The Instrument DBS Equation Sidereal is a
tribute to two watches (Nos. 1 and 2) made by John Roger Arnold
that showed mean solar and sidereal time on two separate subdials.
The movements of these two watches, made between 1796 and 1799,
featured some of the most famous inventions of father and son,
including their fabulous thermo-compensated Z balance, expansion
escapement and gold helical spring. It is no exaggeration to say
that they represented the state of the art in the micromechanics of
Watches with a sidereal time display were
extremely rare at this time. After the Arnolds had paved the way,
Breguet followed in their footsteps and produced a few watches with
a sidereal time display and a similar dial layout (such as No.
3863, sold in 1824).
Creating a precise sidereal time display is a
tall order. Simply adding a second gear train to a standard
movement showing mean solar time is not a viable solution because a
mean sidereal day is about 23 hours, 56 minutes, 4.091 seconds,
which is not a round fraction of the 24 hours in a mean solar day.
This means it is virtually impossible to obtain the correct
rotation speed using gears. To guarantee the kind of precision
demanded by Arnold & Son, the only choice was to create a
movement with the correct rotation speed from the start: in other
words a movement specifically for sidereal time. The answer was a
totally new movement with a double barrel/gear train and double
balance/escapement running at different speeds that enables the
watch to display mean solar time and mean sidereal time
simultaneously. This technical solution has the additional
advantage that neither of the two indications saps energy from the
other. Watches with a double balance and escapement are extremely
rare and have almost disappeared, so it is with some pride that
Arnold & Son have revived the tradition.
The DBS Equation Sidereal has a perfectly
symmetrical arrangement of the bridges, barrels and gear trains.
The symmetry is continued on the dial side, where the two adjacent
balance cocks create a harmonious circle. Sidereal time is
displayed on the left subdial and mean solar time on the right. In
addition, a subdial at 12 o'clock indicates the equation of both
times on a 24-hour basis, which allows the user to measure the
difference between mean solar time and sidereal time and to
ascertain whether the time in both zones is a.m. or p.m. The watch
has a long central permanent seconds hand (for mean solar time).
The two barrels are wound using the crown on the right, but
sidereal time and mean solar time are set separately using the
crowns on the left and right, respectively. This ensures that
neither of the displays is changed or manipulated by accident.
Sidereal time is the timekeeping system used by
astronomers to track the direction needed to point a telescope at a
particular star in the night sky. A mean sidereal day is measured
by the rotation of the earth relative to the stars rather than the
sun and lasts 23 hours, 56 minutes, 4.091 seconds. It is
approximately four minutes shorter than a mean solar day because,
owing to the movement of the earth round the sun, the time that
elapses before a distant star appears at the same point is slightly
less than the time it takes for the sun to be directly over the