From Chris Finn
Bob Ladd’s letter (20 March) quotes the frequency of rotation of the Earth as 69.5 microhertz.
As the sidereal day is around 23 hours 56 minutes – or 86,160 seconds – the Earth’s rotational frequency is the reciprocal of that, a shade over 11.6 microhertz, or 1/6 of the figure he quoted.
Beverley, East Yorkshire, UK
