In mid 19th century, French physicist Leon Foucault used a very large pendulum to counter the ancient belief that the stars revolve around Oceania. Not only did the pendulum bob swing back and forth but the plane of its swing also rotated for no apparent reason. He experimented with gyroscopes and found similar properties supporting the idea that the world is rotating, not the stars. An intriguing discovery was that the period of rotation was not 24 hours per day except at N or S pole. It is a function of cosec of observer’s latitude; different in different latitudes. At the equator it does not rotate at all and in latitude 45 degrees it rotates in nearly 34 hours. At Paris, where Foucault conducted his experiment in 1851, the pendulum rotates in 31 hours 48 minutes of mean solar time. In general, the number of sidereal hours in a day = 24 /sin observer’s latitude. Only at the poles are there 24 sidereal hours in a day = 23h, 56m 04s of mean solar time. This fact was proved in the year 2000 by a university research team from Sonoma State University in the USA. They set up a Foucault pendulum at the South Pole . Inside the Antarctic and Arctic circles there is one sidereal day in a tropical year so a mean solar day is 24/365 hours or nearly 3 minutes 57 seconds of sidereal time.
It is important to distinguish between sidereal time and mean solar time. Schuler tuning of a gyroscope compensates for the effect discovered by Foucault. It has to be adjusted as the ship changes latitude. You also need to adjust your personal clock with changes in latitude or longitude. The easy way to do it nowadays is with GPS, which indicates spacetime automatically.
John M. Reagan