Author Topic: CelNav Eight - Odds and ends...  (Read 694 times)

Offline HenryC

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CelNav Eight - Odds and ends...
« on: September 15, 2010, 04:29:51 PM »
CelNav Eight -- Odds and Ends

Celestial Navigation not only teaches you about mathematics, geography and astronomy, you also learn coordinate systems, piloting and dead reckoning, plotting and chartwork, cartography, and as we have seen, time keeping. You will be an expert in all these areas, each of them useful for a sailor, so it is obviously worth the effort, even if you never use it to find your way at sea. 

As the previous unit demonstrated, understanding time is integral to navigation, and knowing the exact time at sea is essential to determining your longitude.  Without knowing the exact Universal Time, you cannot determine how far E or W of Greenwich you are.  This simply was not possible until the mid eighteenth century, when accurate chronometers were first developed.  Prior to that time, accurate pendulum clocks existed, but pendulums don't work on a rolling ship.  It wasn't until the invention of the chronometer that the east-west portion of the GP, the GHA, could be extracted from an Almanac.  And without the GHA, as we have seen, you can't reduce a sight.

All clocks gain or lose time and have to be reset occasionally, but a good nautical timepiece is designed so it loses or gains AT A UNIFORM RATE,  so many seconds per day, so even if your timepiece has gained or lost time since you last set it, you should be able to figure out how much it has gained or lost after some time and correct for the error.  This unit will teach you how to manage your time.

You can set your timepiece to official US Gov Universal time by calling the US  WWV time service at (303) 499-7111. This will be a toll call. There are other sources of accurate telephone and internet time signals, but you have to be careful because they may not be strictly accurate as they are bounced around by satellite and may be delayed before they get to your phone or computer.  The WWV signal is routed by special comm links and has a guaranteed minimal delay.

The WWV official time is also broadcast by shortwave at 2.5, 5, 10, 15 and 20 MHz.  It is also provided by your GPS unit.  You should reset your timepiece every day while you are underway using these sources, if possible.  You should also keep more than one timepiece as a backup.  I reset my duty clock every chance I get, but I also note the error of all my other clocks in a Chronometer Log so I have plenty of backup.  This also ensures I can keep track of the rate at which each of my clocks gains or loses per day so if I am unable to get a time tick while I'm underway, I can still come up with the exact time, and add a proper correction to my duty timepiece using my other clocks as a source. 

The Chronometer log sould have an entry for each of your clocks, showing the dates it was checked, and the error (+ if it was slow, - if it was fast). If you determine your clock is 1m 37 seconds slow on the day you take a sextant reading, just add 1m 37s to your time of observation.  Pepperday gives you a place to do this.  Monitor several clocks so you can check the clock you use as a backup and make sure its rate of error has remained relatively constant.

For example,  if a clock loses or gains 10 or 20 seconds every day, it is no big deal, as long as it loses or gains the same amount every day.  This way, even even if several weeks or months have gone by since the last time you got a time signal, you can still figure out the exact time.

The good news is that good clocks are easy to get and very cheap these days.  Most modern digital wristwatches lose or gain only a few seconds per day, and their error rate is extremely stable. They also have batteries which last for years. I prefer the Timex "Iron Man" or "Triathlon" model, which is very durable, waterproof, illuminated, and has a button you can program to flash UT on command.  I've never owned one with an error greater than 10s per day.  And the error drift rate remains stable to a second over months at a time.  I find their batteries are good for up to 5 years, but the Velcro wrist bands fall apart after a year or two (I never take mine off, I even sleep and shower with it).  So when the band finally breaks, I throw the still-functioning movement into my Nav kit, and add it to my clock log.  Every few weeks I set my main clock to a time tick and log all my clock errors. The "Iron Man" goes for about $40 at Wal-Mart.

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The last thing I want to talk about is locating stars.  Many navigators neglect using the stars for sights because locating and identifying them seem so difficult.  Unless you're an amateur astronomer and know a lot of them on sight (professional astronomers never bother, they use computers!) or are familiar with star charts and how to use them, facing the thousands of stars visible from a ship on a good night at sea seems like an impossible task.  The Nautical Almanac has a good star map on page 266-267, but unless you are already familiar with the night sky and know how to use a star chart, it is next to useless.  But stars are easy to shoot, and plentiful (the almanac lists SHA and Dec for the 57 main ones on every page, and many backups on pages 268-273).  And you can't really say you can navigate by the stars until you can use the stars.

Fortunately, there are other ways to identify and locate stars.  First off, you can actually learn the sky by using a Planisphere.  You've seen these I'm sure, almost every bookstore or nature/science shop carries them,  A Planisphere is a cardboard or plastic disc with stars printed on it and dates printed along its edge.  It is inside a sleeve with times printed along it's edge.  You rotate the disc in the sleeve and match up the date with the time and the sky as it appears at that time on that date is displayed.  Instructions for its use are printed on the back.  I used one when I was 12 to learn the sky (at least the Northern Sky). 

Pepperday has a star finder table in his little book (a simplified version of his sight reduction table) which can be used to ID stars after you have shot them, but I have never used it myself.  I prefer a using a Rude Star Finder. (It was invented by Capt. Rude).  A civilian version of this device (Model 2102-D) is sold by Weems and Plath.  It can be purchased in many marine stores, or over the internet (Google "Weems & Plath", "star finder", 2102-d).  The star finder comes with a plastic planisphere chart with the northern hemisphere stars on one side, the southern stars on the other. It also comes with a group of clear plastic overlays with grid lines printed on them.  You place the grid overlay closest to your latitude over the white disc with stars on it. Using the Almanac's GHA of Aries for that time and date, and your DR longitude, determine the LHA of Aries and just rotate the overlay until the arrow points to the LHA printed on the edge. Then simply read off the Azimuth and Altitude of each star off the transparent grid  template.  Complete instructions come with the gadget.  You can use the star finder to decide which stars to shoot, or you can use it to determine the names of the stars you have already shot, since the time,  Hs and approximate bearing you wrote down for each shot can be read off the template.

I wrote a short article on this website about this device you might want to check out.

http://cpyoa.geekworkshosting.com/forum/index.php?topic=3090.0

« Last Edit: January 14, 2017, 10:12:25 AM by HenryC »