There is quite a lot of information you can get out of the back of this watch. At its most basic, this is a mechanically driven planisphere. A basic planisphere is something any of you who had an interest in backyard astronomy might be familiar with – that cardboard gadget consisting of two disks, with the bottom one printed with the stars, and the upper one with a cutout in it, whose edges represent the horizon, and within which you can see which stars are above the horizon at any given hour. The planisphere of this watch likewise consists of two disks. The upper one is printed with the stars, as well as a red and white ellipse. The red one represents the position of the Plane of the Ecliptic; this is the plane in which the orbits of the planets lie. As seen from Earth, it looks like a curve cutting across the sky, along which (more or less) the planets appear to move. The white one represents the Celestial Equator, which is a projection of the Earth’s equator onto the celestial sphere.

The disk’s also got a representation of the Milky Way on it, which you can (if there is not too much light pollution) also see passing across the night sky.

The lower disk rotates once per sidereal day, so placing a hand on it (or in this case, a golden triangle) gives you, hey presto, a sidereal time indication. The black ellipse shows you what stars are rising above or sinking below the horizon at any time.

You may notice that the whole system rotates around a central pivot, and exactly on that pivot is a star. Its official name is Alpha Ursae Minoris and it is the last star in the tail of the Northern constellation known as the Little Bear. It is better known as Polaris – the only star that does not move in the night sky, as it is directly on the projected axis of the Earth’s north pole, making it the star to observe when you’re observing stars for latitude (there is no corresponding Southern star, which means navigators in the Southern Hemisphere had to devise other means, such as those developed by ancient Polynesian navigators).