Universal time is a commonplace quantity, and yet it has no generally accepted definition. The expression for sidereal time is often quoted as a definition, but this does not convey much meaning to the average person. Also sidereal time is measured from the equinox of date, and so involves the precession formulae of the stellar reference frame, and hence may not be ideal for other modern techniques of observation. Guinot (1979) has proposed a broad definition of UT1:

“UT1 is an angle which is proportional to the sidereal rotation of the Earth, the coefficient of proportionality being chosen so that UT1, in the long term, remains in phase with the alternation of day and night. In some applications, UT1 can be considered as a non-uniform time scale”.