Sidereal time is a way of measuring time by how fast the Earth turns relative to the distant fixed stars, not the Sun. Astronomers are its main users. Combined with celestial coordinates, it makes it simple to work out where objects sit in the sky, and so which way to point a telescope.
How long is a sidereal day?
A sidereal day is one turn of the Earth measured against the fixed stars. It lasts roughly 23 hours, 56 minutes and 4 seconds. The average solar day, measured against the Sun, runs close to four minutes longer. The gap looks small, but it adds up day after day, so the two times drift steadily apart. A watch that shows sidereal time therefore cannot simply borrow the hours of an ordinary dial. It needs its own count, running at its own rate.
Where does sidereal time appear on a watch?
In watchmaking, sidereal time belongs to the astronomical complications. It shows up in sky charts and in dedicated sidereal displays. Watches with a sidereal complication are niche pieces. Their owners often use them for astronomy or for celestial navigation, where the position of the stars is the point. IWC Schaffhausen and Patek Philippe are among the makers that have offered the complication.
Did older watches show sidereal time?
Between 1796 and 1799, John Roger Arnold made two watches, Nos. 1 and 2. Each showed two kinds of time on separate subdials. One subdial gave mean solar time, the time of daily life, set by the Sun. The other gave sidereal time, set by the stars. Watches with this display were very rare then. Reading the two subdials side by side showed how the two times differ.
How does a watch show both times?
Showing both times is harder than it looks. Simply adding a second gear train to a normal movement does not work. Arnold & Son’s modern DBS Equation Sidereal takes a fuller route. It uses two barrels, two gear trains and two balances. The two balances run at different rates, one for mean solar time and one for sidereal time. In effect, each time has its own power, its own train and its own regulator.
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