Celestial Navigation for dummies

Celestial Navigation is a difficult subject, due to the large ammount of calculation and geometrical concepts involved. My first contact with celnav was back in 1990, when I bought a plastic sextant from Davis Instruments. I had no idea what to do with it, so I went on to read the booklet found inside the box, called "How to Find Your Position With the Master Sextant".

Inside it I found the simplest way to find the position using celnav. It uses a couple sextant readings and only one table (no computer). It is called the simple meridian passage. Today, with the help of a computer, more sofisticated calculations can be performed just as easily, but the meridian passage remains important for the modern sailor as an emergency technique, in case electronics aids fail.

The meridian passage simplified

There are the steps to do the meridian calculation:

1- About 20 minutes before the expected meridian passage time, take a Sun sight with the sextant. Record the time and altitude.

2- At the expected meridian passage time, the Sun will reach the higher altitude for the day, and will appear to hang with constant altitude for a while. Record the culmination altitude.

3- After the passage the Sun altitude will start to decrease. Continue monitoring the altitude until the value observed in step 1 is reached. Record the time.

  • The time of the meridian passage is the mean between the times of step 1 and step 3. Convert to GMT by adding the local time zone value.
  • The altitude of the meridian passage is the culmination altitude, recorded in step 2.

With this single pair of values you can calculate your position.

For more details on the calculation, read the original document from Davis website. Just don't use the table in that document, use the one below which is for the current year (2006). Navigator 2006 Nautical Almanac printer will print the table for any year. Below is the current table

  *** Sun Declination and Equation of Time for 2006 ***
     January          February         March            April            May              June            
   |   Dec.   E.T.  |   Dec.   E.T.  |   Dec.   E.T.  |   Dec.   E.T.  |   Dec.   E.T.  |   Dec.   E.T.  
 1 | 22░59'S E0░53' | 17░ 4'S E3░24' |  7░32'S E3░ 5' |  4░36'N E0░59' | 15░ 7'N W0░43' | 22░ 5'N W0░33'
 2 | 22░54'S E1░ 0' | 16░46'S E3░26' |  7░ 9'S E3░ 3' |  4░59'N E0░54' | 15░25'N W0░45' | 22░12'N W0░30'
 3 | 22░49'S E1░ 7' | 16░29'S E3░27' |  6░46'S E2░59' |  5░22'N E0░50' | 15░43'N W0░47' | 22░20'N W0░28'
 4 | 22░42'S E1░14' | 16░11'S E3░29' |  6░23'S E2░56' |  5░45'N E0░45' | 16░ 1'N W0░48' | 22░27'N W0░26'
 5 | 22░36'S E1░21' | 15░53'S E3░30' |  5░60'S E2░53' |  6░ 8'N E0░41' | 16░18'N W0░50' | 22░34'N W0░23'
 6 | 22░29'S E1░28' | 15░34'S E3░31' |  5░37'S E2░49' |  6░30'N E0░37' | 16░35'N W0░51' | 22░40'N W0░20'
 7 | 22░21'S E1░34' | 15░16'S E3░32' |  5░13'S E2░46' |  6░53'N E0░33' | 16░51'N W0░52' | 22░46'N W0░17'
 8 | 22░13'S E1░40' | 14░57'S E3░33' |  4░50'S E2░42' |  7░15'N E0░28' | 17░ 8'N W0░53' | 22░52'N W0░15'
 9 | 22░ 5'S E1░47' | 14░38'S E3░33' |  4░26'S E2░39' |  7░38'N E0░24' | 17░24'N W0░53' | 22░57'N W0░12'
10 | 21░56'S E1░53' | 14░18'S E3░34' |  4░ 3'S E2░35' |  8░ 0'N E0░20' | 17░40'N W0░54' | 23░ 1'N W0░ 9'
11 | 21░47'S E1░59' | 13░59'S E3░34' |  3░39'S E2░31' |  8░22'N E0░16' | 17░55'N W0░55' | 23░ 6'N W0░ 6'
12 | 21░37'S E2░ 5' | 13░39'S E3░33' |  3░16'S E2░27' |  8░44'N E0░12' | 18░10'N W0░55' | 23░10'N W0░ 3'
13 | 21░27'S E2░10' | 13░19'S E3░33' |  2░52'S E2░23' |  9░ 6'N E0░ 8' | 18░25'N W0░55' | 23░13'N E0░ 0'
14 | 21░17'S E2░16' | 12░58'S E3░33' |  2░29'S E2░19' |  9░28'N E0░ 5' | 18░40'N W0░55' | 23░16'N E0░ 4'
15 | 21░ 6'S E2░21' | 12░38'S E3░32' |  2░ 5'S E2░14' |  9░49'N E0░ 1' | 18░54'N W0░55' | 23░19'N E0░ 7'
16 | 20░55'S E2░26' | 12░17'S E3░31' |  1░41'S E2░10' | 10░10'N W0░ 3' | 19░ 8'N W0░55' | 23░21'N E0░10'
17 | 20░43'S E2░31' | 11░56'S E3░30' |  1░18'S E2░ 6' | 10░32'N W0░ 6' | 19░22'N W0░54' | 23░23'N E0░13'
18 | 20░31'S E2░36' | 11░35'S E3░29' |  0░54'S E2░ 1' | 10░53'N W0░10' | 19░35'N W0░54' | 23░25'N E0░16'
19 | 20░18'S E2░41' | 11░14'S E3░28' |  0░30'S E1░57' | 11░13'N W0░13' | 19░48'N W0░53' | 23░26'N E0░20'
20 | 20░ 5'S E2░45' | 10░52'S E3░26' |  0░ 6'S E1░53' | 11░34'N W0░16' | 20░ 1'N W0░52' | 23░26'N E0░23'
21 | 19░52'S E2░49' | 10░31'S E3░24' |  0░17'N E1░48' | 11░54'N W0░19' | 20░13'N W0░52' | 23░26'N E0░26'
22 | 19░39'S E2░54' | 10░ 9'S E3░22' |  0░41'N E1░44' | 12░15'N W0░22' | 20░25'N W0░50' | 23░26'N E0░29'
23 | 19░25'S E2░57' |  9░47'S E3░20' |  1░ 5'N E1░39' | 12░35'N W0░25' | 20░36'N W0░49' | 23░26'N E0░33'
24 | 19░10'S E3░ 1' |  9░25'S E3░18' |  1░28'N E1░35' | 12░55'N W0░28' | 20░48'N W0░48' | 23░25'N E0░36'
25 | 18░56'S E3░ 5' |  9░ 2'S E3░16' |  1░52'N E1░30' | 13░14'N W0░30' | 20░59'N W0░46' | 23░23'N E0░39'
26 | 18░41'S E3░ 8' |  8░40'S E3░14' |  2░16'N E1░26' | 13░34'N W0░33' | 21░ 9'N W0░45' | 23░21'N E0░42'
27 | 18░25'S E3░11' |  8░17'S E3░11' |  2░39'N E1░21' | 13░53'N W0░35' | 21░19'N W0░43' | 23░19'N E0░46'
28 | 18░10'S E3░14' |  7░55'S E3░ 8' |  3░ 3'N E1░17' | 14░12'N W0░38' | 21░29'N W0░41' | 23░16'N E0░49'
29 | 17░54'S E3░17' |                |  3░26'N E1░12' | 14░31'N W0░40' | 21░39'N W0░39' | 23░13'N E0░52'
30 | 17░37'S E3░19' |                |  3░49'N E1░ 8' | 14░49'N W0░42' | 21░48'N W0░37' | 23░10'N E0░55'
31 | 17░21'S E3░22' |                |  4░13'N E1░ 3' |                | 21░56'N W0░35' |               

     July             August           September        October          November         December        
   |   Dec.   E.T.  |   Dec.   E.T.  |   Dec.   E.T.  |   Dec.   E.T.  |   Dec.   E.T.  |   Dec.   E.T.  
 1 | 23░ 6'N E0░58' | 17░59'N E1░35' |  8░14'N E0░ 1' |  3░15'S W2░34' | 14░28'S W4░ 6' | 21░49'S W2░45'
 2 | 23░ 2'N E1░ 1' | 17░43'N E1░34' |  7░52'N W0░ 4' |  3░38'S W2░39' | 14░47'S W4░ 6' | 21░58'S W2░39'
 3 | 22░57'N E1░ 3' | 17░28'N E1░33' |  7░30'N W0░ 9' |  4░ 1'S W2░44' | 15░ 6'S W4░ 6' | 22░ 7'S W2░34'
 4 | 22░52'N E1░ 6' | 17░12'N E1░32' |  7░ 8'N W0░14' |  4░24'S W2░49' | 15░25'S W4░ 6' | 22░15'S W2░28'
 5 | 22░46'N E1░ 9' | 16░56'N E1░30' |  6░46'N W0░19' |  4░47'S W2░53' | 15░43'S W4░ 6' | 22░23'S W2░21'
 6 | 22░40'N E1░11' | 16░39'N E1░29' |  6░23'N W0░24' |  5░10'S W2░58' | 16░ 1'S W4░ 6' | 22░30'S W2░15'
 7 | 22░34'N E1░14' | 16░23'N E1░27' |  6░ 1'N W0░29' |  5░33'S W3░ 2' | 16░19'S W4░ 5' | 22░37'S W2░ 9'
 8 | 22░27'N E1░16' | 16░ 6'N E1░25' |  5░38'N W0░34' |  5░56'S W3░ 6' | 16░37'S W4░ 4' | 22░44'S W2░ 2'
 9 | 22░20'N E1░18' | 15░49'N E1░23' |  5░16'N W0░40' |  6░19'S W3░11' | 16░54'S W4░ 3' | 22░50'S W1░56'
10 | 22░13'N E1░20' | 15░31'N E1░21' |  4░53'N W0░45' |  6░42'S W3░15' | 17░11'S W4░ 1' | 22░55'S W1░49'
11 | 22░ 5'N E1░22' | 15░14'N E1░18' |  4░30'N W0░50' |  7░ 4'S W3░19' | 17░27'S W3░60' | 23░ 1'S W1░42'
12 | 21░57'N E1░24' | 14░56'N E1░16' |  4░ 7'N W0░55' |  7░27'S W3░22' | 17░44'S W3░58' | 23░ 5'S W1░35'
13 | 21░48'N E1░26' | 14░37'N E1░13' |  3░44'N W1░ 1' |  7░49'S W3░26' | 17░60'S W3░56' | 23░ 9'S W1░28'
14 | 21░39'N E1░28' | 14░19'N E1░10' |  3░21'N W1░ 6' |  8░12'S W3░30' | 18░16'S W3░54' | 23░13'S W1░21'
15 | 21░30'N E1░29' | 14░ 0'N E1░ 7' |  2░58'N W1░11' |  8░34'S W3░33' | 18░31'S W3░51' | 23░16'S W1░14'
16 | 21░20'N E1░31' | 13░41'N E1░ 4' |  2░35'N W1░17' |  8░56'S W3░36' | 18░46'S W3░49' | 23░19'S W1░ 6'
17 | 21░10'N E1░32' | 13░22'N E1░ 1' |  2░12'N W1░22' |  9░18'S W3░39' | 19░ 1'S W3░46' | 23░22'S W0░59'
18 | 20░60'N E1░33' | 13░ 3'N E0░58' |  1░49'N W1░27' |  9░40'S W3░42' | 19░15'S W3░43' | 23░24'S W0░52'
19 | 20░49'N E1░34' | 12░44'N E0░55' |  1░26'N W1░33' | 10░ 2'S W3░45' | 19░29'S W3░39' | 23░25'S W0░44'
20 | 20░38'N E1░35' | 12░24'N E0░51' |  1░ 2'N W1░38' | 10░23'S W3░48' | 19░43'S W3░36' | 23░26'S W0░37'
21 | 20░27'N E1░36' | 12░ 4'N E0░48' |  0░39'N W1░43' | 10░45'S W3░50' | 19░57'S W3░32' | 23░26'S W0░29'
22 | 20░15'N E1░37' | 11░44'N E0░44' |  0░16'N W1░49' | 11░ 6'S W3░53' | 20░10'S W3░28' | 23░26'S W0░22'
23 | 20░ 3'N E1░37' | 11░24'N E0░40' |  0░ 8'S W1░54' | 11░27'S W3░55' | 20░22'S W3░24' | 23░26'S W0░14'
24 | 19░50'N E1░38' | 11░ 3'N E0░36' |  0░31'S W1░59' | 11░48'S W3░57' | 20░35'S W3░20' | 23░25'S W0░ 7'
25 | 19░37'N E1░38' | 10░42'N E0░32' |  0░54'S W2░ 4' | 12░ 9'S W3░58' | 20░46'S W3░16' | 23░24'S E0░ 0'
26 | 19░24'N E1░38' | 10░22'N E0░28' |  1░18'S W2░ 9' | 12░29'S W4░ 0' | 20░58'S W3░11' | 23░22'S E0░ 8'
27 | 19░11'N E1░38' | 10░ 1'N E0░23' |  1░41'S W2░15' | 12░50'S W4░ 2' | 21░ 9'S W3░ 6' | 23░19'S E0░15'
28 | 18░57'N E1░38' |  9░40'N E0░19' |  2░ 5'S W2░20' | 13░10'S W4░ 3' | 21░20'S W3░ 1' | 23░17'S E0░23'
29 | 18░43'N E1░37' |  9░18'N E0░15' |  2░28'S W2░25' | 13░30'S W4░ 4' | 21░30'S W2░56' | 23░13'S E0░30'
30 | 18░28'N E1░37' |  8░57'N E0░10' |  2░51'S W2░30' | 13░50'S W4░ 5' | 21░40'S W2░51' | 23░10'S E0░37'
31 | 18░14'N E1░36' |  8░35'N E0░ 5' |                | 14░ 9'S W4░ 5' |                | 23░ 5'S E0░44'



ęCopr 92-2012
Omar F. Reis - All rights reserved