Astronomy is defined in the Encyclopedia Britannica as the ``science that deals with the origin, evolution, composition, distance, and motion of all bodies and scattered matter in the universe. It includes astrophysics, which discusses the physical properties and structure of all cosmic matter''. Taken at face value this might seem a very daunting task indeed. One needs to remember the point made by a cartoon I once saw. It is a popular cartoon and many may have seen it. It shows a picture of a fairly typical street in New York City, with its fire hydrants, flying bits of paper, half crushed aluminum cans, people rushing past with their great coats buttoned up to their necks. Underneath this picture is the little caption saying, `The Milky Way: detail'. All pursuits of understanding of our physical Universe are simplifications. Blaise Pascal insisted that as parts of the whole, humans were necessarily restricted in their understanding of the Universe. The stated aim of science is to break Physical Reality, as perceived by our sense-perceptions and properly characterized by some methodological harness, down into interactions of simpler self-consistent blocks of physical processes and entities. This is so fundamentally important that it is crucial to understand this point if we are to understand any aspect of science.
It may be simplest to understand if I start by pointing out the differences between religion and science. All religions, humans have had experience with, start off with an a-priori assumption. Some rationalists (viz. Bertrand Russell) have given this the pejorative description of being a dogma. It is an inviolable prior. They all assume that there is some quality, some essence, that is possessed by and is in possession of our Universe. Consequent to this prior there are many corollaries. These may vary from religion to religion. But by and large they tend to be things like authority of revelations, whether personal or public. This is an important aspect of any religion. This is what grants it an orthodoxy. Not in the sense of rigidity or complacency that is often associated with the word, but in the strict meaning of the word, that of being closer, more faithful to the established doctrine. A religion is then the frequently elaborate structure of logic and passion that is built on this foundation of implicit belief.
Science is very different in that it does not have any a-priori beliefs. This may come as a surprise for anyone who has done any mathematics in school. After all, aren't acceptance of Euclid's propositions essential for the proof of his theorems? Of course, but then, mathematics is logic, not science. In fact, the connection between Mathematics and religion becomes easier to understand when we recall that the Pythagoreans kept their knowledge of irrational numbers a secret because it destroyed their vision of the world being a balancing act of fractions. A excellent case of logicians taking on the worst aspects of religious fanaticism. Science on the other hand, simply put, is the study of physical reality. There is no Truth, no essence, no inner meaning. There is nothing above or beyond the world of physical reality as perceived by our senses. This is the supreme governing principle of all good science. This is not to say that science requires there to be no God. It simply is not interested in it. Any question that cannot be put in a way that lends itself to being tested by observations of physical reality as it appears to our sense perception does not concern science. The methodology of science is fairly uncompromising. Imagine having to lay a road on uncharted territory, but the road has to be built to very strict and difficult specifications and every inch has to be constantly tested over and over again. And nothing including the eventual direction has been made known to you, the construction engineer. This is what scientific research is like. Based on various observations a working hypothesis is accepted. Treating these hypotheses as propositions a theory is built. This theory is used to make predictions about future observations. The veracity of these observations confirm or demolish the working hypotheses we had accepted in the first place. Of course the situation is more complex that this suggests. The confirmation (or otherwise) of the predictions of the theory is used to accept not simply the original hypotheses but also logical structures that are used to arrive at these predictions from the hypotheses. The confidence scientists have in their collective work draws a lot from the cumulative positive evidence from varied and repeated observations and is dynamic and is not made or broken by single dramatic experiments or observations.
This then is the method of Science, but what is the aim? This is what I mentioned before. To understand Physical Reality. But what we mean by understanding is very specific. Scientists attach a very functional meaning to the word. When a scientist says she understands a physical process, she means she can describe it in terms of a small set of simple interactions between independent blocks of subprocesses that she previously understood (in the same sense). And now a paradox is apparent. Say the scientist proceeds to break the process into interactions of simpler and simpler pieces. What about the simplest unbreakable block of process? How does she understand that atom of Physical Reality? Once again our sense perceptions come to the rescue. We understand the simplest block of process in terms of observations of Physical Reality. Let me try to explain this by the use of an example. Consider the solar system; a complex physical process. Our scientist can understand the orbits of the planets using Newton's theory of gravitational attraction. How about understanding Newton's theory? Well, she can climb up the same stairs of the tower of Pisa that Galileo was supposed to have and can drop the ball of iron and ball of wood and ensure they fall to the earth at the same time. She can roll little balls on planks of wood and time their speed of descent. She can measure the attractive forces between two balls of steel hung from a balance. And then once she's gone through the same constructs of logic Newton did to arrive at his theory she can claim to have ``understood'' the physical process of the planets orbiting around the Sun.
Naturally the real process of research is never as simple as this. Even the simplest physical process is too difficult to understand by scientists. So to understand them scientists simplify them. This is perfectly acceptable so long as the scientists can characterize clearly and quantitatively the nature of their simplifications and the effect they will have in the final predictions of their theories. Which brings us back to the cartoon. In the understanding of the origin and evolution of the Universe I do not include that of the origin of life on Earth, the dynamics of economical, social or political systems, or those many other questions that occupy many of us. The peculiar questions that we will focus on will become obvious through the course and are those that occupy scientists who describe themselves as astronomers.