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00:00MOVEMENT OF THE EARTH AND THE MOON
00:10We have learnt that planets revolve around the sun in a particular path or orbit.
00:21Let us learn more about the types of movements seen in heavenly bodies.
00:30We shall be studying about the movement of the earth and its natural satellite, the moon.
00:43MOVEMENT OF THE EARTH
00:45The earth shows two types of movements, rotation and revolution.
00:54Both rotation and revolution play a very important part in our lives.
01:04ROTATION
01:05When an object spins around itself on an imaginary line passing through its center, it is said
01:13to be rotating around its axis.
01:21The earth is spherical like a ball and spins around an imaginary tilted axis and this spinning
01:29movement is called rotation of the earth.
01:35The earth completes one rotation in 24 hours.
01:44Different regions on the planet get varied amounts of heat and light from the sun.
01:55EFFECTS OF THE EARTH�S ROTATION
01:59The part of the earth which faces the sun receives sunlight and experiences daytime.
02:08The other side of the earth that does not face the sun experiences darkness and so it
02:16is night there.
02:18Thus, day and night on the earth occur because of its rotation.
02:29If the direction of the rotation is from west to the east, it appears that the sun rises
02:36in the east.
02:41It is only because of rotation that almost all parts of the earth receive sunlight once
02:50every 24 hours.
02:57REVOLUTION
02:58As the earth rotates on its axis, it moves around the sun in a particular path or orbit.
03:07This movement of the earth is called revolution.
03:13The earth completes one revolution around the sun in 365 and a quarter days.
03:23This period is called a year.
03:28To account for the extra quarter day taken by the earth�s each revolution, we have
03:34one day as 29th February after every four years.
03:44This fourth year with 366 days is called a leap year.
03:54PHASES OF REVOLUTION OF THE EARTH
03:58The different seasons on the earth are caused due to the tilt of the earth�s axis and
04:05the revolution of the earth.
04:14THE PHASES OF THE MOON
04:18The moon completes one revolution around the earth in a period of about 28 days.
04:31At different points during this time, different parts of the moon are lit up by light it receives
04:39from the sun.
04:43Just as only half the earth�s surface gets sunlight, similarly, only half the moon�s
04:50surface or the side facing the sun receives sunlight.
04:57However, every night we do not see the entire lit surface of the moon.
05:08Most of the nights we are able to see only a part of it, while the part that is not lit
05:15by the sunlight appears dark.
05:21Once in every 28 days, we see the entire lit surface of the moon.
05:28This is called the full moon.
05:33The full moon looks fully round and shines very brightly, since the side of the moon
05:42facing the earth receives sunlight.
05:49From the full moon onwards, i.e., Purnima, every night we see lesser and lesser lit surface
05:57of the moon.
06:00Since every night the bright surface of the moon appears to decrease in size, this is
06:07called the waning moon.
06:11The waning continues till the night, when there is no moon seen in the sky.
06:18The reason for this is that the side of the moon facing the earth does not get any sunlight.
06:30This is called the new moon, i.e., Amavasya.
06:37After a new moon night, the lit surface of the moon appears in the sky and keeps increasing
06:45in size.
06:47This is called the waxing moon.
06:53It is very important to remember that the actual size and shape of the moon does not
07:00change, but only appears to do so because of the amount of sunlight it receives.
07:10The different shapes of the moon that appear in the sky, from one full moon to the next,
07:18are called the phases of the moon.