Man was born on the earth. During the course of evolution, his life has been indebted to the soil, water, air and landscape of Mother Earth.
When the Universe was first conceived of as an orderly unit, it was called Cosmos, and the studies relating to the cosmos were known as Cosmogony or Cosmology. Today, we speak of them as Space and Space Sciences.
The Universe or the Cosmos consists of millions of Galaxies. A galaxy is a huge congregation of stars which are held together by the forces of gravity. Most of the galaxies appear to be scattered in the space in a random manner, but there are many galaxies which remain clustered into groups.
Our own galaxy, called the Milky way or Akash ganga, which appears as a river of bright light flowing through the sky, belongs to a cluster of some 24 galaxies called the Local group. The Milky Way is made up of more than a hundred billion sparkling stars, which, though quite distant from each other, seem from the Earth as having been placed close together.
The two other nearest galaxies are the Large Magellanic Cloud and the Small Magellanic Cloud, named after Magellan, who discovered them.
The Universe is infinite, both in time and space. It was around sixth century BC that men started enquiring into the mysteries of the Universe in an endeavor to rationally analyse the earthly and the heavenly phenomena.
Ancient Greek astronomers and mathematicians came up with the view that the Earth was a perfect motionless sphere, surrounded by eight other crystalline spheres. The Sun, the Moon, and the five known planets - Mercury, Venus, Mars, Saturn and Jupiter, revolved around the Earth on seven inner spheres. The stars were permanently fixed to the outer sphere that marked the edge of the Universe.
The culmination of Greeks knowledge is associated with the name of Claudius Ptolemy of Alexandria, AD 90 to 168. In second century (AD 140) Ptolemy, a Graeco-Egyptian astronomer, synthesized the various data gathered by the early Greek astronomers. Ptolemy, in his book Almagest, presented his system of astronomy based on a geocentric (Earth-centred) Universe. He maintained that the Earth was the centre of the universe, and the Sun and other heavenly bodies revolved around the Earth.
In 1543, Polish astronomer Copernicus argued that the Sun not the Earth, was the centre of the Universe. Though the Copernicus theory changed the centre of the Universe, it did not change its extent which was still equated with the Solar system.
By 1805, telescopic studies made by the British astronomer Hersohel, made it clear that the Universe was not confined to the Solar system. The Solar system itself was only a part of a much vaster star system called the Galaxy. The Universe became quite extensive comprising millions of stars scattered about the Milky Way.
As the 20th century opened, it seemed that the Milky Way galaxy with its cluster of over a hundred billion stars together with their attendant satellites, the Magellanic clouds, actually represented all there was to the Universe.
In 1925, American astronomer Edwin P. Hubble (1889-1953) pointed out that there were other galaxies in the Universe and that the Universe actually consisted of millions of galaxies like the Milky Way. In 1929, Hubble proved that these galaxies are flying away from each other and that the farther they are, the faster they fly.