Star Fixes at Sea- at Twilight.

Star Fixes at Sea

Greetings
Sextant sights for a navigation and position fixing is an art and a science and doing it gave a sense of  accomplishment and immense  job satisfaction to us Seafarers till GPS took over in the 90s.

In the 60s, esp  on West Coast US port calls, we used to have USCG Sea Charts titled Pacific Pilot issued to ships at U.S.Ports. Apart from weather and current information, the reverse of the chart  had a fund of interesting information. One such was the WW2 experience of naval and merchant navy navigators taking sextant sights after getting eyes adjusted with a special reddish tint goggles to have a delayed time for taking sights,before the nautical twilight. In short for adjusting the "Eyes to a horizon" to fix position by star fixes after identifying suitable  navigational stars.

Twilight in many places is short and so this was a good idea.In any case better that a basic Dead Reckoned Position. Possibly more needed for tracking own vessel's position in submarine infested waters of the Atlantic and Pacific. Historical details of submarine torpedo attacks are spine chilling and also the tonnage sunk by the German Submarines tells; all this, despite the excellent Convoy systems and the effort taken to stay in position and on course and speed.                                   Looking at the definition of Twilight i.e. before actual day break when the sun rises above the Visible horizon.

Astronomical dawn is defined as the moment after which the sky is no longer completely dark. This occurs when the Sun is 18 degrees, below the horizon in the morning i.e. astronomical dawn (and also at dusk)

Nautical dawn is the time at which there is enough sunlight for the horizon and some objects to be distinguishable; formally, when the Sun is 12 degrees below the horizon in the morning & also just after sunset in the evening

Civil dawn is the time at which there is enough light for objects to be distinguishable, so that outdoor activities can commence; formally, when the Sun  6 degrees below the horizon in the morning. The sky is bright and all clearly seen.
But-Taking sights is not practicable then as stars are no longer visible in the bright light.

The duration of the twilight period between dawn and sunrise varies greatly depending on the observer's latitude, from about 15  minutes in the equatorial regions to more in the higher Latitudes. Our data and books tell us that it is many hours in polar regions to weeks at the poles.

                                 Going for morning sun sights (long by chron) at about 0800 hrs

                                                               Plotting the position
Celestial Navigation had been the mode for several centuries and served the shipping well. Allthings change with Time & Technology. I feel like an Ancient mariner when writing this blog post-
TR