Emergency Celestial Navigation

NOTICE: this page is under construction and currently only a draft with a collection of ideas to be elaborated!

Increasing reliance on complex electronic navigation systems, which are nowadays also generally available on pleasure yachts, has changed the perspective of emergency navigation. Today, it is more likely that a navigator will suffer from failure of electronic devices or the associated power supply and be left with little more than a sextant and some tables to navigate with, than that he will be forced to navigate a lifeboat.

In the event of failure or destruction of electronic navigation systems, navigational equipment and methods may need to be improvised. The following sections are a collection of topics related to this.

Timekeeping

Today, precise electronic watches or chronometers have become inexpensive. Even if they rely on a healthy battery, they can operate almost maintenance-free over ten years or more. They must be regulary syncronized to UTC time obtained e.g. from a GPS receiver. Alternatively, the UTC time offset can be recorded as described in the section The Chronometer. Except for the chronometer used for daily naviation, at least one additional such maintained (battery) and synchronized device should be kept as backup in the emergency "grab bag".

Almanac Data

Prerequisite for Celestial Navigation is the availability of some basic almanac data. For the Sun and stars, it is possible to compile compact long-term (10 years and more) almanac data which are accurate enough to make them suitable for rough navigation. An appropriate version of such an almanac can easily be kept in the sextant box. The apparent motion of the Moon and the Planets on the other hand are not regular enough to achieve such compact almanacs. For these celestial objects a "fully-featured" Nautical Almanac is required.
Some details on the construction of long-term Solar Almanac are available.

Combining these long-term Sun Ephemerides, with the simplified Sight-Reduction Method as proposed by Robert Doniol, allows for a basic electronic-free method for Sun-based Celestial Navigation, which consists of only four pages of tables. These will fit in any sextant box and with the knowledge of how to use both the instrument and the required calculation schemes, a complete emergency navigation kit can be assembled.
This can be extended with the tables to determine Latitude by Polaris (three pages), applicable if navigating the Northern part of the world.

Without appropriate Sight-Reduction Tables and electronic calculator it is still possible to obtain a result for the Sight-Reduction calculation with the following graphical method. I have to credit Marco A. Costa, for the insight that there is a simple method of graphically multiplying Sine and Cosine values.

The method is demonstrated in the following Worksheet: Worksheet for graphical Sight-Reduction

Depending on the staring values for Declination, GHA, Latitude and Longitude, the accuracy of the method may be within half a degrees. But this is only achievable if the graphics are done with highest possible accurateness (which contradicts the emergency situation). For unfavourable combinations of Declination, GHA, Latitude and Longitude the achievable accuracy will be in the range of two degrees.

Bygrave Slide Ruler for Sight Reduction

Recently, I received a mail from Tony, who pointed me to a very interesting option for emergency navigation:

Dear Erik,

In my study of the availale sight reduction options I learned about the Bygrave slide rule.
http://fer3.com/arc/m2.aspx/New-compact-backup-CELNAV-system-changed-for-archive-LaPook-feb-2009-g7414
The original user-manual for the Bygrave slide rule:
http://fer3.com/arc/m2.aspx/New-compact-backup-CELNAV-system-LaPook-feb-2009-g7419

Later Mr Robin Stuart posted his nice PostScript files for flat scales printing:
http://fer3.com/arc/m2.aspx/Postscript-code-for-making-Bygrave-Scales-Stuart-jan-2015-g29918

With just 3 (three) slide rule operations and very little arithmetics
in between - one obtains a LoP and intercept without having to have
Ageton's or any other bulky SR tables.
I'm very much impressed!

Warmest regards,
Tony