The star-studded sky on a clear night is a spectacular sight. It has inspired poets and artists to great heights of creativity. Over the ages, philosophers and religious leaders have spent their lives contemplating the starry heavens and wondering about man's place in the vast universe.
But the most common feeling that the sight inspires is one of curiosity. What are these twinkling points of light that we call stars? Are they all alike? How far away are they from us? Why do they shine? What is the status of the most spectacular of heavenly bodies, the Sun and the Moon?... And, if you are extra curious you may also wonder whether or not there are other objects out there which we cannot see with our eyes.
The science of astronomy was born out of this curiosity. And it can well claim to be the oldest of sciences. For, right from the earliest times, man has puzzled over these questions and tried to work out logical answers to them.
We find evidence of such attempts from the archaeological remains of ancient civilizations such as those of Egypt, Babylon, China and India. Manuscripts which have come down from the ancient Greek civilization of more than two thousand years ago, show how man tried to work out the pattern behind the somewhat chaotic movements of a handful of heavenly bodies called the planets.
What is so special about planets? You can find out for yourself if you watch the night sky for several months. First, you will see that stars, as a rule, rise in the east and set in the west, as the Sun does. There is one star, however, which does not seem to move at all. This is the Pole Star that lies towards the north. Now imagine that the night sky is a gigantic sphere with ourselves at its centre, a sphere that rotates from east to west around a line joining our position to the Pole Star. If all the stars were stuck onto this sphere, they would appear to go round this line, from east to west.
This is the picture the Greeks had constructed in describing stars. The planets, however, did not fit into the scheme so well. Instead of being 'stuck to' this revolving celestial sphere, they seemed to have additional motions of their own. This is what you
If we expose a film to a clear sky throughout the night the star-trajectories appear as circular arcs; the smallest one in the picture shows that even the Pole Star is not fixed. It is slightly off the Earth's rotation axis.
will find if, for example, you look at the positions of the planets Venus and Mars in relation to other stars. These planets appear to change their positions, and in no fixed pattern. This explains the name 'planet' which means 'wanderer' in Greek.
Why do planets wander? When confronted with this question human beings found two very different answers: one based on science, the other on superstition.
The superstitious believed that planets wander because they have some 'extra power' which is denied to stars: and out of this belief arose 'astrology'. Astrology assumes that planets are 'powerful' and exert their influence on human destiny.
But those with a scientific bent of mind tried to understand why planets move in this way. The answer was not easy or quick. About two thousand years ago the Greek astronomers, Hipparchus and Ptolemy, demonstrated that there is a pattern in the movement of planets. This pattern, however, appeared complicated because the Greeks had a stubborn belief that the Earth is fixed in space and that everything goes round it. In the sixteenth century Nicolaus Copernicus showed that the pattern of this planetary movement looked much simpler if planets (including our Earth) are assumed
The planetary trajectories appear haphazard when seen against the backdrop of distant stars. The loops in the picture indicate the observed forward and retrograde motions of planets.
to move around a fixed Sun. But his ideas were greeted with hostility.
Earlier, in the fifth century, the Indian astronomer, Aryabhata had stated that the Earth is not fixed, but revolves around a north-south axis and this was why stars appeared to rise and set. But so great was the Greek influence in scientific matters even in India that Aryabhata's correct reasoning did not receive support even from his disciples and successors.
It was only in the seventeenth century, thanks to the researches of Galileo, Kepler and Newton, that it was well established that planets move round the Sun. By 1687 with Newton's laws of motion and gravitation the movements of planets were explained accurately and today the astronomer can predict where a certain planet will be found at any given time in the future.
As in the case of the movement of planets man's surveys of the universe over the centuries have revealed many strange
Johannes Kepler established that a typical planet (P) moves in an elliptical orbit with the Sun (S) at one of the two foci of the ellipse. (S' is the other focus) Kepler's laws tell how the planet moves on this track. Later Isaac Newton's law of gravitation, explained why the planets move in this way.
phenomena. To understand these mysteries, man's best tool has been science. Indeed, our many questions about the heavens have received reasonably satisfactory answers from the laws of science known to us today.
Let us begin our exploration of the universe, armed with the tools provided by science. But first let us look at.these tools.
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