Introduction to Astronomy and Cosmology - Ian Morison

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 Astronomy is probably the oldest of all the sciences. It differs from virtually all other science disciplines in that it is not possible to carry out experimental tests in the laboratory. Instead, the astronomer can only observe what he sees in the Universe and see if his observations fit the theories that have been put forward. 

Astronomers do, however, have one great advantage: in the Universe, there exist extreme states of matter which would be impossible to create here on Earth. This allows astronomers to make tests of key theories, such as Albert Einstein’s General Theory of Relativity. In this fi rst chapter, we will see how two precise sets of obser-vations, made with very simple instruments in the sixteenth century, were able to lead to a significant understanding of our Solar System. In turn, these helped in the formulation of Newton’s Theory of Gravity and subsequently Einstein’s General Theory of Relativity – a theory of gravity which underpins the whole of modern cosmology. In order that these observations may be understood, some of the basics of observational astronomy are also discussed.

One of the first triumphs of observational astronomy was Galileo’s series of obsevations of Venus which showed that the Sun, not the Earth, was at the center of the Solar System so proving that the Copernican, rather than the Ptolemaic, model was correct In the Ptolemaic model of the Solar System (which is more subtle than is often acknowledged), the planets move around circular ‘epicycles’ whose centres move around the Earth in larger circles, called deferents.  This enables it to account for the ‘retrograde’ motion of planets like Mars and Jupiter when they appear to move backwards in the sky. It also models the motion of Mercury and Venus.

Galileo’s observations, made with the simplest possible astronomical instrument, were able to show which of the two competing models of the Solar System was correct. In just the same way, but using vastly more sophisticated instruments, astronomers have been able to choose between competing theories of the Universe.

Looking up at the heavens on a clear night, we can imagine that the stars are located on the inside of a sphere, called the celestial sphere, whose centre is the centre of the Earth.