speed = Distance / Time
or, Distance = SpeedｘTime
Therefore, Distance = 3ｘ10⁸ m/sｘ31536000 s
= 9.46ｘ10⁵ m or 9.46ｘ10¹² km
In the sky, stars appear stationary but they are not. There are moving away from each other based on the Big Bang Theory that you will learn in your higher classes. They simply look stationary to you because of the large distances they are from the earth. One needs years to understand their increasing distances.
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You realized that stars you had identified moved from east to west during the course of your observation. It happened because the earth's surface you were standing on was rotating from west to east on its rotational axis.
It is because of this that the sun, which you now know is a star, rises in the east and sets in the west. There is only one star named Pole star or Polaris or Dhruv Tara that does not appear to move from the earth's surface. It appears stationary because of its location on the earth's rotational axis in the northern hemisphere. It is a member of constellation Ursa Minor about which you will learn in the next section.
Group of Stars
There are two groups of stars in the universe. These are:
1. Galaxies
2. Constellations
Galaxies: A galaxy is a building block of the universe. It is described as a vast system of billions of stars along with a large number of gas clouds, mainly hydrogen gas and dust. The universe is of about 100 billion galaxies and each galaxy is further constituted of the same number of stars, i.e. 100 billion stars, such that the whole universe is constituted of about 10²² stars. This vast number of galaxies in the universe has been classified into three types based on their shapes. These are:
(a) Spiral galaxy
(b) Elliptical galaxy
(c) Irregular galaxy
The spiral galaxies are spiral or oval shaped, while the irregular galaxies do not have any definite
 |
| Spiral galaxy |
shape. The spiral galaxies are the largest of the three types and the elliptical ones are the brightest. There is only a very small percentage of irregular galaxies that are known to us.
 |
| Elliptical Galaxy |
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The galaxy we live is called, the Milky Way or 'Akash Ganga' because in a clear night sky our galaxy appears as a white band of stars faintly whitening the sky from north to south like a stream or river of light running from one corner to another.
 |
| Irregular Galaxy |
Constellations: A constellation is a group of a limited number of stars that appear in a closed group to form a recognizable pattern or shape. There are 88 constellations known, called 'Nakshatra' by our ancestors. Each constellation has been assigned a name based on an object, an animal or a human being. Some constellations are:
Ursa Major (Great Bear): In Hindi, it is called Saptarshi because of the seven stars, it consists of.
These seven stars are so arranged that they form a pattern resembling a bear. The three stars in the row represent the tail of the bear and the quadrangle of four stars represents the body of the bear. The constellation is easily visible in the northern sky in the months of July and August.
 |
| Ursa Major |
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 |
| Ursa Minor |
Orion (Hunter or Mriga or Vyadha): It resembles the structure of a hunter. It is easily visible
during the winter season. It consists of seven bright stars. The four out of these seven stars are arranged in the form of a quadrangle. One of the largest of stars, Betelgeuse, is situated in one corner while on the opposite corner is the star called Rigel and the other three appear to be on a straight line in the middle of it.
 |
| Orion |
Taurus (Bull or Vrish): It resembles the shape of a bull. It is easily visible in the winter season. The pattern of arrangement shows the front half part of a bull with horns.
 |
| Taurus |
Libra (Tula or Tarazu): It resembles the shape of a simple physical balance. It is easily visible in the winter season. It comprises of four brightest are arranged in the form of a quadrangle, the upper two stars represent the arm of balance while the lower two represent the pan of the balance.
 |
| Libra |
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Scorpio (Vrashchika): It resembles the shape of Scorpion. It is easily visible in the summer season. The pattern that consists of seven stars represents a scorpion with tail and pincers.
 |
| Scorpio |
Solar System
It is a system of eight planets revolving around the sun in specific orbits along with their satellites. It also includes asteroids, meteors, and comets that revolve around with the sun in the centre.
 |
| Our Solar System |
| Note: Until recently 2006, Pluto was considered as the farthest planet of the solar system. In an International Conference of Renowned astronomers, it was decided not to include Pluto in the list of planets because of its small size and distance from the sun. |
Sun: It is a medium-size star of average brightness about 1.5ｘ10⁸ kilometre from the earth's surface.
It has a group of eight planets revolving around it.
 |
| Sun |
Planets: On a clear night when you look at the sky, some celestial objects do not twinkle. These celestial objects are called planets. The planets appear bright like stars only because they reflect the sunlight falling on them.
There are eight planets in our solar system. These eight planets have been group into two types as terrestrial planets and jovian planets.
Terrestrial planets: The first four planets, the nearest to the sun have been group as terrestrial planets. These planets have a rocky crust like earth, a thin atmosphere and have neither no or few numbers of satellites (moons).
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Jovian planets: This planet lack the rocky crust. They are gaseous with a ring and many satellites.
The sun compared to earth is very big with its diameter about 100 times the diameter of earth and mass about a million times the mass of earth. It is not a solid body like the earth but it is a sphere of hot gases, mainly hydrogen and helium. These gases are responsible for the emission of a tremendous amount of energy from the sun by the nuclear fusion of hydrogen into helium that generates a temperature of 6000º C at the surface of the sun. You will study about this in detail and your higher classes.
Almost all the celestial objects you see in space reflects the light of the sun falling on them. It is the bright sunlight that unables you to see stars during the day, despite their presence in space.
Different Planets of Solar System
Let us study about the eight planets of the solar system.
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| Mercury planet |
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| Venus planet |
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| Earth |
1. Presence of all essential elements such as nitrogen, carbon dioxide, and oxygen in its atmosphere.
2. Presence of liquid water in the ocean, rivers, lakes, and ponds.
3. Presence of a protective layer of ozone that prevents harmful radiations such as ultraviolet radiation to enter in the earth's atmosphere.
4. Presence of appropriate temperature conditions required for the sustenance of life because of greenhouse gases such as CO₂ and CH₄ in the earth's atmosphere.
 |
| Earth |
The earth, as a planet, shows two types of motion called rotational motion and revolutionary motion.
(a) Rotational motion: The earth rotates on its rotational axis that passes through the North pole and the South Pole despite the fact that it is tilted along its own axis. It completes one rotation in 23 hours 56 minutes and 41 seconds from west to east. It is the rotational motion of earth that results in day and night bringing one part of the earth in front of the sun and simultaneously, taking the other part behind to bring darkness or night, behind its own shadow.
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| Rotational motion of the Earth |
(b) Revolution motion: It is the second motion shown by the earth. In this motion, the earth revolves round the sun and its specific orbit that it completes in 365.25 days. This time period is called a year. It is because of the fact that the earth completes its one revolution in 365.25 days, every fourth year is a leap year having 366 days.
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| Revolution motion of the Earth |
The tilting of the earth along its rotational axis and its revolution round the sun bring changes in its position with respect to the sun to help bring seasonal changes you experience throughout the year and also the difference in the seasonal changes experienced in the northern and the southern hemisphere. When the southern hemisphere is tilted towards the sun, it is summer there but winter in the northern hemisphere and vice versa. The seasons of spring and autumn occur when the earth is in between the two extreme positions of its tilt towards the sun.
Important dates in a year caused because of the revolution of the earth round the sun: The earth revolves round the sun but being tilted at its own axis rotational axis, few days of the year have their own significance. These days are:
(i) 21st June: When the North pole is inclined towards the sun, days are longer than nights. The north pole experiences a day of 24 hours and the South pole experiences a night of 24 hours. The position of the earth is called summer solstice.
(ii) 22nd December: When it is winter in the Northern hemisphere and summer in the southern hemisphere, the day length is the longest in the southern hemisphere and the shortest in the northern hemisphere. The position of the earth is called winter solstice.
(iii) 21st March and 23rd September: These are days when the length of day and night is equal in both hemispheres such that the seasons are either autumn or spring.
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| Mars |
Recently, in the last decade, the astronomers have happened to notice some changes on Mars's surface that have made them think of the presence of water and life in some form on Mars. However, so for all the efforts have gone in vain although the investigations are still in progress.
Jupiter: It is called Brahspati in Hindi. It is the first of the jovian planets, fifth in distance from the sun and the largest of all the planets of the solar system. It is composed mainly of hydrogen and helium gases along with other gases such as methane and ammonia found in crystalline form.
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| Jupiter |
In brightness, it comes next to mercury and venus. When observed with a telescope, it appears very colourful with the faint ring around it. The 'Red Spot' developed because of some long-lasting storm in its atmosphere, is its characteristic feature. The astronomers so for believe the Jupiter to have 28 satellites.
Saturn: It is called Shani in Hindi. It is almost similar to Jupiter in size, mass, and composition but is more cooler owing to increased distance from the sun to the sixth planet. The planet is characterized by the presence of three distinct rings along with many faint rings around it. It is known to have 30 natural satellites.
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| Saturn |
Uranus: William Herchel in 1781 discovered Uranus. It is different from all other planets of the solar system as it rotates from east to west on its rotational axis, unlike other planets which rotate west to east. It is a very cold planet with 21 natural satellites and an atmosphere consisting of hydrogen, helium, and methane.
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| Uranus |
Neptune: The last planet of the solar system was discovered by U.J. Leverier, who after the discovery of Uranus, on the basis of size calculations, predicted the presence of Neptune and materialized it. It is almost the same as Uranus in size and atmosphere, being very cold with a surface temperature of -190º C.
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| Neptune |
Note: Pluto has been stripped off its status as a planet of the solar system on 24 August 2006, when scientists redefined 'planet'. Now our solar system has eight planets only. |
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Moon
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| Moon |
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| Moon |
Full moon day (Purnima): It is the day when the earth lies between the sun and the moon making the full face of the moon visible.
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| Full moon day |
New moon day (Amavasya): It is the day when sunlight falls on that side of the moon which is invisible, so you cannot see the moon.
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| New moon day |
Between the full moon day and the new moon day, you always happened to see a part of the moon, small or big which is called the crescent of the moon. The different phases of the crescent moon have different shapes. From the new moon day until full moon day it increases and then, after full moon day, until next new moon day, it continuously reduces.
Other Members of the Solar System
The solar system along with sun, planets and their natural satellites contains some other celestial bodies also. The celestial bodies are asteroids, meteors, and comets.
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| Asteroid |
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| Asteroid |
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| Meteor |
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| Meteor |
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| The ground hit by a meteor (meteorite), Place: Arizona |
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| Comet |
Satellites (Natural and Artificial): A heavenly body that revolves around a planet in its orbit is called a satellite, such as a moon is the natural satellite of the earth. The other planets in the solar system also have their own satellites with Saturn having the maximum number of satellites or moons.
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| The moon revolves around the Earth, hence, it is a natural satellite of the Earth. |
In modern times when you talk of a satellite, you need to be cautious about which type of satellite you are talking about natural satellite or artificial satellite. The artificial satellites are the man-made satellites that are launched into space by rockets to carry out the desired functions for mankind. At present only six countries in the world have the technology for developing the artificial satellite. India has launched many satellites into space. These include INSAT, IRS and Rohini series of satellites. Aryabhatta was the first artificial satellite to be launched by India in 1975.
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| Artificial satellite |
The artificial satellites, based on the type of orbit in which they revolve around the earth have further been classified into two types, polar and equatorial orbit satellite. The polar orbit which passes over the north and south poles of the earth, the equatorial orbit lies around the equator of the earth.
Advantage of Artificial Satellites: The use of artificial satellites has influenced the life of every individual, whether a scientist or layman, to an extent that the people have started calling the present age as space age with the tremendous improvement in the quality of life ranging from the weather forecast to the use of cellular phones. Some applications of artificial satellites include:
1. Satellite Communication: It is the communication that uses a satellite to make long-distance calls, television transmission, and radio broadcast, and sending of the telex and fax messages. This is achieved by satellites orbiting in equatorial orbit at the speed of earth's rotation and remain stationary at a point with respect to earth to receive and transmit the messages. These satellites are also called geostationary satellites.
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| Satellite communication |
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| Satellite communication |
Moon is the natural satellite of the earth but it cannot be used for long-distance communication because of limitation which includes its long distance from the earth and the difference in its period of revolution and rotation of the earth.
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| Weather monitoring |
3. Remote Sensing: It is the taking of photographs and collection of information about a place from a distance without coming in physical contact. These satellites are launched in polar orbit for groundwater survey, assessment of drought, crop yield, and survey of coal, oil or ore reserves.
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| Satellite taking photograph of a place: Remote sensing |
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| Space exploration |
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| Space exploration |
*Useful Words
Universe: All the space consisting of galaxies and other celestial bodies.
Galaxy: A group of about 100 billion stars.
Stars: They are the luminous bodies found in the universe.
Solar system: The system of eight planets revolving around the sun with other celestial bodies such as comets, asteroids, and meteors.
Planets: The non-luminous celestial bodies that revolve around the sun.
Constellation: A small group of stars positioned with respect to each other to give some definite shape.
Comet: A small celestial body with an elliptical orbit around the sun.
Meteors: The celestial bodies that happened to enter the earth's atmosphere.
Meteorites: Meteors that happened to land on the earth's surface in the form of rocks.
Asteroids: The celestial bodies like small planet that revolve around the sun between the orbits of Mars and Jupiter.
Satellites: The celestial bodies that revolve around a planet.
Artificial satellite: The man-made satellite that revolves around the earth.
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Now, Answer time!
| Answer the questions given below. Tell us your answers through the comment. |
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| 1. Why does the pole star appear stationary from the surface of the earth? 2. Where will you look for to see asteroids in the solar system using a telescope? 3. Name the two main gases that constitute the stars including the sun. 4. How is Uranus different from the rest of the planets of the solar system? 5. Why the comets in their orbits can only be viewed when close to the sun? |
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