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.
Summary of Contents for COSMOS
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