WW2 Royal Navy HS 7 Navigator's Sidereal Timer

A good WW2 Royal Navy issue stopwatch calibrated for Sidereal timing. This adjustment allows navigators to use the watch to time Stellar observations.Sidereal Time factors that the earth takes 23 hours and 56 minutes of Solar time for it to complete a revolution - Not 24 hours. This difference is equivalent to 10 seconds per hour, and navigators need to account for this. This is why the Hydrographic Survey adjusted their stopwatches specifically for this task and marked them HS7 accordingly.Presented in crisp condition, this watch made by Lemania, is fully working.