Girard-Perregaux COSMOS, User Manual

INSTRUCTION FOR USE

ASTRONOMY AS A TIME MEASUREMENT INSTRUMENT

The measurement of time finds its origins in the observation of astronomical phenomena such as 
the motions of celestial bodies in the sky. Most of the calendars have been developed according to 
the movements of the Sun as Man observed a regular alternation of diurnal and nocturnal periods 
of time. Thus, it is customary to measure a day according to the position of the Sun with respect 
to the Earth.

A sidereal day and a solar day must be differentiated in terms of duration. A sidereal day lasts 23 
hours, 56 minutes and 4 seconds, which corresponds to a 360 degrees full rotation of the Earth 
on itself, independently of its revolution around the Sun. Meanwhile, a solar day has an average 
duration of 24 hours corresponding with the time interval passing between two culminations of 
the Sun on a given meridian. This positive difference of 3 minutes and 56 seconds is due to the 
revolution of the Earth around the Sun, leading to a rotation of more than 360 degrees of the 
former.