Astronomy is mostly (footnote - but not exclusively...) concerned with analysing the electromagnetic radiation emitted and absorbed by the constituents of the universe. In that sense it is an observational rather than an experimental science: we have little control over the circumstances in which the radiation is produced or influenced on its travels to the telescope. In practice, modern astronomy is concerned with the measurement of the whole spectrum from high energy (very short wavelength) gamma radiation to the lowest frequency (wavelengths of metres) radio waves. In this short essay we will use the terminology associated with optical light: electromagnetic radiation with wavelengths ranging from about 0.4 to 0.7 microns (thousandths of a mm). The concepts and measurements can, however, be carried over to other wavebands - although the instruments used to make the measurements will look different.

The radiation we collect with our telescopes carries a set of information with it which we are able to separate and measure with instruments. The direction from which the radiation arrives is determined by the pointing and spatial resolution of the telescope. This is the science of imaging and astrometry and the precision with which the measurements are made is determined by the resolution of the telescope and, if the telescope is located on the ground, by the properties of the atmosphere. The variation of the intensity of the radiation with wavelength - the spectrum - is measured with a spectrograph, an instrument which 'disperses' the light into its constituent colours. The spectrum contains a huge richness of information about chemical composition, temperature, density, state of motion and esoteric properties such as magnetic field of the source of radiation and the material through which it has passed on its journey to the telescope. Spectroscopy is the foundation of the science of astrophysics and has been the driving force behind the construction of ever-larger telescopes on the ground and in space.

Although most objects studied by astronomers are very large, very far away and therefore usually unchanging, there are celestial sources which vary on timescales as short as thousandths of a second. Some of these variations are intrinsic to the source - which may be very small and intense - and some are due to influences operating during the propagation of the radiation to us. Both processes carry astronomical information. Pulsars, pulsating and eclipsing stars, active galactic nuclei and gamma-ray bursters are all examples of these time-dependent phenomena.

The properties of direction, colour and time variation would seem to cover most of phenomena of interest to astronomers. These quantities are not, however, a complete description of the light entering our telescopes.