The purpose of this section is to give a very brief guide to the major dynamical properties of some real astronomical systems. It is not meant to be a review of the latest observations, for these change almost daily and often a long time must pass before they can be put into proper perspective. Specific astronomical observations, for understandable reasons, are seldom repeated. Often the pressures of time on large telescopes, or astronomical careers, are too great. The result is that, unlike laboratory physics experiments, it is difficult to gauge the real uncertainties (in contrast to the formal error limits) which surround particular observations. Another feature of astronomical observations is that a good deal of theory is usually needed to make the observation itself. This is because the systems we observe are complicated and seldom yield a basic physical quantity in a straightforward manner. The measurements we want must usually be strained through a network of interpretation.

Steady advances in observing technology mean that, at any given time, the most exciting frontier observations will often be near the limits of available instruments. Frequently these observations will just be able to rise above the ‘noise level’, and astronomers then speak of ‘a two- or three-sigma effect’, sigma being the standard error added by noise.