Solar System - Our Solar System 2020 ( Indian astronaut)

Our Earth and Our Solar System

            

         When we look up from an open ground, we see the sky. In a clear night sky we can see many stars. They are very far away from the earth. Some stars are prominent while some are tiny and faint. If we look at them carefully, we find that many of them twinkle, but some do not. The sun and the moon are comparatively close to the earth. So, we can see their round shapes clearly. The sun, the moon, the stars, the plants, etc. are all know as heavenly bodies. Observe the sky on two clear nights, keeping an gap of about a week between them. Base your observation on the following points :

• The brightness of the heavenly bodies 
• Whether they twinkle
• Their color and size
• Changes in their positions

        On both nights, draw a picture of the illuminated portion of the moon and note how it changes from day to day.

• The brightness of the heavenly bodies : It would seem that the brightness of the heavenly bodies will not be increased at this renewal, For this renewal as regards the lower bodies will be caused by the cleansing fire. But the cleansing fire will not reach the heavenly bodies. Therefore the heavenly bodies will not be renewed by receiving an increase of brightness. Further, just as the heavenly bodies are the cause of generation in this lower world by their movement, so are they by their light. But, when generation ceases, movement will cease as stated above . Therefore in like manner the light of the heavenly bodies will cease rather than increase. Further, if the heavenly bodies will be renewed when man is renewed, it follows that when man deteriorated they deteriorated likewise. But this does not seem probable, since these bodies are unalterable as to their substance. Therefore neither will they be renewed when man is renewed.

• Whether they twinkle : Stars twinkle because ... they're so far away from Earth that, even through large telescopes, they appear only as pinpoints And it's easy for Earth's atmosphere to disturb the pinpoint light of a star. As a star's light pierces our atmosphere, each single stream of starlight is refracted - caused to change direction. because they're closer to Earth and so appear not as pinpoints, but as tiny disks in our sky. You can see planets as disks if you looked through a telescope, while stars remain pinpoints. The light from these little disks is also refracted by Earth's atmosphere, as it travels toward our eyes.

• Their color and size : Outward from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, followed by the dwarf planet Pluto. Jupiter's diameter is about 11 times that of the Earth's and the Sun's diameter is about 10 times Jupiter's Pluto's diameter is slightly less than one - fifth of Earth's. The planet are not shown at the appropriate distance from the Sun.

• Changes in their positions : From the perspective of an Earth - based observer, the planets constantly appear to change positions in the sky ------ a fact reflected in the word "planet " itself, which comes from the ancient Greek for "wanderer." These apparent motions can be explained by assuming that the planets move on near - circular orbits around the sun. The dimensions of these orbits have remained constant throughout human history.

Stars : The heavenly bodies that twinkle are called stars. Stars have their own light. The sun is a star. It is closer to us than any of the other star. Hence, it appears big and brilliant. In its bright light, during the day, we cannot see other stars. 

Planets : The heavenly bodies that do not twinkle are called planets. Planets do not have light of their own. They get light from the stars. Planets revolve around a star, even as they rotate around themselves. 

The solar system : Our earth is a planet. It gets its light from the sun. It moves around the sun. Its movement around the sun is called the revolution of the earth. Besides earth, there are seven other planets that revolve around the sun They are Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. Every planet in the solar system revolves around the sun along a specific path. This path is knows as that planet's orbit. The sun, which is a star, and the planets that revolve around it are togethers called the solar system. Besides the planets, the solar system also includes various other heavenly bodies. The Solar System has evolved considerably since its initial formation. Many moons have formed from circling discs of gas and dust around their parent planets, while other moons are believed to have formed and were later captured by their planets. Still others, as the Earth's Moon, may be the result of giant collisions. Many collisions between bodies have occurred, and have been important to the evolution of the Solar System. In the early stages, the positions of the planets sometimes shifted, and planets have switched places. This planetary migration is thought to have been responsible for much of the Solar System's early evolution.

           Other heavenly bodies in the solar system.

Satellites : Some heavenly bodies revolve around plants. These are called satellites. Satellites too get their light from  the sun. We see the moon at night. It revolves around the earth. Hence, it is called a satellite of the earth. Most of the planets in the solar system have satellites. The plants revolve around the sun around the sun  along with their satellites.

Dwarf planets : There are some smaller heavenly bodies that revolve around the sun. They are called dwarf planets. Of these, the most well-knows is Pluto. Dwarf plants revolve independently around the sun. They have an orbit of their own.

Asteroids : Between the planets Mars and Jupiter, there is a band of numerous small heavenly bodies. They are called asteroids. Asteroids also revolve around the sum.  Compared to the sun, other heavenly bodies in the solar system are much smaller. The moon is closest to the earth. That is why, it appears to be so big although it is actually very much smaller than the sun. A diagram of the solar system is given below. In it, you see the sun in the center, the heavenly bodies that revolve around it and their orbit. The planets, satellites, dwarf planets and asteroids are all parts of the solar system.

Gravity : All heavenly bodies exert a force of attraction or a pull on one another. This force is called the force of gravity. The sun exerts a gravitational pull on all planets whereas the tendency of the planets is to move away from the sun. As a result of these two forces, a planets keeps revolving around the sun at a fixed distance in a fixed orbit . In the same way, satellites revolve around their planets. Due to the earth's gravity, all things on the earth remain on it. Even if we throw something upwards with great force, it finally falls down to he ground.

Space: The emptiness between and beyond the stars and planets is called space or outer space. People have always been curious about the heavenly bodies in the sky. They have always wanted to reach them. However, to send some object from the earth into space, it must be given power against the force of gravity. Rocket technology or space launch technology is used for the purpose.

          One of the Diwali firecrackers is called a 'racket'. It is packed with explosive substance. The explosives burn rapidly and produce a lot of energy. The design of the rocket is suck that the firecracker is pushed in a certain direction at a great speed. Very powerful rockets are used to send a spacecraft into space. A tremendous quantity of fuel is burnt in rockets so that spacecraft weighing thousands of tons can be launched into space. In the twentieth century, a few countries of the world developed space technology and sent hundreds of spacecraft into space. Our country is well - known for the development of its space launch technology. Some spacecraft remain in space. Some are brought back to earth while some land on other planets or satellites. In some missions, scientists also travel in the spacecraft. They are called astronauts.

Do You Know ?

      Indian Astronaut : Rakesh Sharma became the first Indian astronaut to go into space in 1984. He spent eight days on a space station for a joint mission of the ISRO and the Soviet Intercommons. Seeing India from space, he said that it looked 'Saare jahan se achha !' 

Find out More about the work of Kalpana Chawala and Sunita Williams, astronauts of Indian origin.

Man - made satellites : Man - made satellites provide useful information for agriculture, environment, weather forecasting, making maps, and searching for water and mineral wealth on the earth. They are also used for telecommunication. They are put into orbit around the earth. They can remain in space for many years.

Always Remember - Space scientists have not yet been able to find a single other planet which has life on it. Therefore, our earth, is an invaluable planet. The degradation of its environment for any reason will be a threat to the existence of life itself.

Do You Know?

India's Space Missions : On 22 October 2008, the Indian Space Research Organization, ISRO, launched a spacecraft to the moon. The mission is knows as Chandrayaan - 1  Mangalyaan is another important Indian space mission. It is knows as M.O.M. or [ Mars Orbit Mission]. It was launched on 5 November 2013. It got established in an orbit around the planet Mars on 24 September 2014. ISRO achieved this feat in its first attempt. Both these missions are unmanned, i.e. there were no people on board these spacecraft. The objective of the missions is a deeper study of the moon and Mars.

The solar system :  Solar System assemblage consisting of the Sun an average star in the Milky Way Galaxy - and those bodies orbiting around it : 8 (Formerly 9 ) planets with about 210 known planetary satellites (moons); countless asteroids, some with their own satellites; comets and other icy bodies; and vast reaches of highly tenuous gas and dust known as the interplanetary medium. The solar system consists of the sun, the planets revolving around the sun, their satellites, dwarf planets, asteroids, comets and meteors. Mercury, Venus, Earth and Mars are called inner planets and Jupiter, Saturn, Uranus and Neptune are called outer planets. Inner planets have hard crust while outer planets have gaseous outer cover and rings around them. The Solar System also contains other things. There are asteroid belts, mostly between Mars and Jupiter. Further out than Neptune there is the Kuiper belt and the scattered disc. These areas have dwarf planets, including Pluto, make , Haumea, Ceres and Eris. There are thousands of very small objects in these areas. There also comets,  centaurs, and interplanetary dust. In Ancient Greece, Aristarchus of Samos proposed the heliocentric model of the Solar System, where the Sun, is at the center of the known universe. He is sometimes known as the "Greek Copernicus". Six of the Planets and three of the dwarf planets are orbited by moons. There are about 200 moons in the Solar System. Mercury and Venus have no moons, and Jupiter and Saturn have the largest number of moons. The largest moon is Ganymede which is a moon of Jupiter. Furthermore, planetary dust orbit the gas giants. Many other system like the Solar System have been found. Each of the billions of stars in the Milky Way galaxy might have a planetary system. 

Composition of the solar system : 

        Located at the center of the solar system and influencing the motion of all the other bodies through its gravitational force is the Sun, which in itself contains more than 99 percent of the mass of the system. The planets, in order of their distance outwards from the sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Four planets - Jupiter through Neptune - have ring systems, and all but Mercury and Venus have one or more moons. Pluto had been officially listed among the planets since it was discovered in 1930 orbiting beyond Neptune, but in 1992 an icy object was discovered still farther from the sun than Pluto. Many other such discoveries followed, including an object named Eris that appears to be at least as large as Pluto. It became apparent that  Pluto was simply one of the larger members of this new group of objects, collectively known as the Kuiper belt.

       There are eight plants in the solar System. From closest to farthest from the Sun, they are : Mercury, Venus, Earth, Mars, Jupiter Saturn, Uranus and Neptune. The first four planets are called terrestrial planets. They are mostly made of rock and metal, and they are mostly solid.  The last four planets are called gas giants. This is because they are much larger than other planets and are mostly made of gas. Titan, one of Saturn's moons, is the only moon in the solar system to have an Atmosphere.

Satellite : The celestial bodies revolving around the planets are called satellites. They do not revolve around the sun independently. Satellites also rotate around their axes. The satellite of the earth is the moon, which completes its rotation and revolution both in 27.3 days. Mercury and Venus do not have satellites, remaining planets have satellites.

Asteroid : Some small sized bodies formed when the solar system was created, are asteroids. They too revolve around the sun but they could not become planets. An asteroid belt is seen between the Mars and Jupiter.

Dwarf planet : Dwarf planet is a celestial body revolving independently around the sun, but is of small size. e.g. pluto : 248 years revolution period and 6.39 days rotation period.

What are the types of comets and on what basis are they classified:

Comets are of two types: 1) Long period comets : 2)Short period comets : This classification is based on the time taken by them to make a complete revolution around the sun. Long period comets take more than 200 years for a revolution while short period comets take less than 200 years to make one revolution around the sun.

Why do comets look as if they have a feathery tail :

A comet is formed of ice or frozen matter and dust particles. They appear as a small point when they are away from the sun. But when they come closer to the sun they are seen clearly due to sun's heat and shorter distance. When they ae close to the sun, the frozen matter in them is converted into gases. These gases are thrown in a direction away from the sun, Therefore, certain comets look as if they have a long, feathery tail.

Comet : 1) A comet is a celestial  body formed of ice and dust particles. It too revolves around the sun and is a part of the solar system. 2) When they are away from the sun, they appear as small points. But when they come closer to the sun, they are easily visible to us . 3) When they come closer to the sun. the frozen matter on them is converted to gases due to solar heat. These gases are thrown in the opposite direction away from the sun. This appears as a long feathery tail. 4) They have long elliptical orbits, their appearance in the sky is very rare and taking place after a very long time period.

Types of comets : a) Long period comets : Comets that take more than 200 years to complete one revolution around the sun. b) Short period comets : Comets that take less than 200 years to complete one revolution around the sun.

Meteor fall : Rocky pieces originating from the asteroid belt, fall on the earth's surface. This is called a meteor fall. When we see a falling star. it is actually a meteor falling down. Meteor : When small rocky pieces enter the earth's atmosphere, they get completely burnt due to friction with air. Sometimes they do not burn but fall down. They are called  meteorites.

 The different types of stars.

There is a great variety in stars. Some are very bright, some are faint. There are many colors among the stars. e.g. Blue, white, yellow and reddish. Some stars have varying brightness. Their luminance changes constantly. According to these characters, the stars are of the following subtypes : Sun-like Stars, Red Giants Stars, Super Nova, Binary or Twin Stars and Variable Stars.

What is a special characteristic of the planet Mars:

1) Mars is the fourth planet in our solar system. 2) It appears reddish as the soil on the Mars contains iron. Hence it is also known as the red planet. 3) The highest and largest mountain in our solar system called Olympus Mons is present on the planet Mars. 4) Mars has two satellites. Its rotation is in about 24 hours and ir takes about 1.88 years to revolve around the sun.

What are the characteristics of the planet Neptune :

1) Neptune is the eighth planet of our solar system. 2) One season on the Neptune planet lasts for about 41 years. 3) The winds blow at a very high speed on Neptune. 4) Neptune has 13 satellites. It takes approximately 16 hours and 11 minutes to rotate around itself and 164 years for one revolution around the sun.

Meteors and Meteorites :

Meteors are rocky pieces originating from the asteroid belt. Smaller rocky pieces are completely burnt due to friction with air when they enter the earth's atmospheric layers. Meteorites are the rocky pieces that do not burn completely but fall down to the surface of the earth.

Questions Time  :

What is the solar system ?

It's the Sun's family ! The solar system consists of the sun with its eight planets : Mercury, Venus, Earth ,Mars,  Jupiter, Saturn, Uranus and Neptune; orbited by more than 140 moons. The solar system also has asteroids meteors, comets, dwarf planets and other space objects orbiting the sun, with sun at as center, our solar system is always in motion.

Where is the Solar System ?

It's all around you. The Solar System is the Sun, eight planets and everything else that moves through space with the Sun. 

Where does the Sun go at night ?

The Sun doesn't go anywhere - it's the Earth that is moving !

Is the Sun hotter than an oven ?

The Sun's surface is over 20 times hotter than a regular oven The center is even hotter - thousands of times hotter than an oven. It would melt the oven !

What is a nebula ?

It is where stars begin their life ! A nebula is a cloud made of dust and gases. Stars are formed when a nebula collapses under its own gravity it is mainly made of hydrogen and helium Nebula look like fluffy clouds that are many light tears away from our Earth.

What Are Sunspots ?

The sun is marked by irregularly - shaped dark spots, called sunspots not visible to the naked eye. These areas are cooler than the real of the sun, giving less light. Most of these are larger than the size of the earth !

Why do I see different shape of the moon ?

The moon does not really change its shape. It just seems to change it, or go through phases, because we only see the parts of it that are lit up by the Sun. As the moon orbits Earth every month, Earth blocks the sunlight hitting the moon at different angles every day. As this happens, you see different portions of the moon.

Try This :

If you weigh 48 kilograms on Earth how much would you weigh on the moon ? Remember moon's gravity is 1/6 th of Earth's gravity.

Why do I see a bunny on the moon ?

The surface of the moon makes you see the bunny ! There are deep holes, called craters, and high hills on the moon's surface. When it seems that you see a bunny on the moon, you are actually seeing the uneven surface of the moon.

How does the Sun appear from other planets ?

It appears either big or small. The size of the sun would be different from all the planets in the solar system . The sun appears much large from planets that are closer to it (Mercury , Venus ) than the Earth, whereas it looks smaller from planets that are far away (Jupiter, Saturn , Uranus or Neptune ) from it . For example, from Mercury the sun would appear 2.5 times larger than it appears from Earth , but it would look like a tiny star from Neptune.

How do astronomers know the secrets of the sun ?

By studying its phenomena ! Astronomers study movement of particles in the Sun from which they gather information about its density and temperature. They use spectrometers to continuously monitor changes in the Sun's luminosity. From this data they know what is going on in the Sun.

What is a galaxy made of ?

Stars, dust, and dark matter, all held together by gravity ! A galaxy contains planets, star  systems , star clusters and types of interstellar clouds. In between them is a sparse interstellar medium of gas, dust and cosmic rays. Some galaxies may also contain objects like Quasars, the most energetic bodies in the universe, at their cores.

What gives our galaxy its milky appearance ?

Its stars ! Our galaxy appears as a dim glowing band arching across the night sky. This is where most stars are located which gives  that fuzzy milky  appearance. This is why our galaxy is called the Milky Way Galaxy.

The Sun, The Moon and The Earth

The Moon's Motions : Like the earth, the moon, also has axial and orbital motions. The moon while rotating around itself, revolves around the earth and the earth revolves around the sun. As a result, though the moon does not revolve around the sun independently, indirectly, it makes revolutions around the sun. The time it takes to make one revolution around the earth and one rotation around itself is the same. That is why, we constantly see one and the same side of the moon.

           The moon's orbit of the revolution is also elliptical as the of the earth. Hence the distance of the moon from the earth is not the same everywhere along its orbit while revolving. When it is the closest to the earth it is said to be in perigee and when it is at the farthest the position is called apogee.  You have studied the phases of the moon. You know how it waxes from the new moon day to the full moon day and how it wanes from the full moon to the new moon day. Which shows the different phases of the moon. It shows the relative position of the moon on both the quarters, the full moon and the new moon days with respect to the earth and the sun. The moon in the sky from the earth. They are the illuminated portions of the moon. They become visible due to the sunlight reflected from the moon. While revolving around the earth, earth, the moon is on the opposite side of the sun on a full moon day, whereas on a new moon day, it is positioned between the sun and the earth. On the first and the third quarter days, the moon, the earth and the sun make an angle of 90° . At these positions, we see half the portion of illuminated moon. Hence in the sky, it appears semicircular in shape.

Eclipses : The orbital path of the earth and that of the moon are not in the same plane The mon's revolutionary orbit makes an angle of about 5° with that of the earth. As a result, the moon intersects the planes o the earth's orbit twice during one revolution. On each new moon day, the lines joining the earth and the sun and the moon make an angle of 0° whereas on each full moon day, this angle is of 180°. Even so, the sun, the earth and the moon may not be in one straight line in the same plane on every new moon or full mon day. Hence, eclipses do not occur on each new moon or full moon day. However, sometimes, on a new moon day or a full moon day,  the three fall in one line and are in the same plane. Eclipses occur on such occasions. Eclipses may be solar or lunar.

        A condition of the sun disc getting covered by the moon OR the moon disc getting shadowed by the earth. Solar eclipse occurs when the moon is positioned between the earth and the sun and covers the sun disc. Lunar eclipse occurs when the moon passes through the shadow of the earth. This situation becomes possible if the sun, the moon and the earth come in one straight line, Such a situation occurs only on the full moon or new moon days. However as the planes of the orbit of the moon and the earth intersect each other at an angle of  5•, the eclipses do not occur on every full moon or every new moon day.

Solar Eclipse : If the moon gets positioned between the earth and the sun and if these three are in a straight line, the shadow of the moon falls on the  earth. As result the sun becomes totally or partially invisible in the shadow zone. This condition is called solar eclipse.

Solar eclipse: If the moon is between the sun and the earth, the shadow of the moon falls on the earth. At such a time these three celestial objects are on the same plane and fall in one line. Hence the place on the earth where the moon's shadow falls, experiences a solar eclipse. This shadow is of two types. The central portion of the shadow is darker and the periphery is light. In the area of dark shadow on the earth, the sun becomes completely invisible .This condition is known as total solar eclipse. However, during the same period, at the places where the shadow  is lighter the sun disc appears partially covered. This condition is described as partial solar eclipse. Total solar eclipse is see in a very limited region of the earth. Sometimes the moon is in apogee position. This means it is at is farthest from the earth, As a result, the deep shadow of the moon is cast in space and does not reach the earth, From a very small region of the earth, only an illuminated edge of  the sun disc is see in the form of a ring. This is called annular eclipse. Annular eclipse is a rare phenomenon.

Solstices : Specific positions of the earth on its elliptical orbital path. These positions occur on two days - 21 st June and 22nd December. These are the solstice days. On 21st June, the north pole of the earth records maximum inclination i.e., 23• 30', towards the sun. On this day, the sunrays are perpendicular at the Tropic of Cancer. On 22nd December, the south pole of the earth records maximum inclination, i.e., 23• 30', towards the sun. On this day, the sunrays are perpendicular at the Tropic of Capricorn. 21st June and 22nd December are summer and winter solstices respectively. 21st June is the longest day in the northern hemisphere whereas 22nd December is the , longest day in the southern hemisphere.

Dakshinayan: March of the sun towards the south. This start from 21st June and every day the sun appears to be moving southwards slowly. The southward march of the sun comes to an end on 22nd December when it starts moving towards the north. In reality, the sun does not move but due to the revolution of the earth and the inclination of its axis, the sun appears to be moving towards north or south.

Try This : 

• Take a ball of clay or mud and keep it at the Centre of a table 
• Fix a pencil vertically in the clay ball in such a way that its sharp end points upwards.
•  Fix a small plastic or sponge ball on the sharp end of the pencil.
•  Draw a circle along the midline of the ball. Let this ball be your moon.
•  Now place a large plastic or rubber ball at a distance of 10 to 15 cm. From this moon. Let this larger ball be the earth. Draw a circle along the midline of this ball too using a pencil in the Centre. Let this be the equator. 
• Keep the ball steady using a rubber or cloth ring.
• Arrange the balls representing the earth and the moon in such a way that the circle drawn on the moon will be in the plane of the equator of the earth.
• Take a torch (the sun). Hold it at a distance of about 30 cm from the moon.
•  Direct the light beam on the moon. 
• Observe the shadow of the smaller ball on the earth to understand how solar eclipse occurs.

Circle of illumination : Sunlight divides the Globe into an illuminated and a dark part. The line separating the two parts is the circle of illumination. This is not an imaginary line, it exists on the surface of the earth all the time. The circle of illumination is a great circle. Due to the rotation of the earth, its position seems to change all the time.

Lunar eclipse : A lunar eclipse occurs when the moon enters the shadow of the earth, while revolving around it. At this time, it is necessary that the earth is between the sun and the moon, and all three of them are in the same planes. On a full moon day, the moon's path of revolution   passes through the thick shadow of the earth. If the moon is totally hidden within the shadow, we see a total lunar eclipse and if only a part of the moon is in the shadow, we see a partial lunar eclipse.

Annular Eclipse : The sun disc does not get totally covered by the moon if the moon is in its apogee position (at the farthest distance from the earth). Moon's shadow does not reach up to the earth. As a result in such a situation only the rim of the sun disc remains visible from a very narrow portion of the earth. This illuminated rim appears like a ring. Hence this type of solar eclipse is called annular solar eclipse.

Partial eclipse : A type of solar or lunar eclipse when a part of the sun disc is covered by the moon or part of the moon disc is shadowed by the earth.

Perigee : A specific position of the moon on its orbital path with respect to the earth. In this position, the moon is at its nearest distance from the earth.

Characteristics of solar eclipse : A solar eclipse occurs on a new moon day, but not on every new moon day  If and only if the sun, the moon and the earth are in the same plane and fall in one line, the solar eclipses occur. The maximum duration of a total solar eclipse is 7 minutes and 20 seconds (440 seconds).

Characteristics of lunar eclipse: A lunar eclipse occurs on a full moon day, but not on every full moon day.  A lunar eclipse occurs if and only if the sun, the moon and the earth are in the same plane and fall in one line. The maximum duration of a total lunar eclipse is 107 minutes.

Eclipse - an astronomical event :  A solar eclipse or lunar eclipse is just an astronomical event.. There is nothing 'auspicious' or 'inauspicious' about these events. It is just a result of the sun, the earth and the moon being in specific positions. There is a lot of curiosity about eclipses because an eclipse is not a regular occurrence. For scientists working in the field of astronomy, eclipses and particularly total or annular solar eclipses present great opportunities for study. Scientists from all over the world   make it a point to visit the locations where such events are going to occur. They carry out elaborate studies of eclipse conditions.

Total Eclipse: A type of solar or lunar eclipse when the sun disc is completely covered by the  moon or the moon is completely shadowed by the earth.

Equinox : Specific situation of the earth's location on its orbital path with respect to the sun. In the situation, both the ends of the earth's axis face the sun and are equidistant from the sun. This situation is observed on two days in a year. On these two days, the circle of illumination coincides with the meridional great circles. This situation is observed on 22nd March and 23rd September. On these days, sun rays are perpendicular at the equator. The duration of daytime and nighttime is the same and it is of 12 hours. 

Fortnight: A Period of 15 days from new moon day to full moon day (and from full moon day to new moon day).

Perihelion : A Position of the earth on its elliptical orbital path at the minimum distance from the sun. This position occurs in the month of January.

Perihelion : A Position of the earth on its elliptical orbital path at the minimum distance from the sun. This position occurs in the month of January.

Period of waning moon : The fortnight from the full moon day to the new moon day. In this period, the illuminated portion of the moon appears to be getting progressively reduced as observed from the earth.

Period of waxing moon : The fortnight from the full moon day to the new moon day. In this period, the illuminated part of the moon keeps on increasing every day as observed  observed from the earth.

Phases of the moon : The illuminated portion of the moon disc observed from the earth that keeps on changing every day within a lunar month.

Period of waxing moon: The fortnight from the new moon day to the full moon day. In this period the illuminated part of the moon keeps on increasing every day as observed from the earth.

Antipodal Point : A point that is located diametrically opposite to a given point. Such a point is fixed with reference to an imaginary line in the form of the diameter of the earth. For any or every point on the surface of the earth, there exists an antipodal point on the earth's surface.

Aphelion : A position of the earth on its elliptical orbit when it is at the maximum distance from the sun. This position occurs in the month of July.

Apogee : A specific position of the moon on its orbital path with respect to the earth. In this position the moon is at its farthest distance from the earth.

Autumnal Equinox : One of the positions of the earth on the orbital path, relative to the sun. This position occurs on 23rd September. In this position both the poles of the earth are equidistant from the sun and the sunrays are perpendicular at the equator. The duration of nighttime and daytime are equal all over the earth.

Measurement of Time : A day, a month and a year are the basic units of the measurement of time. A day and a year are the results of the axial and the orbital motions of the earth respectively. The month is a result of the revolution of the moon around the earth.

Neap tide : The range of tides id lowest on the first and third quarter days. On these days the sun and the moon are positioned at right angle to each other with respect to the earth and hence the force of their attraction do not complement each other.

Spring or Vernal equinox : One of the positions of the earth on the orbital path relative to the sun. This position occurs on 23rd March. In this position both the poles of the earth are equidistant from the sun and the sunrays are perpendicular at the equator. The duration of night time and day time are equal all over the earth.

Spring Tide : The high or low tide occurring on the full moon and the new moon days. As the sun and moon are on the same side of the earth on this (new moon) day, maximum tidal range is observed. The tidal range on full moon day is also high but it is somewhat less than that on new moon day.

Always remember -  While observing a solar eclipse, it is necessary to view the sun disc through dark glasses or special goggles made for that purpose, otherwise  the intense light of the sun can be harmful to the naked eye.

       During the period of a solar eclipse, a large number of birds and animals get confused due to the untimely darkness that sets in.  As this is an event that does not suit their biological clock, their response to the event is also unusual. Try to observe the responses of the birds and animals at the time of such events and record your observations.

Do You Know ? Occultation and Transit : Like eclipses, some other specific conditions occur with reference to the sun or the moon.  These are called occultation and transit respectively. Occultation occurs with reference to the moon whereas the transit is associated with the sun.

Uttarayan : March of the sun towards the north. This starts from 23rd December and every day the sun appears to be moving northwards slowly. The northwards march of the sun comes to an end on 21st June when it starts moving towards the south. In reality, the sun does not move but due to the revolution of the earth and the inclination of its axis, the sun appears to be moving towards north or south.

Gravitational force: In any two objects, there exists an attraction towards each other. This is called gravitational force. The gravitational force depends on the mass of the two objects and the distance that separates them The planets revolving around the sun along their specific orbit is the combined effect of the gravitational force of the sun and the centrifugal force associated with planets. The value of gravitational force is computers as M1 M2 / D2 , Where M1 and M2  represent the mass of object 1 and 2 and D represents the distance between them.

Occultation : This is a typical event occurring in space. The moon revolves around the earth. While doing so, it obscures a star or a planet and that celestial body appears to hide behind the moon. This is called occultation. The total solar eclipse is actually a kind of occultation. During this the sun disc gets hidden completely because of the moon.

Transit : If an inner planet like Mercury or Venus comes in between the line of the earth and the sun, a transit occurs.  At that time, a small dot appears to move across the sun disc. Transit is a type of solar eclipse.

Horizon : A line where it appears that the sky and the land are meeting. In reality, it is the plane of a tangent line assumed to be drawn at a given place. 'N' number of such tangents to the earth surface can be assumed. When the celestial objects like the sun become visible above this line, we say it is a sunrise, When it moves below this tangent line it is said to be Sunset.

Horizontal distribution : The values of different elements of climate are not the same  at various places on the earth, As a result, the values of temperature, air pressure, rainfall etc. differ from place to place near the earth's surface. This distribution of climate elements in a horizontal direction is called horizontal distribution.

Motions Of The Earth

           The top turns around itself. Any object that turns about itself actually turns around a certain imaginary line. The turning of an object around itself is called 'rotation' and the imaginary line around which it rotates is called the 'axis of its rotation'.

           The plumb - line and the earth's axis are at an angle to each other. That, is, the earth's axis is inclined. The earth rotates with its axis inclined like this. The line NS in the picture shows the earth's axis. It passes through the Centre of the earth. The points N and S are called the poles of the earth. N is the north pole of the earth and S is the south pole. If a circle were draw around the surface of the earth exactly in between the north and south poles, it would divide the earth into two equal parts. This imaginary circle is called the 'equator'. The two equal parts it makes of the earth are called the northern hemisphere and the southern hemisphere respectively.

The Sun and the earth : The earth gets light from the sun. The earth's shape is like a very large ball. While going around the sun, the earth turns around itself. The spinning of the earth around itself is called the rotation of the earth.

         The light of the sun does not reach the entire earth at one and the same time. Half of the earth gets sunlight while the other half remains dark. We say that it is day in the half that gets light, while in the other half where sunlight does not reach, it is night. Every day we see day chasing night and then night chasing day. This cycle goes on without stopping.

Sunset and Sunrise :  Stick a red bindi on the globe. Set up the previous model of the globe and the candle. Turn the globe anti - clockwise. Note when it is sunrise, noon and sunset at the location of the bindi. After one sunrise, note when the next one occurs at the bindi. You will see that this happens when the earth completes one rotation, that is, when it makes one complete turn around itself.

        This period of time that the earth takes to complete one rotation is called a day. day has two parts, daytime and nighttime or simply day and night. For the purpose of measuring time, we divide the whole day into 24 parts, each of which is called an hour.

A Leap Year : In the Gregorian calendar, the year is taken to have 365 days. It means that it counts 6 hours less every year. That makes 24 hours or one day in every four years. To make up for this lost one day, the month of February in the Gregorian calendar has an extra day every fourth year. That year is called a leap year and it has 366 instead of 365 days and February has 29 instead of 28 days.

Do You Know ?

         You know that the length of day and nigh is not always equal. This happens because of the earth's inclined axis and its revolution around the sun. In the northern hemisphere, between 22 March and 23 September, the days are longer than the nights. Therefore, it is warmer there. That, it is summer in the northern hemisphere. However, during this same period in the southern hemisphere, the nights are longer than the days. The earth gets less heat in these parts and therefore it is winter in the southern hemisphere. In the period from 23 September to 22 March, the days are longer than the nights in the southern hemisphere, It gets more heat and it is summer there. In this period in the northern hemisphere, it is the nights that are longer. The northern hemisphere gets less heat and it is winter there. 

    

        Note that there may be differences in these dates due to the leap year. In India, summer, the rainy season and winter are considered to be the main seasons. We also divide the year into six seasons, namely, Vasant, Grishma, Varsha,Sharad, Hemant and Shishir. This cycle of six seasons is called the 'rituchakra'. Many of our festivals are connected with the seasons. Many of our songs and games are also related to the different seasons.

The full moon and the new moon : The moon revolves around the earth and the earth revolves around the sun. However these two orbits intersect. Hence, the sun, the moon and the earth are not always along  a straight line. We see half of the moon's surface which faces the earth. That is, from the earth we see only one side of the moon. The moon has no light of its own. We can see the moon because of the sun's light that falls on it. On a full moon that faces the earth. On a new moon night, we cannot see any of it. From the full moon to the new moon the illuminated part of the moon seen from the earth becomes smaller and smaller. From new moon to full moon it again grows bigger and bigger. These different shapes of the moon that we see are called the phases of the moon.

The lunar month and days (tithi): You know that it take 14 or 15 days from new moon to full moon. This is the fortnight of the 'waxing' moon. After the full moon, the moon appears smaller and smaller and after 14 - 15 days it is new moon again. This period is the fortnight of the 'waning' moon. Thus, the period from one new moon to the next is of 28 - 30 days. It is called the lunar month. Every day of the ,lunar month is called a tithi.

Always remember ---- The rotation of the earth gives rise to day and night . The revolution of the earth and the inclination of of its axis give rise to the cycle of seasons.