The number N of low tides in a given 6-hour interval of the day varies typically by a factor of order 2. A simple formula is derived relating Nmax/Nmin to the ratio R of the solar and lunar semi-diurnal tidal constituents. This is shown to be quite accurate provided that the diurnal constituents are not large, and that the shallow-water tides are negligible. Hence R could often be deduced from observations of Nmax/Nmin alone, with the aid of only a timepiece.