Invented by Longines in 1938, the ?Siderograph? measures Greenwich (0 longitude) Sidereal or Civil Time in hour angles, minutes and minutes of arc, and can be considered the final evolution of the marine chronometer as well as the angle hour watch. These timepieces allow the user to navigate at night as well as during the daylight hours more accurately than the Weems and Lindberg systems for nocturnal navigation that were heavily reliant on radio time signals. The ?Siderographs? were based on the chronometer calibre 21.29 which was at the time Longines?s most accurate movement. The caliber was modified with the addition of the split-seconds mechanisms and anti-magnetic beryllium pallet fork. The movement was then cased in a heat-toughened aluminum case. This material was chosen due to the fact it is a non-magnetic, light, non-ferrous material perfectly suited for use for use in both aeronautic and maritime applications. The present lot is one of a handful of examples known to exist in a gimballed deck box, an indication it was intended for navigational use aboard a ship.

Here follows an extract from British patent No. 525,767 which explains how the Siderograph functions. ?The subject of the present invention is an Horological instrument (watch of the like) to determine, particularly but not exclusively in aviation and navigation, the hour angle of the sun or of another star in relation to the Greenwich meridian. The object of the invention has the advantage of giving directly in angular value Greenwich time, solar mean-time or sidereal time, according as its movement is regulated to Greenwich civil time or to sidereal time. It has thus for its object the simplification of calculations, particularly with a view to modifying the Ephemeral tables of the stars, suggested in aeronautical circles, by introduction into these tables of the right ascensions of the star in an angular value. Modern aerial navigation methods tend in effect, more and more, by simplifying the entry of the tables known as the ?Connaissance des Temps? or of the Nautical Almanac to reduce and simplify geographical coordinates. The Horological instrument, whether regulated to civil or to sidereal time, comprises a watch movement actuating at the centre of rotation and each being displaced in relation to a fixed circular scale. These three scales are disposed concentrically in relation to the centre of rotation of the hands and comprise, the first, 10 equal divisions each corresponding to 1 degree of arc, the second 36 equal divisions each of which corresponds to 10 degrees of arc, and the third 300 equal divisions each of which corresponds to 2/10 minutes of arc. When the Horological instrument is regulated to civil time and is used for observations with the sun, the hands corresponding to these scales make a complete rotation, the first in 40 minutes over the inner scale, the second in 24 hours over the intermediate scale and the third in four minutes over the exterior scale. The annexed drawing illustrates by way of example one embodiment, in the form of a watch, in the front view of the object of the invention, regulated to civil time. The watch illustrated encloses a movement capable of being rewound by means of the stem h. This movement actuates three hands a, b, c, having a common centre of rotation and each being displaced in relation to a circular scale d, e, f, the three scales being concentric in relation to the centre of rotation of the hands. The hand b, is displaced on the inner scale e, and makes one complete rotation in 40 minutes in civil time, say one rotation for 10 degrees of arc: this scale is divided into 10 equal parts each corresponding to 1 degree of arc. The hand b has for its object to indicate the degrees of arc contained between two consecutive divisions of the middle scale. The hand a makes one complete rotation in 24 hours, say one rotation for 360 degrees of arc, the scale d comprising 36 equal divisions each corresponding to an arc of 10 degrees. The hand c displaced on the outer scale f makes one complete rotation in four minutes, say one rotation in 1 degree of arc, this scale comprises 300 equal parts each corresponding to 0.2 minutes of arc. The hand c has for its object to indicate the minutes of arc comprised between two consecutive stages of the interior scale e. The time indicated in an angular value by the watch is represented by the sum of the readings effected on each of the three scales d, e, f. Due to the rapid movement of the hand c, one begins always by reading this hand, the two other readings being effected in any order. In order to determine the Greenwich apparent hour angle of the sun, the watch is regulated to civil time. By introducing into the reading of the watch the equation of time given by the tables of the ?Connaissance des Temps? there is obtained the Greenwich hour angle of the sun directly in angular value. For example, on the 20th January 1935, the Ephemerides of the sun give the following readings:

Civil time by true noon of Greenwich: 12h. 10m. 57.33s Equation of time: 12h. 10m. 57.33s-12h. 10m. 57.33s In decimal value: +10.9 min. In angular value: 2?? 43? of arc. Greenwich hour angle of the sun: 130° 9.2? civil time Equation of time: 2° 43.5? apparent time Greenwich hour angle of the sun: +127° 25.7? -+127° 25?.42? apparent time The watch gives the Greenwich hour angle in angular value of civil time, exclusive of the equation of time: the reading is effected first on the hand c due to its rapid movement.

Hand c: 9.2? Hand b: 130 degrees Hand a: 0 degrees In order to determine the Greenwich hour angle of a star, the watch will be regulated to sidereal time, the sidereal day being about 23h. 56 m. By introducing into the reading of the watch the right ascension of the star, given by the tables, there is obtained, directly the Greenwich hour angle of this star. Let us determine the Greenwich hour angle for Ursa Major the 20th January 1935.

Right ascension Ursa Major: 10h. 59? 47.82? in sidereal time. In angular value: 164° 56? 53.7?=164° 56.9? The reading of the watch gives us: Hand c: 9.2? Hand a: 130 degrees Hand d: 0 degrees Sidereal time for Greenwich: 130° 9.2? expressed in angular value Right ascension: 164° 56.9? Greenwich hour angle for Ursa Major: 34° 47.7? =-34 ° 47? 42? Modern aircraft moving at such a speed, it is sufficient to take an approximation of minute, of a decimal, and to reduce it then to seconds. In the examples given above, two different watches are referred to, one regulated to civil time and the other regulated to sidereal time, the two types could be mounted on a table, comprising in addition an ordinary instrument board watch indicating mean time or civil time in hours, minutes and seconds. The watch could be provided with a chronograph watch movement or with a modified split-second hand arrested only whilst a push button is being operated. Thus the watch will be provided preferably with a simplified split-second hand mechanism and will comprise for this reason four hands of which two hands c move over the outer scale f. One of these hands c will be in continuous movement, while the second, a split-second hand, will be operated by a split-second push piece, (without chronograph). The two hands are superposed and move together. When an observation is made, the push piece is pressed and the split-second hand is stopped for the reading, while the other hand continues its movement. As soon as pressure on the push piece is released, the split-second hand will rejoin the continuously moving hand and the two hands will move again together. It is to be understood that the hand c could be a usual chronograph or stopwatch hand, operated by a known mechanism, by means of one or two push pieces.?