It gave mean solar time, mean sidereal time, equation of time, and could chart the phases of the moon.
If it misses this window, further opportunities occur at intervals of 23 hours and 56 minutes a period known as a sidereal day.All this may seem a lot of effort to confirm a theory that most physicists assume is true.
To track an object, the telescope's polar axis is driven smoothly by an electric motor at a sidereal rate namely, at a rate equal to the rate of rotation of Earth with respect to the stars.
Because the Earth rotates once a sidereal day, or 360° approximately every 24 hours, its rate of rotation may be expressed as 15° per hour, which corresponds to the rate of rotation of a Foucault pendulum at the North or South Pole.
Venus spins very slowly, taking about 243 Earth days to complete one rotation with respect to the stars the length of its sidereal day.
If the stars are used, then the interval is called the sidereal day and is defined by the period between two passages of a star (more precisely of the vernal equinox, a reference point on the celestial sphere) across the meridian: it is 23 hours 56 minutes 4.10 seconds of mean solar time.
These discoveries were earthshaking, and Galileo quickly produced a little book, Sidereus Nuncius (The Sidereal Messenger), in which he described them.
At the North Pole, latitude 90° N, the relative motion as viewed from above in the plane of the pendulum's suspension is a counterclockwise rotation of the Earth once approximately every 24 hours (more precisely, once every 23 hours 56 minutes 4 seconds, the length of a sidereal day).
This period of constant length, far more convenient for civil purposes, is the mean solar day, which has a duration in sidereal time of 24 hours 3 minutes 56.55 seconds.
It is longer than the sidereal day because the motion of the Earth in its orbit during the period between two transits of the Sun means that the Earth must complete more than a whole revolution to bring the Sun back to the meridian.
In the Commentariolus, Copernicus postulated that, if the Sun is assumed to be at rest and if Earth is assumed to be in motion, then the remaining planets fall into an orderly relationship whereby their sidereal periods increase from the Sun as follows: Mercury (88 days), Venus (225 days), Earth (1 year), Mars (1.9 years), Jupiter (12 years), and Saturn (30 years).
