Support for GPS Backup with a Mark 3 Sextant

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Video illustrating the process.

Expanded Tables (to remove or minimize interpolation)

Practice with Dec and GHA

PRACTICE Finding sun's declination using the Backup Sun Almanac

PRACTICE Finding Sun's GHA using the Backup Sun Almanac

Finding UTC of LAN

Finding UTC of LAN using the Starpath Custom Sun Almanac

Finding UTC of LAN using an Analemma

Hi-res analemma graphic

Arc to Time Table

PRACTICE Finding UTC and WT of LAN

Miscellaneous related articles

Accuracy of Back up Cel Nav Using a Mark 3

Progress to Weather ... a topic that comes up in some exercises that use this book

The book mentions putting together a backup kit made up of this book, a Mark 3 sextant, and a rated watch. We have such a kit available which includes all other aids needed to for ocean navigation. See contents at Starpath GPS backup kit.

PRACTICE with Latitude by Polaris

Example 1: With the sky below...

July 19, 2021, UTC = 05:56:28
DR: 44N, 141W
Hs Polaris = 43° 23.4'
IC=0
HE=10ft
What is your latitude? Solution 1

Example 2: with the sky below...

July 14, 2020, UTC = 14:42:44
DR: 25N, 151W
Hs Polaris = 25° 29.4'
IC=0
HE=10ft
Find your latitude. Solution 2

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Errata (back)

Page 16. Discusses Lon uncertainty of ± 8' (7.5' rounded up) or so, assuming we can determine the time of LAN to within 30 seconds. That is true, but it is difficult in most cases to find the middle of the LAN curve to that precision. One minute is a more conservative estimate of the uncertainty following the methods outlined, and this would lead to a Lon uncertainty of ± 15'. So We might best think of the Lon uncertainty as more generally equal to ± 10' to 20'. This is discussed in our blog: Accuracy of Backup Cel Nav using a Mark 3 Sextant.

Page 35, Example 1, August 25 should be August 15.

Page 8, Step 3, first paragraph, discussing rocking the sextant, the axis should be "eyepiece-to-horizon-mirror" not index.