When teaching navigation seminars for the Ocean Navigator School of Seamanship, I always preach the benefit of getting out and practicing skills. Skills most often used in coastal situations (visual bearings, plotting, and set and drift calculations, to name a few) are best practiced in home waters on board the boat (perhaps after some review at the kitchen table). However, I am often asked where and how to practice celestial navigation. My answer is always to do so on land, before heading out on the boat.
On land, the platform is stable, the weather is always nice (or one has the option of selecting only the good days) and most important, it is very easy to pinpoint a position with an inexpensive GPS to which results can be compared. Plus, the speed of celestial calculators offers immediate feedback on each sight because the GPS position can be used as the Assumed Position, yielding an intercept that directly relates to where the navigator is on land.
There are several ways to approach land-based celestial practice, including artificial horizons (Davis Instruments has sold one for many years) and bubble sight tubes (Celestaire offers one). Here we will focus on the simplest of all, getting down to the beach with sextant in hand and just taking sights.
So, after setting your watch to GMT, pack everything you need into the sextant box, grab a beach chair if you want to be extra comfortable and head to the beach. My list includes a watch, sextant, notebook, pencil, GPS and a Celesticomp. Obviously, it is important to find a spot with a clear view of the horizon and the celestial body. It’s easiest to start with the sun, although practicing with other celestial bodies is possible. First turn on the GPS and let it find itself. Then estimate your height of eye above sea level and jot this down in your notebook next to the GPS position. You’re ready to go.
Begin by establishing your index error. If the sextant has been in the box for a long time, this is vital. Recording the index error is all that is necessary, but if the error is deemed unacceptable, correct it on the spot by adjusting the mirrors (procedures for this are found in books such as The Sextant Handbook by Bruce Bauer). Next take a series of three sights, recording the GMT and altitude information for each in the notebook. Then, using the calculator, reduce the sights to see how close to (or far off of) the beach you are. Repeat until you are satisfied with the results.
Some tips on using the Celesticomp for this purpose:
· Make the Sight Time and Fix Time the same; use the time of the sight.
· Enter the GPS position as the DR position.
· Set Speed and Course to zero.
The beauty of the Celesticomp (or your chosen calculator) is that in less than five minutes the results from the three sights can be obtained, with the intercept telling the navigator precisely how accurate each sight was compared to the position of the beach chair! No salt spray, no time down below crunching numbers, no heaving deck. Just quality practice and meaningful results. Of course, for more practice, bring along a set of tables, or bring the sights back to the house to reduce using traditional methods.
If a horizon is not readily available, because your chosen beach happens to be on a lake where the opposite shore is only a mile away, just acquire a copy of Table 22 in Bowditch (Dip of the Sea Short of the Horizon). After checking on a local chart for the distance to the nearest land that marks your horizon, enter the table with that distance to find the dip correction to apply. When taking a sight in this situation, simply bring the lower limb of the sun, for example, down to the water’s edge of the nearest land.
Not only is practice on land fun, but accuracy is noticeably enhanced with consistent realistic practice using a true horizon and the sextant’s standard telescope. Once the basic skill of taking a sight is renewed, continuing on board the boat in home waters is the next step. Practice prior to an offshore race (like the Trans-Pac or the Marion-Bermuda Race, which still maintain certain celestial practices) or long passage, builds confidence for the navigator while rebuilding valuable skills that may have become a bit rusty.
Andrew Howe is Ocean Navigator’s celestial navigation seminar instructor.