GPS Backup with a Mark 3 Sextant

David Burch is a Fellow of the Royal Institute of Navigation in London as well as a Fellow of the Institute of Navigation in Washington, DC, from which he received the Superior Achievement Award for outstanding performance as a practicing navigator. He has logged more than 70,000 miles at sea, including twelve transoceanic yacht races, with several first place victories and a passage record for boats under 36 feet that lasted 16 years. He also navigated the only American entry in the storm-ridden 1993 Sydney to Hobart Race.

On the academic side, he is a former Fulbright Scholar with a PhD in physics. As Founding Director of Starpath School of Navigation in Seattle he has designed courses and taken part in the teaching of marine weather and navigation for more than 30 years. He continues to work on the development of online training materials, which are presented at starpath.com. His articles on special topics in navigation and weather appear at starpath.com/articles.

contents

1. OVERVIEW 3

2. SEXTANT SIGHTS WITH THE MARK 3 SEXTANT 3

       Reading the Sextant Scales 4

       Adjusting the Index Mirror 6

       Index Mirror Adjustment Using Only a Sea Horizon 6

       Index Mirror Adjustment with a Star 9

       Index Mirror Adjustment with Cloud Edges 10

3. TAKING SUN SIGHTS 11

       Aligning the Sun on the Horizon 12

4. A COOKBOOK APPROACH TO FINDING POSITION 13

       Correcting the Sextant Height to get the Observed Height 13

       Finding Latitude and Longitude From the Sun 14

       The Recipe 14

       Finding the Peak Height of the Sun 15

       Finding the Time of LAN 15

       Sailing Under the Sun 16

5. WORKED LAN EXAMPLES 16

       Latitude Example 1 16

       Easy LAN Rule 18

       Longitude Example 1 18

       Latitude Example 2 19

       Longitude Example 2 19

6. LATITUDE BY POLARIS 20

       Latitude by Polaris Example 22

7. PLOTTING POSITIONS AND DR TRACKS 23

       Setting Up a Universal Plotting Sheet 23

       Using the Plotting Sheet 24

       Getting by Without Dividers 24

8. BACKGROUND INFORMATION 26

       8.1 Timekeeping in Navigation 26

       8.2 Lat, Lon, nmi, and DR 27

       8.3 Adjusting the Index Mirror on Land 28

       8.4 Predicting the Time of LAN From Your DR-Lon 28

       8.5 Principles of the Noon Sight for Lat and Lon 30

       8.6 Universal LAN Rule 33

9. GPS BACKUP KIT—THEME AND VARIATIONS 33

10. REFERENCES 34

11. TABLES 34

       Sun Almanac 35

       Refraction Table, Dip Table, Polaris Table 88

       Increments Table 89