From Steve Jones
Matthew Chalmers perpetuates a fallacy that a day is the time taken for the Earth to “turn once about its axis” (7 February, p 39). That is actually the sidereal day. However, it is not the source of our Babylonian-influenced clock units, nor what is commonly called a day. It is in fact the solar day, or the average time taken for the Earth to rotate once relative to the Sun. The difference, of a little less than four minutes, comes from the necessity of the Earth to rotate almost 361 degrees in a 24 hour period in order to compensate for its orbit around the Sun.
Maidenhead, Berkshire, UK
