Principles of Celestial Navigation

A Day in the Life of a Navigator » Preparing for a Morning Observation

nautical twilight is defined as the time of day (morning and evening) when stars are visible but there is enough light to also discern the horizon. morning and evening three-star fixes are done during nautical twilight.

Before navigators can start a series of observations, they need to decide which objects will be visible, and which will give the best lines of position. To take a sight using stars, it must be light enough to see the horizon, but dark enough to see the stars. The period when the sky brightness allows for this is referred to as "nautical twilight." The times of twilight and sunrise are determined by using The Nautical Almanac; strip forms can be used to help.

photo of Rude star finder set for moring three-star fix

The decision of which objects to observe must be made well before morning twilight. Two methods are often used to identify the best objects. We could use a 2102-D Star Finder, also known as a Rude Star Finder, which is a plastic celestial map that shows the positions of stars used for navigational purposes.

photo of Pub 249, Sight Reduction Tables for Air Navigation

Or we could use the book, "Publication 249 Sight Reduction Tables for Air Navigation (Selected Stars)," also known as "Pub 249." Both the Star Finder and Pub 249 indicate which objects are best to observe, and also give us their rough positions in the sky so we know where to look for them.

photo of Pub 249, Sight Reduction Tables for Air Navigation with the Rude Star Finder

Pub 249 is also known as AP 3270 outside of the U.S.

Using the Rude Star Finder and Pub 249 are not difficult, but are outside the scope of this module. For our example location and date, the three best stars are Capella, Deneb, and Fomalhaut.