Product Review

Celestial Navigation with the S Table
by Mike Pepperday (1992)

My copy of the S-Table was purchased from Celestaire, Inc., available from their online catalog.

Like most navigators, I originally learned how to perform sight reductions with the venerable Sight Reduction Tables for Marine Navigation (HO 229). For those not familiar with this work, it is a set of six hard-cover volumes, each 10" x 12" and weighing about 3 1/2 pounds each. Many small boat sailors prefer the more compact Sight Reduction Tables for Air Navigation (HO 249) in three volumes, each the same size and weight as HO 229, although the reduced format was paid for by certain restrictions on use and generality. The complete HO 249 can be purchased for approximately $120, while its larger cousin costs over twice as much. Although both of these items are available in smaller paperbound commercial editions at a substantial discount, we are still talking a substantial investment and a lot of valuable space, particularly for the mariner who only sees celestial as a backup to his electronics.

Today, most celestial navigators use pocket calculators to reduce their sights, either dedicated models specialized for this purpose or programmable devices with user-written or commercially published software. Some sailors have been known to use laptops or even desktop PCs with full-blown "Navigation Packages" installed. I always thought this was like putting an air conditioner in a stagecoach, but... to each his own.

The primary justification of celestial navigation, other than the pride taken in even having the ability and skill to do it, is as an independent backup to GPS, and as much as possible to be able to overcome the vulnerabilities of that system. These include power or battery failure, mechanical malfunction, hostile jamming or friendly spoofing of the signal in wartime; or even potential hazards such as solar flares, nearby supernovae, or electromagnetic pulse due to nuclear explosions or lightning strike. If the situation ever gets to the point where we cannot rely on the government to properly operate and maintain GPS, then celestial skills may be a matter of life and death. Some of these problems may seem a bit far-fetched, but the ability of the sea to wreak havoc on even the best electronic gear is not. Having a sextant on board tied to any piece of integrated circuitry is defeating its whole purpose. Consequently, any celestial navigator, even one who relies on mechanical assistance with his calculations as I do, will probably be interested in some sort of tabular backup, even if he has decided from the start to rule out HO 229/249.

So what are the alternatives? There are literally hundreds of tabular methods for solving the astronomical triangle, but a few come to mind as more than just historical curiosities. There are the compact sight reduction tables included in the Nautical Almanac, Ageton's tables (HO 211) in Bowditch (prior to the 1995 edition), and several other methods each with its own adherents and champions. I have to admit I've only worked with the Nautical Almanac tables, and the price you pay for their compactness is that they are awkward and hard to use unless you practice with them all the time. But I do have a favorite, and from what I can tell, both by my experience with it and by comparison of specifications with other tables, the Pepperday S-Table is the only way to go. It is a folded and streamlined version of HO 211, yet is actually complete for all angles. If you have a better alternative, I'll be delighted to check it out.

• The S-Table is compact, only 9" x 5", and at 30 pages is the smallest tabular package available. It easily fits in the sextant case, or tucked into the Nautical Almanac as a book mark. Furthermore, most of the booklet is devoted to very clear and well-written instructions and examples: the table itself is only nine pages long! I made photocopies of those pages and laminated them; I keep them in my NAV kit, and the booklet with the sextant as a backup.

• The S-Table can be used as a look-up table, or you can use its pre-printed sight reduction form with a pocket calculator as a check to your tabular work. This is a great way to learn the forms and give yourself the security of knowing you have two independent methods available to you at all times.

• The S-Table comes with its own sight reduction form, which you can copy and use for practice and then on the job. The sight reduction form looks a bit baffling at first, but with a little practice you will find that it is extremely well thought-out and that each step leads you logically to the next. In fact, it is a masterpiece of design; it is obvious that a lot of field testing went into its final layout. I am struck with admiration at the amount of intelligence exhibited in the form: anything that can possibly go wrong has been anticipated and a warning or note has been provided where you will are most likely to see it.

• The S-Table comes with an abridged star list and mini-table and a special form for star identification. It uses the same principle as the main table, but is not as accurate and is designed to ID a star after you have observed it. I have not actually used this feature, since I use a Star Finder, but I'm glad to have it as a backup. As with all other features, it has excellent instructions and examples in the text.

• Using any tabular method "cold" in an emergency may be impossible for the navigator who has not drilled with it extensively before. However, the S-Table might be the best one to have if you are ever forced into this situation. Though not the easiest method to use, the instructions are extremely thorough and cover even non-tabular aspects of the sight reduction process, such as some Almanac operations, observing tips and plotting tricks. Although no substitute for prior training and drill, these instructions are perfect for the experienced navigator who may be rusty and not have manually calculated a sight in a long time.

• For me, the most important and valuable feature of the S-Table is the way it handles Assumed Position: it doesn't! There is no need to plot each LOP at a different AP as with other methods. This is because the S-Table is a general solution for any astronomical triangle, not just the even degrees. You don't have to fudge an AP for each LOP to make it come out a round number.

  You can use any convenient point on the chart near your DR position (I usually pick the nearest even degree lat-lon grid intersection and plot all my LOPs from that single point, with the azimuths like spokes on a wheel). This immediately eliminates the major source of potential errors at the plotting step. If you're doing a running fix, DR the LOPs with your parallels, instead of the APs.

• The S-Table has no limitations or exclusions. It works for all latitudes and altitudes, and all angles to the nearest minute of arc. It is easy to learn and easy to use, and best of all, it only costs about $10. Outstanding!

In conclusion, I heartily recommend the S-Table for all intermediate and experienced navigators. Perhaps the novice may be better off with some other method, that is, maybe it is easier to learn on some other table, or easier to work without making mistakes with some other method. I really don't know, since I've completely forgotten how to use HO 229 and 249. However, I am convinced that the virtues of Mr. Pepperday's little book are certainly worth its modest purchase price.

Epilogue: My copy of the S-Table was copyrighted in 1992. It seems ironic that this handy tool for celestial navigators was released to the world right about the time that reliable and inexpensive GPS units became ubiquitous on all but the smallest boats, and celnav started down the road to obsolescence. There's some kind of moral here.

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