The major benefit of these complications is that they can be sensitive to hemisphere, but they have one fundamental inaccuracy: they do not account for the difference between mean solar time and true solar time. In other words, you might have your clock set to GMT, but if you are on the far eastern or the far western point of the GMT time zone, you will have significantly different experiences of the apparent motion of the sun and the northerly direction in relation to it. Remember, the hour angle principle, whereby the sun moves at 15º of arc every hour, therefore any location separated by exactly 15° of longitude will be exactly I hour of true solar time apart. For example, let’s assume that it is 12:00 in Greenwich, England. Although they share the same GMT mean solar time zone, at the far eastern point (say Inekar, Mali) and far western point (say Reykjavik, Iceland) the local true solar time will be 12:12 and 10:36 respectively. It is understandable therefore how, despite both being in the northern hemisphere and sharing a time zone, the position of the sun and the respective true north will be a little off from either location. One special watch was designed to handle this issue.
The Arnold & Son True North Perpetual allows the user to adjust for local longitude, as well as the equation of time, in order to identify the true north using the watch. It is calibrated to the northern hemisphere. First, the crown at 8 o’clock is used to adjust to exact local longitude. This causes the internal rotating bezel to rotate and is read by tracking the small arrow at 6 o’clock against the degrees indicated in the black section of the ring. If you are located 5° east of the formal longitude of the time zone (GMT=0°, GMT+I=15°,etc.), you rotate the bezel so that is moved to 5° from True North in the easterly direction of the bezel. Then it is as simple as pointing the 24-hour sun hand to the direction of the Sun and you have yourself a True North bearing. The watch is even capable of adjusting the difference between the mean solar time and the true solar time. This is important because, depending on the time of the year, the apparent sun will cut a faster or a slower path across the sky versus the mean solar time. The watch has an equation of time register at 7 o’clock, which indicates how many minutes deviation there is at any given day. What is particularly special about this watch is the way this equation adjusts the reading of the sun hand. The thin disc marked “Sun Slow/Sun Fast” at 12 o’clock is connected to the outer disc, and will move the respective true solar time according to the equation. So a+10 minute equation will mean that the true solar time is running ten minutes faster than mean solar time, and this will be translated to a two-notch clockwise adjustment of the chapter ring.
The Richard Mille RM 60-01 is a very useful compass complication, able to be used in both hemispheres. The central red UTC hand is adjusted by the pusher at 8 o’clock. In addition to the orienteering functions the watch has subsidiary seconds at 3 o’clock, a big date at 12 o’clock, month indicator at 4 o’clock, central flyback chronograph seconds, with a sub-dial at 9 o’clock that can be read as a 60 minute countdown register (reading the yellow dial against the yellow arrow) or regular 60-minute chronograph register (reading the yellow 60 marker against the green register).
