Advanced Physics Topics like quantum mechanics and relativity have revolutionized our understanding of the universe.
What is a Satellite? and What are they used for?
Just as a celestial body which revolves around the Sun (or any other star) is called a planet, in the same way, a celestial body which revolves around a planet is called a satellite. Thus : A satellite is a celestial body that revolves around a planet (see Figure). Earth is a planet. Since the Moon revolves around the Earth,
therefore, Moon is a satellite of the Earth. Please note that the Moon is a natural satellite of the Earth. As we will learn after a while, many artiíìcial satellites (or man-made satellites) are also revolving around the Earth these days. Please note that though we commonly call the Earth’s natural satellite as Moon, the natural satellites of all other planets are also called their ‘moons’
Out of the eight planets of the Solar System, the first two planets. Mercury and Venus, do not have satellites. All the remaining six planets have one or more satellites. The satellites revolve around the planets due to the gravitational pull of the planets. The satellites have no light of their own. Fhe satellites shine and become visible to us because they reflect the light of the Sun falling on them. We will now describe the Earth’s natural satellite called Moon in detail.
The moon is a natural satellite of the Earth (see Figure). The Moon revolves around the Earth on a definite regular path—the Moon’s orbit (see Figure). The gravitational attraction of the Earth holds the Moon in its orbit. The Earth along with Moon, revolves around the Sun (see Figure). The Moon also rotates on its axis. The Moon is about one-fourth the size of the Earth in diameter but its mass is only about one-eightieth that of the Earth. The distance of Moon from the Earth is 3,84,000 km. Moon is the closest celestial object to the Earth. Moon appears to be much bigger than the stars because it is much more nearer
to the Earth than the stars. Actually, all the planets and stars are much bigger than the Moon. Moon is a huge ball of rocks. Moon’s surface is covered with hard and loose dirt. There are many craters of different sizes on the surface of Moon (see Figure). The surface of Moon has also a large number of steep and high mountains. We can now say that the surface of Moon is covered with hard and loose dirt, craters and mountains. The Moon has no air (or atmosphere). The Moon has no water. Since there is no air or water on the Moon, therefore, there is no life on the Moon. On the Moon, days are extremely hot and nights are extremely cold.
Man has landed on the Moon. The first man to land on Moon was an American astronaut Neil Armstrong in July 1969. He was followed by another astronaut Edwin Aldrin shortly afterwards (see Figure).
The Moon does not produce its own light even then we are able to see it shining in the night sky. We are able to see the Moon because the sunlight falling on the Moon gets reflected towards the Earth (see Figure). When the sunlight falls on Moon, then a part of this sunlight is reflected by the surface of Moon towards the Earth. When this reflected sunlight enters our eyes on the Earth, to us it appears as if the light is coming from the Moon itself (see Figure).
Since the sunlight reflected by Moon enters our eyes, we are able to see the Moon. Thus, Moon shines with the sunlight reflected by it. We can, however, see only that part of the Moon from which sunlight is reflected towards us on the Earth. Please note that though the Moon reflects sunlight towards us even during daytime but we cannot see the Moon during daytime due to the glare of the Sun. We can see the Moon only during night time when the Sun is not present in our sky (see Figure). The Moon always shows the same face (or same side) to the Earth as it rotates on its axis. This is because the Moon completes one rotation on its axis in the same time which it takes to complete one revolution around the Earth (taking into account the movement of Earth around the Sun). So, we never see the back side of the Moon from the Earth. We will now discuss the phases of Moon.
If we observe the Moon continuously every night for a month, we will find that there is one day in the month when the Moon cannot be seen in the night (even when the sky is clear and there are no clouds, etc.). The day (or rather night) on which the Moon is not visible at all is called the new Moon day [see Figure]. New Moon day is called ‘Amavasya’ in Hindi. We have a very dark night on this day of the month (because there is no moonlight at all). On the next day, only a small, curve like portion of the Moon appears in the night
sky [see Figure (b)], This is known as the Crescent Moon. Thereafter, every night, the size of the bright, visible part of the Moon appears to become bigger and bigger, giving us many shapes including half Moon and more than half Moon [see Figures (c) and (d)]. After fifteen days (from the new Moon day), we can see the whole bright disc of the Moon in the night sky. So, there is also one day in a month when the Moon is visible as a perfectly round ball of light in the sky. The day (or rather night) on which the whole bright disc of Moon is visible to us on the Earth is called the full Moon day [see Figure (c)], The full Moon day is called ‘Purnima’ in Hindi.
We have the maximum moonlight during the night on this day of the month. Thereafter, every night, the size of the bright, visible part of the Moon goes on becoming smaller and smaller [see Figures (f), (g) and (h)). And after fifteen days (from the full Moon day), the Moon is not visible again [see Figure 30(A)]. We have the new Moon again. This new Moon will again change into full Moon after another fifteen days, and the process is repeated endlessly. From this discussion we conclude that the shape (or appearance) of the visible part of the Moon changes everyday over the whole month. And this change is repeated again and again, month after month. The different shapes (or appearances) of the bright, visible part of the Moon as seen from the Earth (during a whole month) are called phases of the Moon. Some of the phases of Moon (as seen from the Earth) are shown in Figure 30. We will now describe why Moon changes its shape everyday. In other words, we will explain the formation of phases of the 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. 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). Depending upon the relative positions of the Sun, the Moon and the Earth, we see different amounts (or portions) of the sun-lit surface of Moon from the Earth. So, as Moon revolves around the Earth once every month and moves around the Sun alongwith Earth, different amounts of its sun-lit surface are turned towards the Earth leading to a change in the appearance of Moon and formation of phases of the Moon.
From the above discussion it is clear that the phases of Moon occur due to its continuously changing position with respect to the Earth and the Sun. This is shown in Figure.
(i) When the Moon is on the side of Earth nearest to the Sun (see position 1 in 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 and hence cannot be seen. This is called new Moon.
(ii) 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 positions 3 and 4, the sun-lit 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.
(iii) After fifteen days from new Moon day, the Moon reaches in position 5 which is on the side of Earth farthest from the Sun (see Figure). In this position, the 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.
(iv) 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.
Please note that after the new Moon day, the size of sun-lit part of Moon visible from the Earth increases everyday. On the other hand, after the full Moon day, the size of sun-lit part of Moon visible from the Earth decreases every day. 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). The time period between one full Moon and the next full Moon is actually 29V2 days. In the moon-based calendars (called lunar calendars) this period of 29Vi days is taken as a month.
Some Other Members Of The Solar System
In addition to the Sun, planets and satellites, the Solar System also includes asteroids, comets and meteoroids. Asteroids, Comets and Meteoroids revolve around the Sun just like the planets but they are much smaller than the planets. The motion of asteroids, comets and meteoroids is also governed by the gravitational force of the Sun. So, asteroids, comets and meteoroids are considered to be the members of the Solar System. We will now describe all these members of the Solar System in detail. Let us start with asteroids.
There is a wide space (or gap) in-between the orbits of planets Mars and Jupiter. A large number of small objects made of rocks revolve around the Sun in the wide space between the orbits of the planets Mars and Jupiter in the Solar System. These are called asteroids. Thus, asteroids are small celestial objects which revolve around the Sun between the orbits of Mars and Jupiter (see Figure). Asteroids are smaller than the planets.
Asteroids are also called minor planets. The asteroids are rocks of various sizes. The biggest asteroids are hundreds of kilometres in diameter whereas the smallest asteroids are only hundreds of metres in diameter. There are millions of asteroids which orbit around the Sun between the orbits of Mars and Jupiter. Each asteroid has its own orbit around the Sun. Asteroids can be seen only through large telescopes. Asteroids are the members of the Solar System. Astronomers believe that asteroids are the pieces of matter (or rocks) that were formed at the same time as the rest of the planets which somehow could not assemble to form a major planet. Sometimes the orbiting asteroids collide with one another, break into pieces and form meteoroids which give rise to meteors (shooting stars) or meteorites.
Comets are very small celestial objects made of gas and dust which revolve around the Sun in highly elliptical orbits and become visible only when they come close to the Sun. As a comet approaches the Sun,
it develops a long, glowing tail and becomes visible to us (see Figure 33). A comet becomes visible on coming close to the Sun because the Sun’s rays make its gas glow which then spreads to form a tail millions of kilometres long. The head of a comet is small and appears as a bright ball of light but its tail is very, very long. Thus, a comet is a collection of gas and dust which appears as a bright ball of light in the sky with a long glowing tail when it comes close to the Sun. The tail of a comet always points away from the Sun (see Figure). The tail of a comet grows longer as it comes nearer the Sun but it disappears when the comet moves far away from the Sun (see Figure).
Comets are smaller than asteroids. Comets are distinguished from asteroids by the presence of a long glowing tail. Asteroids do not have any glowing tail.
Comets are also the members of our Solar System. Comets revolve around the Sun just like planets. Each comet follows its own orbit around the Sun. The orbits of comets are very, very large and highly elliptical, due to which they remain very far off from the Sun most of the time and can be seen very rarely. The period of revolution of a comet around the Sun is very, very large. For example, Halley’s comet has a period of revolution of about 76 years. That is, Halley’s comet is seen after every 76 years. Halley’s comet was last seen in 1986. Comets do not last for ever. Each time a comet passes near the Sun, it loses some of its gas and ultimately only the dust particles are left in space. When these dust particles enter into the Earth’s atmosphere, they burn up due to heat produced by air resistance and produce meteors (or shooting stars).
Some people think that comets are the messengers of disasters such as wars, epidemics (widespread diseases) and floods, etc. These are, however, myths and superstitions (false beliefs). The appearance of a comet is a natural phenomenon. It has nothing to do with natural or man-made disasters. We should not be afraid of sighting the comets.
Meteoroids : Meteors and Meteorites
Meteoroids are celestial objects which range in size from tin) sand grains to big boulders of several hundred tonnes and revolve around the Sun in their orbits. Meteoroids are mainly left behind by the disintegration of asteroids and comets. Meteoroids are much smaller than asteroids and comets. Meteoroids are present throughout the Solar System. Meteoroids are members of the Solar System because they revolve around the Sun. When (due to some reason), a meteoroid breaks away from its orbit around the Sun and falls towards the Earth, it becomes a meteor. We can now say that meteoroid is a small body in the Solar System that would become a meteor if it entered the Earth’s atmosphere. It is the visible path of a burning meteoroid that enters the Earth’s atmosphere which is called a meteor. We will now describe the meteors in detail.
When the sky is clear and the Moon is not there, many times we see a streak of light in the sky during night which disappears within seconds. It is called a meteor or shooting star. Meteors are the celestial bodies from the sky which we see as a bright streak of light that flashes for a moment across the sky (Figure). The meteors are commonly called shooting stars.
When a meteor enters into the atmosphere of Earth with high speed, a lot of heat is produced due to the resistance of air. This heat burns the meteor and the burning meteor is seen in the form of a streak of light shooting down the sky, and it falls on the Earth in the form of dust. A meteor (or shooting star) lasts for a very short time because the tiny rocky pieces (of which it is made), burn and vaporise completely in a few seconds due to the excessive heat produced by atmospheric friction. When the Earth crosses the tail of a comet, swarms of meteors (large number of meteors) are seen. These are known as meteor showers. Some meteor showers occur at regular intervals each year.
A meteor is commonly known as a shooting star though it is not really a star. A meteor is called a shooting star because, viewed from the Earth, it looks like a streak of starlight shooting across the night sky. The main difference between a star and a shooting star is that a star has its own light but a shooting star has no light of its own. The light of a shooting star is produced when its particles burn on entering the Earth’s atmosphere.
Most of the meteors are small and burn up completely on entering the Earth’s atmosphere. They fall on the Earth in the form of dust. If, however, a meteor is big, then a part of it may reach the Earth’s surface without being burned up in the air. A meteor which does not burn up completely on entering the Earth’s atmosphere and lands on Earth, is known as a meteorite.
Meteorites are sort of stones falling from the sky. The meteorites falling on the Earth from the sky range in size from small pebbles to big blocks many tonnes in weight. More than 3000 meteorites, weighing around a kilogram or more, fall on the Earth each year in different parts of the world (see Figure). Meteorites are made of rock (or metal). Meteorites are believed to be the pieces of asteroids or comets which some how strayed away from their orbit around the Sun and fell to Earth. They are thought to have been formed at the same time as the planets of the Solar System. So, by studying the composition of meteorites, the scientists can get valuable information about the nature of the material of which the various planets (including the Earth) are made.
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 shown in Figure. An artificial satellite is placed in orbit around the Earth with the help of a launch vehicle called rocket. Rocket carries the artificial satellite from the Earth to a height of a few hundred kilometres (or a few thousand kilometres) above it and gives it a sideways push. This makes the satellite move in an orbit around the Earth. The motion of artificial satellite around the Earth is maintained by the gravitational pull of the Earth. In this way, the artificial satellite keeps on revolving around the Earth continuously, without stopping. The artificial satellites revolve around the Earth just like its natural satellite Moon.
The main differences between the artificial satellites of the Earth and its natural satellite Moon are as follows :
(i) The artificial satellites are much nearer to the Earth than its natural satellite Moon.
(ii) The height (or distance) of the artificial satellites from the Earth (and hence their speed around the Earth) can be adjusted according to our needs. This is, however, not possible with the natural satellite of Earth called Moon.
The artificial satellites carry a large variety of equipment and instruments inside them. For example, they may carry sound and picture relaying machines, cameras, infra-red sensors, telescopes and many other type of instruments required for doing different jobs. The electricity required for running the equipment in an artificial satellite is provided by solar cells. The solar cells convert sunlight into electricity.
Before we give the various applications (or uses) of artificial satellites, we should know the meaning of the term remote sensing’. The technique of collecting information about an object from a distance (without making a physical contact with the object) is called remote sensing. Remote sensing satellites can scan the Earth’s surface very closely with their cameras and infra-red sensors, even while orbiting high above the Earth. They can see the details on the Earth even up to a fraction of a metre. We will now describe the important applications (or uses) of artificial satellites.
Artificial satellites have many practical applications (or uses) which help our lives in many ways. The important applications (or uses) of artificial satellites are given below:
1. Artificial satellites are used for communications such as long distance transmission of television programmes, radio programmes, telephone calls and internet. The artificial satellites used for communications purposes are called Communications Satellites.
2. Artificial satellites are used for weather forecasting (such as rain fall, snow-fall, etc.) and for giving advance warning of floods and cyclones, etc. Weather forecasting is done by using artificial satellites called Weather Satellites (or Meteorological Satellites) which are a kind of remote sensing satellites.
3. Artificial satellites are used for surveying the natural resources of the Earth like minerals, agricultural crops and potential fishing zones in the sea, etc. This is done by using the artificial satellites called remote sensing satellites.
4. Artificial satellites are used for spying for military purposes (like observing the movement of enemy troops and military equipment, taking pictures of enemy air-fields and harbours, etc). Remote sensing satellites are used for spying for military purposes.
5. Artificial satellites are used to collect information about other planets, stars and galaxies, etc. The
artificial satellites used for studying celestial objects are called Astronomy Satellites’.
India has built and launched many artificial satellites. The first artificial satellite launched by India was Aryabhatta’ (see Figure). It was launched in 1975. Some other Indian satellites are : Bhaskara, INS AT, IRS, Rohini, Kalpana-1 and EDUSAT (INSAT stands for Indian National SAtellite, IRS stands for Indian Remote-sensing Satellite whereas EDUSAT stands for EDUcation SATellite). The Agency responsible for the development of space science programmes in our country is ‘Indian Space Research Organisation (ISRO).