We Turn Private Boat Owners into Private Boat Captains
Vincent Pica Chief of Staff, First District, Southern Region (D1SR) United States Coast Guard Auxiliary
GPS is Out!Radio is Out!And I’m WAY Out!Home is..???
When I wrote recently about the medical kit and using scotch whiskey instead of alcohol for cleansing my wound, it reminded me that it is the sail boaters (“rag tops” as they are sometimes called by the “stink pot drivers”, powered boaters) that often have the best safety procedures, ideas and home remedies.Why?Because when you’re 2,000 from land, you are “IT” when it comes to fixes and rescues, at least for quite a while.Well, if you’re fishing the canyons 100 miles out, you’re just about as lonely.What if your electronics get fried by a storm?What then?This column is about that – celestial navigation with “string and bailing wire.” Water, Water Everywhere – and No Street Signs! Face it.If some of us lost our electronics, we’d be hard pressed to figure out how to get home. And, if your electronics are fried, you can’t call the USCG since your radio is gone and your cell phone isn’t within 30 leagues of a cell tower.Back on August 27, 2008, the column was about why you need a simple compass (see “I have a GPS – Why Do I Need a Compass”, SSP, 8/27/08.)In that article, I gave you a quick and dirty way to get home when your GPS AND compass have failed.It is reprinted here so you have it all in one place:
My GPS Has Failed and I Don’t Have a Compass!Well, happily for this sorry skipper, there is a way to create a crude compass with a watch if you find yourself in such a state.Simply point the hour hand at the Sun.Halfway between the hour hand (the Sun) and 12 on your watch lies South.If you know where South is, you know where North, East and West are…Don’t have an old fashioned watch..?Draw one and line it up as if it were on your wrist.It works!
But what if you need more information than which way is South?How about where you are in terms of latitude and longitude, at least within a couple of miles on God’s GreatSea..?Read on…
The Poor Man’s GPS!
If all you had left aboard was a wrist watch, a piece of paper, a ruler/straight-edge, a poem, something to use as a Sun-dial-like shadow caster (a gnomon in tech-speak) and 4 simple numbers, you can get a pretty good fix of where you are (OK, keep this article folded in the back of your log book.)A critical item in getting home is knowing where you are starting from!So, let’s start.
First, set your watch to Greenwich Mean Time (GMT), also called Universal Time Coordinated (UTC), or Zulu-Time.For we New Yorker’s, that is local time + 5 hours when on Standard Time and +4 hours when on Daylight Savings Time.Next, using your ruler/straight edge, reduce your piece of paper a ratio of 5 x 6.Huh?Well, a standard piece of paper in the US is 8 ½ x 11.Take 1 ½ inches off that piece of paper and now it is 8 ½ by 10 – almost “spot on” 5 x 6 ratio!
Making Your Quadrant
Now you are going to create a Quadrant to figure out the Sun’s position relative to the Equator (its Declination.)(Sextants, Octants and Quadrants are all devices for measuring angles so you can figure out where you are!)To make lining things up easier, draw a line from one corner to the diagonal opposite, “long ways”, and then fold the long side down to the drawn line. Take the new long edge and fold it again to the drawn line.See Figure #1.
Figure #1 - click to enlarge
Figure #2 - click to enlarge
Now you have 4 sectors of 10o each.Soon we will have 9 folds of 10o, adding up to 90o.Stop!How can making 4 folds in half of the paper, implying 8 folds in total, add up to 9 folds and 90o?The answer lies in the proportion of 5 x 6.If you just take folded side of your Quadrant and fold it across the drawn diagonal line, you will see a “tail” sticking out the other side (see figure #2.)
Now just keep folding the sectors over each other, left to right in this diagram, until you have the 8 original sectors folded over each other and the 9th “tail” sector sticking out to the right.Make a crease where the 9th sector meets the other “8” and you have the 9 10o sectors needed.See figure #3.
Figure #3 - click to enlarge
Figure #4 - click to enlarge
Now, using the centimeter side of your ruler, try to draw 10 hatch marks across each sector of the Quadrant that you’ve created where the lengths are approximately the same.Each of those hatches will be very close to 1o apart.See figure 4 and 5 for clarity.
Accuracy is important but understand that you are still likely to get a more accurate answer when all said and done than Captain Cook did in his day… and he was quite the mariner….Now, you want to mount the shadow-maker at the nexus of all those lines.Ideally, you can mount your Quadrant onto a board and put a nail or screw into the top corner to be the shadow-maker/gnomon.
Hold your Quadrant upright at right-angles to the direction of the Sun and have the upper edge of the Quadrant line up with the horizon ahead.
Figure #5 - click to enlarge
The key is to hold the Quadrant at right angles to the Sun and upright and aligned with the horizon.This may be hard on a moving deck so take multiple readings and average them.Patience is a virtue! Calculating the Sun’s Declination – Its Position Relative to the Equator OK, now, we need to get back to one of those basic requirements you need to do this – 4 simple numbers.To do so, remember this – “Help’s On The Way”What?OK, the Sun crosses the equator on its northward leg around March 21 and on its southwards leg around September 23.The Sun reaches its maximum declination of 23.45° North (this point is called the Tropic of Cancer) and South roughly 92 days later (correspondingly, the Tropic of Capricorn.)
click to enlarge - With permission of and thanks to Tony Crowley, Herts, England
The latitudes that the Sun will have reached each 20 days it is from the equator are 8° (H!); the next 20 days to reach a declination of 15° (O!); the next 20 days to reach a declination of 20° (T!); and lastly the next 20 days to reach a declination of 23° (W!); and the remaining 11-12 days to reach a declination of 23.45°.But why is this important?With a calendar and simple math, you can figure out your latitude within a ½ degree pretty easily!For example, April 24 is 34 days after March 21. This is fourteen days or 70% of the next 20 days in the table. 70% of the amount between 8° and 15° is about 12.9°. The declination on April 24, therefore, is about 12.9°, North. Some readers may know that over a four-year period, there are annual variations in the Sun's daily declination. Except for Leap Year (an extra day), these variations can be ignored for this level of accuracy. The Equation of Time – Or The Little Poem OK, back to the top.I said another requirement was remembering a little poem – and it has to do with the scientific fact that the Earth wobbles a little around its axis.This means that Sun passes over Greenwich, England at noon – but not exactly.It passes overhead at sometime between 11:44 GMT and 12:14 GMT, depending on the time of year.So, what is the adjustment…?It is in the poem… 14 minutes late around Valentine's Day 4 minutes early in the middle of May 6 minutes late near the end of July 16 minutes early when Halloween's nigh These variations last two weeks either side of those four peaks. Again, with a calendar and some simple arithmetic, the Equation of Time can be calculated for specific days to within a minute of the correct figure. Let’s go back to the April 24th example above.We have to calculate the number of days from two weeks after Valentine's Day to two weeks before the middle of May, i.e., about 61 days. April 24 is 55 days into this period, i.e., about 90% of 61 days. The (absolute) range between Valentine’s Day (+14 minutes) to mid-May (-4 minutes) is 18 minutes.90% of the 18 minutes range is about 16 minutes. So the Equation of Time is 14 - 16 minutes = -2 minutes. So, on April 24th, the Sun crosses the Greenwich Meridian at approximately 11:58am (noon less 2 minutes!) OK, Now Let’s Find Our Latitude We’ve assembled all the pieces.The rest is math.First, measure the Sun's altitude when the Sun is bearing due south (or north, if you are in the Southern Hemisphere) with your Quadrant and deduct it from 90°. The result is the angle between your zenith and the Sun. In navigation, this angle is known as the Zenith Distance.To calculate your latitude, think about where the Sun lies in relation to you and the equator. If the Sun is between you and the equator (as it would be in our climes), add the Sun's declination (12.9o in our example above) to the Zenith Distance. (If the equator is between you and the Sun, deduct the Sun's declination from the Zenith Distance.) So, let’s say that at noon on our April 24th example, the Sun has a declination of 12.9° North. In our example, the Sun's altitude is found to be, e.g., 61° via our Quadrant so its Zenith Distance is 29° (90-61.) The Sun lies between the observer and the equator, so the declination is added to the Zenith Distance. The latitude, therefore, is 29° + 12.9° = 43.9° North. Next, Finding Longitude – Where is it Noon? The Earth rotates 15° each hour or about 1° every 4 minutes. Knowing this, the rest is, again, math.If it is noon where you are, the difference in time between the Sun's transit over the Greenwich Meridian (noon, Equation of Time above) and the time it took to your position can be converted directly to a longitude. Remember we noted that GMT is 5 hours ahead of Eastern Standard Time at the top of the column?So, how many degrees is 5 hours of time when 1° every 4 minutes?Five hours is 300 minutes, which 5 goes into 75 times, or 75o – but few people sit exactly on the meridians so you need to make measurements.Timing the Sun's exact transit over your meridian, however, is not easy. Using the simple equipment described here, the best method is to measure the Sun's height a half-hour or so before noon and note the time of the observation and continue to do so periodically until the Sun has descended to an equivalent height a half-hour after noon. Average the results and this will be very close to the actual measurement at noon. (It also assumes that your position has remained pretty much unchanged in the intervening period.) So, again using our April 24th example, if we averaged times of 1630 UCT over our meridian and we know from above that noon at Greenwich was 1158 UCT, we have 4 hours, 32 minutes (272 minutes) of Longitude to account for.272 minutes divided by 4 (1° every 4 minutes) means we have 68° of west (after) Longitude.So, pulling all the examples together, we would have found ourselves on April 24th at 43.9° North by 068° West, or just south of the Sea Buoy “TBI” at the entrance to Penobscot Bay up in Maine! Admittedly, this wasn’t easy.But it wasn’t hard either – it was simply detail intensive but if that results in knowing where you are, the tedium turns into satisfaction and some degree of excitement to be “at one” with the world’s great navigators and explorers!And hopefully it gives you an answer to the “all systems failed – now what?” question!It is also an appreciation of how wondrous the GPS system is (see “Gee, How Does GPS Do It?”, SSP, 1/2/08) since all this and much more is done instantly.And we’ve passed along important celestial navigations techniques and concepts, which we can’t forget…! I would like to thank Tony Crowley, who lives in Herts, England, for inspiring this article.Tony is a published author.He wrote a book called The Lo-Tech Navigator which was published by Sheridan House, and is a contributor to Ocean Navigator magazine, http://www.oceannavigator.com.
BTW, if you are interested in being part of USCG Forces, email me at JoinUSCGAux@aol.comor go direct to the D1SR Human Resources department, who are in charge of new members matters, at DSO-HR and we will help you “get in this thing…”
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