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A bit of Celestial Navigation

This voyage has given me the opportunity to revisit the joys and art of navigating without GPS - in particular adding sun sights to my armoury of objects that can be used to get a positive fix on position at sea. 

The first thing James, our Skipper, recommended was to read a book called Longitude by Dava Sobel - essentially the history of the chronometer. It was a great way for me to get an understanding of the principles and possibilities of navigating with celestial bodies as well as a good history story. 

Seafarers have navigated using the stars for millennia, essentially because their position relative to each other never really changes.

It is not the same for our sun, moon and planets (which the ancients thought of as wandering stars). However their movements relative to the spinning of the Earth are easily predicted so by knowing what time it is exactly, and some jiggery pokery with angles and tables they can be used to get a position line. 

Any reader who remembers old fashioned coastal navigation will recall how a bearing on something charted, perhaps a lighthouse, castle or church spire can give a position líne and how two or better still three well chosen crossing lines generally yield a fix.

With the sun it just happens that you are using the same object to create those multiple position lines by taking different sightings of it as it moves across the sky. 

This post is not supposed to be a celestial navigation lesson, instead this post is just intended to encourage anyone who remembers dead reckoning and coastal navigation of old that celestial navigation is not rocket science and if you ever get the opportunity I have just received it can be satisfying (and not impossible) to learn. 

Anyway some pictures

1. This is Par, a man of many talents using Ruth II's sextant to get as accurate angle as possible of the sun to the horizon. With a clever arrangement of mirrors, a telescope and quadrant with micrometer type screw you can line up the sun (or star) exactly on the horizon and read the angle as accurately as 0.1 of a minute of one degree. Of course a flat calm helps, it becomes a bit more judgemental in a seaway but I certainly found it far less judgemental than a hand bearing compass sight of old. Not visible, but on Pars left wrist is a watch set to the accurate time. 

2. The books needed. The almanac gives each days sun (and other) predictions, most importantly it's declination from the equator (for Latitude) and angle from Greenwich (for Longitude) tabulated hourly with ways to add increments and corrections for minutes and seconds. Very detailed, but think of daily tide tables. The other book, Sight Reduction Tables, essentially gives a quick and clever way to translate your sight to something plotable on a sheet of paper. The other items in the photo are the usual, except as you will see I recommend a full supply of rubbers!

3. A book of plotting sheets is very handy. This side of the sheet is formatted to work through 3 daily sun sights.
The first and third sight are similar in how they are worked, the sextant angle and time are converted to zeniths and azimuths from chosen positions (don't worry about it!) by lots of jiggery pokery and clever tables - essentially yielding plotable position lines. The second sight, the Meridian Pass is at your own boat noon when the sun is highest for the day, adjusting for the sun's declination will directly give your latitude - another position líne.

4. The other side of the sheet. Ocean charts are too large scale to be useful so you basically draw up latitude and longitude scales to suit.  
Each sighting ultimately becomes a position line. Finally the highlighted lines are essentially to bring it all together at midday (the boat was traveling while all the jiggery pokery was happening) and taking the center of the highlighted triangle is our fix. On this day I was delighted - I was only 2 miles out on the latitude and 3 on the longitude according to the very quick, modern and extremely accurate GPS.

Anyway sorry about all that if you feel inflicted upon - it was a hard post for me too.

However if it encourages someone else somewhere to give it a go it will have been worth it. 

Also again thanks to Skipper James for his patience and encouragement, not to mention Par who kept sending me back out in the hot sun when I was ready to give up!



  1. Thanks for sharing this and well done .. Enjoying reading the Blogg and missing Ruth 11.. fair winds for the next leg.


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