Longines Watch Co., Movement No. 7513094, Case No. 7513094, Cal. 21.29, 140 x 160 x 58 mm, circa 1945

An aviator's extremely rare navigation watch with split seconds chronograph and 36h-power reserve indication, a simplified version of a "Siderograph"
Case: aluminium. Dial: silvered, inner turnable bezel. Movm.: 2/3 plate movement, Glucydur screw balance, Glucydur lever.

The Siderograph takes its name from the word "sidereal" or star time. The mechanism was originally developed for astro-navigation.
To measure sidereal time, an ordinary watch is speeded up by adjusting the the mechanism so that it will indicate 24 hours of sidereal time for approximately every 23 hours 56 minutes approximately of solar time. The Siderograph is a clock and a navigation instrument at the same time; it keeps the Greenwich time as a reference.