My Science School

           The earth is round, and so are the sun and the moon. Even the planets, their satellites and the stars are also spherical. Do you know why all these heavenly bodies are spherical in shape?

           According to a law of science, every physical system in the universe tries to attain a state of minimum energy for maximum stability. It is an established fact that if a sphere is made from a given volume of any material it will have the least surface area compared to any other form. Consequently the spherical bodies have the minimum surface energy. Substances try to acquire a round form like a football in order to achieve the state of minimum energy, that is, the state of maximum stability. This is why the sun, the moon, the stars, the earth and all other heavenly bodies have a spherical shape.

           The rain drops are also spherical. It is so because the surface of the spherical drops has the minimum energy and maximum stability. Whatever be the process of producing the water drops, they immediately try to acquire a spherical shape.

           Atoms and molecules are also spherical in shape. Even the electrons, protons and neutrons present in the atoms are also spherical.

          You might have observed that every man tries to earn the most with the least effort. Every student tries to secure maximum marks after putting in the minimum labour. Thus we see that the tendency to come to the minimum energy state is found in all the living and non-living bodies. As soon as a substance acquires the minimum energy state, its shape becomes spherical.

 

 

          We see millions of stars twinkling in the sky during night. They radiate their light in all the directions. Even though they appear very small yet in reality they are very big. Most of the stars are many times bigger than our earth. They look smaller only because of their great distance from the earth. Do you know why the stars twinkle?

          Our earth is surrounded by a thick cover of air known as the atmosphere. There is vacuum beyond the atmosphere. The gases present in the atmosphere are in constant motion. Because of the movement of gases the density of air in the atmosphere is not uniform everywhere. As such the refractive index of air varies from place to place. When the light from a star enters our atmosphere, it gets deviated from its path several times before reaching our eyes because of the changing density and the consequent change in the refractive index of the air.

          The deviation in the path of light in its passage from one medium to another is called ‘refraction’. Because of this refraction, the light reaching our eyes from the star varies. Due to this variation the stars appear to be twinkling.

          Now the question arises: why don’t moon, sun and other planets twinkle like the stars? This is so because compared to the stars, sun, moon and the planets are very near to earth and as such they appear bigger than the stars. Hence the angles subtended by the moon, sun and the planets at our eyes are larger than the angles subtended by the stars. Because of the larger angles, our eyes are not able to detect the deviation in the path of light from the sun, moon and the planets and hence they do not appear to be twinkling. 

          On the full moon day, the moon appears like a shining disc but it goes on waning till it disappears on the new moon night. And then it again starts increasing in size and becomes a full bright disc, on the full moon day. Changes in the moon’s size are called the ‘phases’ of the moon. Do you know why it occurs? 

          The fact is that the moon neither increases nor decreases in size. Its size simply appears to be changing because it changes its position with respect to the sun. We all know that the moon is the only satellite of the earth and revolves round the earth. It travels around the earth about 384,400 km away. This orbit takes 27 days and eight hours. It is illuminated by the sunlight. Only one side of the moon faces the earth. We do not see its other side. When the moon comes in between the earth and the sun, its bright side is not visible to people on the earth and only its dark side faces the earth. As a result the moon cannot be seen. This is the new moon day. As the position of the moon changes due to its motion from east to west with respect to earth, some illuminated part of its surface is visible to us. A week after the new moon, almost half of the illuminated lunar disc becomes visible to us. The bright part of the moon’s surface goes on increasing gradually and on the full moon day the whole disc, seen from the earth, is fully illuminated. On this day the earth is in between the sun and the moon. In the next fifteen days the bright portion of the lunar surface goes on decreasing and on the fifteenth day it disappears completely. In short, the changes in the apparent shape of the moon depend upon how much of its illuminated surface is visible to us.

          The cycle continues like this and we see the different phases of the moon. On the full moon day it appears just after the sunset and disappears only at the sunrise. The time between new moon phases is called the synodic month.