Types of satellites
Solar system of Class 8
Types of satellites
A satellite is a small body revolving around a planet.
Satellites are of two types:
(a) Natural satellite (b) Artificial satellite
NATURAL SATELLITE (MOON):
The moon, our nearest neighbour in space, is a natural satellite of the earth. A satellite is a heavenly body that revolves around a planet. Of the nine planets, only mercury and venus have no satellites or moons associated with them. Mars is believed to have 2 moons, jupiter 28 moons, saturn 30 moons, uranus 21 moons, neptune 8 moons of their own. The earth, of course, has only one moon and that, being our nearest neighbour, has been explored and studied the most by the astronomers.
The moon has no air or water on it. Its diameter is about (1/4th) of the diameter of the earth and its mass is about (1/81th) that of the earth. It has no light of its own. Its observed silver glow is only due to the light of sun reflected by its surface.
The moon is very much smaller than stars but it appears bigger because of its nearness. This near neighbour is, however, still about three lakh eighty four thousand and four hundred kilometers (3,84,400 km) away from us. Its surface is covered with hard and loose dirt and it has many uplands (mountains) and valleys that are filled with dry rock materials.
The moon has a very interesting feature associated with it. It takes same time (27 days and 7 hours) to revolve around the earth as well as to spin, or rotate, once about its own axis. We on the earth, therefore, always seen only one side (the `front' side) of the moon. Its other; or `black' side is not seen from the earth.
Phases of moon:
Moon has no light of its own. Moon shines and becomes visible to us because it reflects sunlight falling on it towards the earth. At any point of time, half of the moon's surface is in direct sunlight and the other half is in shadow. But since the moon revolves around the earth and the earth (alongwith moon) revolves around the sun, we cannot see all of the sun-lit surface of moon from the earth all the time. We can see only that part of the sun-lit surface of moon which is towards us (on the earth). Thus, we see different amount(or portions) of the sun-lit surface of the moon from the earth depending upon the relative positions of the sun, the moon and the earth. Thus, sometimes we see almost no face of moon (called new moon); sometimes we see less than half the face of moon (called crescent moon); sometimes we see half of moon; sometimes we can see more than half of moon; and sometimes we see the whole circular face of moon (called full moon). So, as moon revolves around the earth once every month, the moon's appearance varies according to how much of its illuminated surface is turned towards the earth (from where we see it). The different views of the moon (as seen from the earth) are called phases of the moon. Some of the phases of the moon are shown below in the figure.
On a new moon night, the moon is a dark bail and hence the new moon is not visible to us in the night sky. On a full moon night, the moon appears to be a bright bail of light. In-between a new moon and full moon, and then between the full moon and the next new moon, we see different shapes of the moon which are called phases of the moon.
Formation of the various Phases of the Moon:
(A) When the moon is on the side of earth nearest to the sun (see position 1 in the figure), then the side of moon which is lit by sun is away from earth. And the side of moon which is towards the earth is in darkness, in this position, moon appears to be in darkness from earth. This is called new moon. The new moon day is called `Amavasya'. In fact, we cannot see the moon at all on the new moon day (or Amavasya). So, the new moon night (or Amavasya Night) is very dark having no moonlight at all.
(B) As the moon moves in its orbit around the earth from position 1 to 2, we can see a small sun-lit portion of its surface. This is called crescent moon. As the moon moves further from position 2 to position 3 and 4, the sun-fit portion of moon facing the earth becomes bigger and bigger giving us half moon and more than half moon. This is called the waxing phase (increasing phase) of the moon.
(C) After two weeks time from new moon day, the moon reaches in position 5 which is on the side of the earth farthest from the sun. In this position whole sun-lit side of the moon is towards the earth, and we see the moon as a full round disc of bright light. This is called full moon. The full moon day is called `Purnima'. The full moon night (or Purnima Night) is very bright because we have the maximum moonlight on that night.
(D) As the moon moves around the earth further from position 5 to positions 6, 7 and 8, the sun-lit portion of moon facing the earth becomes smaller and smaller. This is called the waning phase (decreasing phase) of the moon. And ultimately moon completes the revolution around the earth and again reaches position 1. So, we have new moon once again. Pfease note that we can have one new moon and one full moon during a month (which is the time taken by moon to complete one revolution around the earth).
From the above disscussion we conclude that we have 'Amavasya' on the new moon day which changes in two weeks' time into Purnima. And during the next two weeks, Purnima changes into Amavasya again. This process is repeated again and again.
A man-made space-craft placed in orbit around the earth is called an artificial satellite. The artificial satellites are also known as man-made satellites. An artificial satellite is placed in orbit around the earth with the help of a launch vehicle called rocket.
India has built and launched several artificial satellites. Aryabhatta was the first Indian satellite. Some other Indian satellites are INSAT, IRS, Kalpana, EDUSAT, etc.
Artificial satellites have many practical applications. They are used for forecasting weather, transmitting television and radio signals. They are also used for telecommunication and remote sensing.