Information on the Emergency Sextant Navigation Method


GPS has now become the primary method of navigation, I strongly feel that sextant navigation should be implemented as an independent backup to GPS.
My intention was to develop a selfcontained emergency method, yet still use the modern but timetested methods of altitude intercept, and the noon sight. This system uses five basic formulas with presolved examples, and requires only a low cost battery or solar powered scientific pocket calculator. Simple approximate emergency tables for the declination and GHA of the sun are included with sufficient accuracy for emergency use. An hourly mental interpolation can also be added. Use the diagram to help picture the navigation concept.
Altitude intercept sights formulas 2, 4, & 5Altitude intercept sights can be used at any time of the day that the sun is visible, as illustrated at 13 hours GMT. Two separate LOP’s taken a few hours apart will also give a complete running fix where they cross on your chart or plotting sheet. Formulas 4 & 5 can also be used to determine your initial great circle course and Z Distance to a distant port. Use Dec. and Lat. N+ S rules for Formula 4. Use quadrant circle near Solved Formula 5, to determine azimuth from your location. GHA = Longitude in western hemisphere
Noon sight formulas 1, 2, & 3The historically famous and simple noon sight works only at local noon as the sun crosses or transits your local meridian or longitude, giving the highest sextant reading of the day. This is illustrated at 15 hours GMT. Latitude is very accurate with this method, and is limited only by the accuracy of the emergency declination table, sextant adjustment, and observation techniques. However, with this method, Formula 2 longitude is only approximate due to observational timing difficulties near noon. In the tropics, additional rules may apply. Therefore, disregard all Dec. and Lat. N+ S rules for Formula 1, and use your DR to determine the correct hemisphere if necessary. Use Formula 3 to determine GMT of the noon sight.
Use Declination and E Interpolation for best accuracyThe sun declination and E, the equation of time + 5º are shown in table 4 at 00 hours GMT each day and should be brought forward and corrected for the current sight GMT for best accuracy. The equation of time, hourly change, can be interpolated from comparison of the next day E. The declination daily change is listed to the right of the DEC. table, and should be interpolated hourly for best accuracy. Either correction can be plus or minus during the year. Mental interpolation for declination of ¼ of the daily change for each six hour period during the day can also be used. Formula 1 is in degrees and min. Formulas 2 through 5 are in degrees and decimal min. To convert, divide .min by .6 To convert dec min to min multiply by 60. For 90 º, rewrite as 89º 60´ and 180º as 179º 60´ for easy subtraction. The maximum daily change in DEC. is about 24´ or 24 NM, and reduces to about zero around June 21 and Dec. 21. The equation of time has a maximum daily change of about 8´ of arc. The equation of time itself is zero on April 16th in the solved example, as E. = 5º + the equation of time.
A good source for sextant adjustment and observation techniques can be found in a Davis sextant owners manual, located on their website: www.davisnet.com Remember to correct for sextant index error on all observations.
A few hours of selfstudy will give you the confidence to practice this valuable method of emergency sextant navigation.
If you have any questions, please contact me.
Gary Davis
gdavisKD9SB@sbcglobal.net Rev. 82514
My intention was to develop a selfcontained emergency method, yet still use the modern but timetested methods of altitude intercept, and the noon sight. This system uses five basic formulas with presolved examples, and requires only a low cost battery or solar powered scientific pocket calculator. Simple approximate emergency tables for the declination and GHA of the sun are included with sufficient accuracy for emergency use. An hourly mental interpolation can also be added. Use the diagram to help picture the navigation concept.
Altitude intercept sights formulas 2, 4, & 5Altitude intercept sights can be used at any time of the day that the sun is visible, as illustrated at 13 hours GMT. Two separate LOP’s taken a few hours apart will also give a complete running fix where they cross on your chart or plotting sheet. Formulas 4 & 5 can also be used to determine your initial great circle course and Z Distance to a distant port. Use Dec. and Lat. N+ S rules for Formula 4. Use quadrant circle near Solved Formula 5, to determine azimuth from your location. GHA = Longitude in western hemisphere
Noon sight formulas 1, 2, & 3The historically famous and simple noon sight works only at local noon as the sun crosses or transits your local meridian or longitude, giving the highest sextant reading of the day. This is illustrated at 15 hours GMT. Latitude is very accurate with this method, and is limited only by the accuracy of the emergency declination table, sextant adjustment, and observation techniques. However, with this method, Formula 2 longitude is only approximate due to observational timing difficulties near noon. In the tropics, additional rules may apply. Therefore, disregard all Dec. and Lat. N+ S rules for Formula 1, and use your DR to determine the correct hemisphere if necessary. Use Formula 3 to determine GMT of the noon sight.
Use Declination and E Interpolation for best accuracyThe sun declination and E, the equation of time + 5º are shown in table 4 at 00 hours GMT each day and should be brought forward and corrected for the current sight GMT for best accuracy. The equation of time, hourly change, can be interpolated from comparison of the next day E. The declination daily change is listed to the right of the DEC. table, and should be interpolated hourly for best accuracy. Either correction can be plus or minus during the year. Mental interpolation for declination of ¼ of the daily change for each six hour period during the day can also be used. Formula 1 is in degrees and min. Formulas 2 through 5 are in degrees and decimal min. To convert, divide .min by .6 To convert dec min to min multiply by 60. For 90 º, rewrite as 89º 60´ and 180º as 179º 60´ for easy subtraction. The maximum daily change in DEC. is about 24´ or 24 NM, and reduces to about zero around June 21 and Dec. 21. The equation of time has a maximum daily change of about 8´ of arc. The equation of time itself is zero on April 16th in the solved example, as E. = 5º + the equation of time.
A good source for sextant adjustment and observation techniques can be found in a Davis sextant owners manual, located on their website: www.davisnet.com Remember to correct for sextant index error on all observations.
A few hours of selfstudy will give you the confidence to practice this valuable method of emergency sextant navigation.
If you have any questions, please contact me.
Gary Davis
gdavisKD9SB@sbcglobal.net Rev. 82514