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3rd Class Chapter No 13 - The Sun, Moon And Stars in Science for CBSE NCERT

Moon

         The various shapes of the bright part of the moon as seen during the month. They are known as the phases of moon. Moon is a non luminous body.

This means that the moon does not have its own light. But it shines. It shines because it reflects the light falling on it from the sun. If you observe the moon with naked eye, you see dark patches on it. These dark patches present on moon indicate the mountains on the moon.

An American astronaut named Neil Armstrong and Aldrin who landed on moon with the help of a vehicle named Apollo 11 in July 1969. The two scientists noted down that the surface of the moon is a rocky desert. There was no water or vegetation present on the surface of moon. It was a rocky desert. That is why life is not possible on moon and only 1/6th of gravitational pull that we have on earth is present on moon. If we talk about the temperature of moon during day time it is 100 degree Celsius and in night it is 168 degree Celsius because of such inhabitable conditions. Life is not present on moon.

Now, if we talk about the phases of moon, we have noticed this that the shape of the moon keeps on changing. Someday it is big and the other day it is small, this is because the moon is a non luminous body. The shape which we see is that part of the moon on which the sunlight falls. That is why we see moon in different phases as it reflects only that much light which it receives from the sun.

The day on which moon receives full light from the sun it reflects full, resulting into a full shape. This day is called full moon day. Likewise the day when it is fully dark as the light does not fall on moon then that day is called new moon day. The days when moon is partly visible means when there is some light falling on the moon and it is partly visible that day is known as crescent day.

The Full Moon day

When the whole disc of moon facing towards the earth is visible to us. It is known as the full moon day. It generally occurs on the 14th day.

full moon day

New moon day

It occurs on the 15th day and the moon is not visible because it receives no light from the sun.

Crescent moon

The next day of the new moon day, when only a small portion of the moon is visible in the sky. This is known as the crescent moon.

So, we can say that the phases of moon play an important role in our life because almost all festivals in India are celebrated according to the phases of the moon.

Diwali is celebrated on the new moon day, Budh Poornima and Guru Nanak’s birthday are celebrated on full moon day, Maha Shivratri is celebrated on thirteenth night of waning moon, Eid -ul- Fitr is observed on the day following the sighting of crescent moon.

Q. Why phases of the moon occur?

A. The moon does not produce its own light, whereas the Sun and other stars do. We see the moon because the sunlight falling on it gets reflected towards us. We, therefore, see only that part of the moon, from which the light of the Sun is reflected towards us.

Position of the moon in its orbit and its corresponding phases.

As we have read, the various stages such as the stage when the moon is not visible, it is called new moon day. Then comes the crescent stage and then the full moon day.

The moon revolves around the earth and the earth revolves around the sun. So, we can say that in an indirect form both moon and the earth revolve around the sun.

Now if we talk about the surface of the moon. The 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. Some of these are as high as the highest mountains on the Earth. The moon has no atmosphere. It has no water. Therefore, no life exists on the moon.

Stars

So, let’s discuss about the stars. When its day what we can see in the sky is the sun but as the night comes, we start seeing some objects which twinkle. So, those twinkling bodies are stars.

Star is a luminous body because it can emit its own light.

The size of star is bigger than that of earth. Some of the stars are even bigger than sun. As they are far away from us, hence, they look so tiny. We can see them in night only because during day, the light of the sun is so intense that it becomes difficult to see them. Likewise we have some star like objects present in our sky which do not twinkle but they keep on shining. They are known as planets. But before we study about the planets we will first talk about stars.

The sun which is also a star is 150,000,000 kilometers (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

As it is not easy to express such a long distance in Kilometers, so the unit used to express the distance between the objects of universe is ‘light year’.

How we define light year?

It is the distance travelled by light in one year.

So, the next question is why does the sun appear to rise in the east and set in the west?

The answer is that as the earth rotates from west to east on its axis, so, the Sun appears to rise in the east and set in the west.

We have a star named Pole star. It is also called a stationary star because it does not move and it is the brightest star. It is situated on the axis of earth that is why it appears immovable.

Sometimes we see a cluster of stars which we can relate with the shape of other things. This group of stars is known as a Constellation.

Constellations

The stars forming a group that has a recognizable shape is called a constellation. The shapes of constellations resemble objects familiar to those people.

Ursa Major

One of the most famous constellations which you can see during summer time, in the early part of the night is Ursa Major. It is also known as the Big Dipper, the Great Bear or the Saptarshi. There are seven prominent stars in this constellation. It appears like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl.

Another constellation which we have is Orion

Orion

Orion can be seen during winter season in the late evenings. It also has seven or eight bright Stars. Orion is also called the 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.

 

Cassiopeia

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

So, now as we know about the various celestial bodies. Now we will talk about 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 Earth

The Earth, as you know, also revolves around the Sun. It is a member of the solar system. It is a planet. There are seven other planets that revolve around the Sun. The tilting of earth on its axis and its revolving around the sun causes seasonal changes on earth.

 The eight planets in their order of distance from the Sun are

  • Mercury,
  • Venus,
  • Earth,
  • Mars,
  • Jupiter,
  • Saturn,
  • Uranus and
  • Neptune.

Sun

The Sun is the nearest star from us. It is continuously emitting huge amounts of heat and light. The Sun is the source of almost all energy on the Earth. In fact, the Sun is the main source of heat and light for all the planets.

 

Posted in 3rd on February 13 2019 at 03:29 PM

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