Hi Nicolas,
I could fumble for a proper explanation and history or filch from Sotheby's from a cased example....
"Longines developed the Siderograph to satisfy the increasing need for accurate aeronautical night navigation during the 1930s. Existing systems had relied upon radio signals but the Siderograph enabled navigation by the stars, (astronavigation), which had previously been very difficult in a fast-moving vehicle. Using Longines’ most accurate movement, the Siderograph is designed to operate on Greenwich sidereal time with the dial calibrated in hour angles, and minutes and seconds of arc. Used in conjunction with a sextant, and a chronometer set to Greenwich Mean Time, the navigator uses the split-second functions of the Siderograph to set the vernal equinox hour angle and thus pinpoint exact position."
I think Goldberger says there were 4 or so made in the version shown....