Today, watches with double barrels and double gear trains are employed mainly for the purpose of conserving energy, with one gear train generally driving the chronograph and the other, regular hours and minutes. Separating them extends the power reserve for each. Arnold & Son has gone a step further and used the concept to create two separate running times altogether – one for sidereal time and the other for mean solar time. Sidereal time is based on the Earth’s rate of rotation measured relative to the stars, which is slightly less than the mean solar day of 24 hours.
Arnold & Son was one of the first brands to create a watch with sidereal time, around 1796. The new Equation Sidereal is part of the DBS collection, which is inspired by the timepieces produced during this era, when they played a crucial role as navigational aids and were considered scientific instruments.
The brand determined that creating a precise sidereal time display required more than simply adding a second gear train to a standard movement showing mean solar time. 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 absolute precision, Arnold & Son created a separate movement with the correct rotation speed, one created specifically for sidereal time. Thus, the watch displays mean solar time and mean sidereal time simultaneously – with the additional advantage that neither of the two indications saps energy from the other.