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-   -   Celestial navigation (https://www.pprune.org/pacific-general-aviation-questions/543935-celestial-navigation.html)

Ozdork 20th Jul 2014 06:06

Celestial navigation
 
Are there any (old?) aviators out there in Ppruneland who are versed in the fine old art of Celestial Navigation? If so, where do I start? I have been trying to get my head around it for a while now (for my boating life, not my working life I must add) and despite owning an el-cheapo training sextant (Davis MkIII) and numerous books on the subject, without a hands-on mentor available to actually SHOW me what I should be doing, I'm still basically stumped. Any assistance in where I should start, from a practical perspective rather than a theoretical perspective, would be appreciated.

mickjoebill 20th Jul 2014 06:29

Checkout this documentary, a voyage across the North Sea from Norway to Shetland with Sir Robin Knox Johnston on a replica Viking boat without modern nav aides.
https://www.youtube.com/watch?v=dgH5fmwASs0

He used a sun compass to set off then combination of celestial and direction of waves for the night time navigation.

PM if you want more details of the doc:ok:

Mickjoebill

Brian Abraham 20th Jul 2014 06:36

If you have any Navy organisation near by you could try them, ex RAAF Nav, or find an old time yachtie at the local club. Think the RAAF don't teach it on nav courses any longer, all GPS.

You could get a Polynesian sailor, they didn't have or need sextants for deep water sailing nav.

compressor stall 20th Jul 2014 07:08

You can pick up ex WW2 astrocompasses in the US for around $100 if you want to try.

Ozdork 20th Jul 2014 07:18

Thanks for the suggestions - now I need to google 'astrocompass'. Unless you want to give me a quick rundown?

Ozdork 20th Jul 2014 07:22

And any suggestions for what celestial bodies I should be learning to identify? Is there some sort of list of 'must knows'? Most of the stuff I've read is northern hemisphere biased and my little boat won't be heading further north than Lizard Island. For now.

enicalyth 20th Jul 2014 07:27

Boat Stuff?
 
Do you live in NSW 2250?

500N 20th Jul 2014 07:32

OzDork

I am sure someone mentioned this on the military forum a short while back as it was used by the RAF - I think.

4Greens 20th Jul 2014 07:42

Aircraft celestial navigation was known as astronavigation. It used a device called a bubble sextant and as such didn't need a visible horizon. Shooting stars from a plane at altitude was easy as it was so stable. A far cry from the moving deck of a yacht. Star sights from a boat are generally done at sunset as there is a visible horizon and a star at the same time.

Been there, done that.

Mach E Avelli 20th Jul 2014 08:29

Ozdork if you are ever in Tassie I would be pleased to show you how to use that Davis sextant. It will take all of 30 minutes to master it.
For what it's worth, the Davis Mark 2 (the one with the vernier scale) is quite adequate for use on a small boat. In fact easier to hold than heavy metal jobbies when all is pitching and tossing, and if you do drop it in the drink it won't break the bank.

As for tables, a guy named Prof. Bennett from one of our Aussie Universities published his own version of an almanac good for (I think) 10 years. Once you can get a reasonable sight with the sextant, and have accurate time, all you need is Bennett's Almanac and an ability to add and subtract.
Forget the theory......just follow the directions in the Almanac.

As for the bodies, start with sun, moon and planets. Stars are not as useful as you would hope, because you need a clear horizon and by the time most stars are visible, the horizon is not. However, Sirius usually rises early enough and it is nice to know the easily identifiable stars in the Southern Cross and Orion's Belt.

compressor stall 20th Jul 2014 08:41

Ozdork, an astrocompass uses the sun to find true north.

Of most use in high latitude navigation where mag variation is high.

A separate (and easier!) skill to sextant use.

Ozdork 20th Jul 2014 08:46


Boat Stuff?
Do you live in NSW 2250?
Nup, Qld 4020:cool:

criticalmass 20th Jul 2014 10:18

For stars, use "Selected Stars" (AP 3270/NP 303) Epoch 2010.0, and calculate Local Hour Angle of Aries at nautical twilight (time in GMT for nautical twilight taken from the Nautical Almanac) for your Assumed Position thus:-

LHA Aries = GHA Aries (for the Hour GMT, from Nautical Almanac) + Increment (Minutes, again from Nautical almanac) + Longitude East (OR - Longitude if you are West or Prime Meridian). i.e.

LHA Aries = GHA Aries
+Inc
+ Longitude East OR
- Longitude West

If it's less than 360 degrees then add 360. If it's greater than 360, subtract 360 degrees. Now you have the Local Hour Angle of Aries.

Then, find the page for your your Latitude (in whole degrees) in "Selected Stars" and look down the column on the extreme left marked LHA Aries until you get to your calculated LHA Aries. In that row you will find the tabulated Altitudes and Azimuths (True) for seven selected navigational stars at various azimuths. You don't need to know them by sight, the sextant will find them for you using the following procedure.

To get a star shot (at dawn or dusk), calculate the local time of Nautical Twilight (for dawn or dusk), and at that time (or preferably a few minutes earlier,) pre-set your sextant to the altitude of the star you wish to shoot, then go to your compass (gyro or magnetic, in which case you will have to correct for magnetic variation), line your sextant up on the bearing (i.e. azimuth) and look in your eyepiece. Somewhere in the field you should see a small bright spot...that's the star! You might not be able to see it with your naked eye, but the sextant star-telescope will help. Using the adjusting-drum, bring the star to the horizon, rock the sextant to make sure the star just grazes the horizon, and note the time of the shot to the nearest second. Accurate timing is vital!

Now, perform your Marc St Hilaire sight-reduction using the values of time, Sidereal Hour Angle and Declination for the star for the time of your shot obtained from the Nautical Almanac,and compare it against the observed altitude (corrected for Dip or height-of-eye, sextant Index-Error and refraction) for your assumed position. There will be an error of some number of minutes of angle (i.e. nautical miles distance).

The difference is your error in position, which you will plot on a plotting-sheet. Remember the useful acronym "Coast Guard Academy" - Computed Greater Away. In other words, if your calculated altitude is greater than your corrected observed altitude, then you are further away from the star along the azimuth. Your Assumed Position is the origin of the azimuth-line on which you plot your line of position (LOP). Plot the position-line thus obtained from the comparison of calculated versus observed.

Repeat for some other stars, and try to get shots of stars which give a good angle of "cut". Work from the East to the West because in the evening you lose the eastern horizon first, and in the morning you get the eastern horizon first.

When several such stars are plotted they will not intersect in one point but will define a smallish area called a "cocked hat". You are somewhere inside that "cocked hat". That's your fix by stars.

I still practice this, but it's been a long time since I stood on the bridge of a ship.

Feather #3 20th Jul 2014 20:18

Note too that "Selected Star" are available online. Also the Nautical Almanac for your daily data.

G'day ;)

SkinFricti0n 21st Jul 2014 05:18

Very interesting subject I would like to learn more too.

aussie027 21st Jul 2014 05:26

Ozdork,

While a few peoples posts in this thread obviously show a good deal of knowledge on this subject and there is certainly plenty to learn to try and gain even a basic level of competency and understanding the accuracy of this navigation method whilst using instruments such as a sextant etc I don't think is particularly good.

Years ago I saw several documentaries talking to RAF navigators from Bomber Command and they said that using the instruments they had, bubble sextant etc for astronav as 4Greens mentioned, that the best they could get as far as positional accuracy was to fix the position of the airfield where they were at, whilst on the ground, within 5 miles of its actual location.:uhoh:
In the air it was far less accurate and as someone mentioned on a pitching boat deck likely to be less so again.

I remember at the time thinking that 5 odd mile radius figure wasn't very good especially if you were standing still when taking the star shots etc, but maybe that is considered very good for this form of navigation??

Can anybody comment on what sort of accuracy could be expected under ideal conditions ??

For those Bomber Command guys +/- 5 miles may not be bad on a long flight, especially at nite with no navaids or lights etc to help keep you on a general course etc but in those days with all the nav errors compiling during the flight and the navigator using an airplot method rather than a trackplot method there wasn't much accuracy in the bomb dropping for eg, especially at night as was proven at the time in published RAF accuracy studies and assessments.

When astronav done with electronic equipment its quite a bit more accurate,as we would expect, LOL. for eg the SR71 recon aircraft astronav equipment, from the mid 1960s-

  • Astro tracker located behind the RSO would track three stars groups on a clear day within 30 seconds after leaving the hanger. Precision location of the aircraft could be confirmed.
  • Guaranteed within 300 feet anywhere in the world traveling at 2200 mph.:ok:
Not too shabby for 50 years ago, LOL. Anyway back to you and your sextant etc, keep us posted on your progress, this form of nav is an obviously dying art and in certain applications it shouldn't be.
The stars cant be switched off or interfered with like GPS, all you need is clear skies, the tools and the know how to use them. :ok:

Mach E Avelli 21st Jul 2014 06:52

Five miles is achievable in good conditions in a small boat using a plastic sextant.
When conditions are really bad, there is usually too much cloud cover for astro to be counted on anyway.
Fortunately, when you are on the open sea and only doing five or six knots, missing a day's noon sight or even sights for 48 hours (not unusual), is unlikely to mean 'game over'. Pre GPS, it was normal to go to DR for as long as it took and most of us stayed out of trouble simply by being conservative and staying well offshore.
Five, ten or even 15 miles will normally get you in sight of land, when navigation switches to 'pilotage'. Here, the sextant is still useful for measuring distances off landmarks, lighthouses etc. And surprisingly accurate - easily to within half a mile - which of course you need close in to obstacles. Boats and ships rarely come to harm in the open sea - most wrecks occur within sight of land.

ForkTailedDrKiller 21st Jul 2014 06:58

I have been using celestial navigation for the last 15 yrs or so! :ok:

I have several GPS antennas pointed at the stars! :E

Dr :8

500N 21st Jul 2014 07:05


Can anybody comment on what sort of accuracy could be expected under ideal conditions ??
Try Danny42C on the military forum. Worth asking anyway. Their is a really long long thread where he is detailing what occurred during the war.
Others who were in it have chimed in as well.

criticalmass 21st Jul 2014 07:34

Perhaps we need to reflect on the purpose for which celestial navigation was developed...for the fixing of position of ships at sea, out of sight of land. In the middle of a four-thousand mile wide ocean an error of five miles is insignificant, and we realised this, but it in no way prevented us from being scrupulously accurate in our timing as much as our mathematics.

This was as true in the eighteenth century as it was in the twentieth, until the availability and reliability of GPS systems became such that resort to celestial navigation was considered no longer necessary. The US Navy no longer teaches it to officer trainees, but it survives due to the dedication of mariners of diverse backgrounds who learned the craft and wish to perpetuate it so it doesn't become a lost art.

It is prudent to reflect that in spite of all guarantees to the contrary, The US is perfectly capable of disabling civilian GPS signals, as are the Russians for their equivalent system. A guarantee that it will not be turned off is something only the foolish accept without at least one or two "grains of salt."

Celestial navigation can be viewed as a craft rather than a science, like wood-carving, or hand-thrown pottery. Certainly there are machines which will make more uniform products, faster and cheaper, but they are not hand-made products,with the flaws and imperfections that show their hand-made nature.

So too with position-fixing. GPS is clinically clean, swift and efficient...but the user is totally divorced from the stars, accurate time-keeping and "simple sums for simple sailors". I distrust technology basically because I have been using it since about 1977, and I know the errors and Achilles Heel of such systems. By all means let the mariner of today use his GPS receiver for fixing his position, but the prudent seaferer keeps one foot firmly planted in the past and uses his sextant to check the GPS. The sea is no respecter of technology, nor does it respect the works of man.

Celestial navigation also required the development of an accurate timepiece...accurate not in term of absolute accuracy per se, but inasmuch as its rate of deviation from Greenwich Mean Time was known and repeatable at any stage of the discharge of the mechanical escapement (the fusee detente) which powered it. Hence the development of the Marine Chronometer, a mechanical timekeeper with a rate which was more or less constant, relatively small, and for which allowance could be made.

Such importance was placed on accurate timing of sextant observations that a Chronometer Rate Book was kept on the bridge, and the deviation from standard time noted each day (in more recent times using a radio time-signal). At the annual Deck Survey, the marine surveyor would re-set the chronometer against standard time, and the rate-keeping would start anew. This was standard practice until I left the sea in 1992. All that had changed were our chronometers in their glass-topped recesses in the chart-table were quartz-crystal ones rather than the famous Frodshams, Mercers, Dents, Ulysse Nardins and Hamilton 21s of yesteryear.

These quartz marine chronometers are fantastically good rate-keepers. I have four such units, the best of which is a Seiko QM-10 which is only 8 seconds fast after 1470 days continuous running...a daily rate of 0.0054 seconds per day. My East German Glashutte is just 14 seconds slow after 847 days running, and the West German Wempe is 26 seconds slow after 1199 days, slightly worse than its cross-border counterpart. The poorest is a relatively cheap Tamaya, which is 20 seconds fast after 322 days...not all that good compared to the three others, but still far in excess of any of the best mechanical chronpometers. Even the iconic Hamilton 21, arguably the most technically-perfect, mass-produced fusee-detent marine chronometer ever made, is rarely able to attain a rate of less than half a second per day.

Mariners have a saying..."you can lend your wife but never lend your sextant."
A German C. Plath or Cassens & Plath sextant is truly a piece of ultra-precision equipment designed to operate in the hostile environment of salt-air, and varying temperatures. The Japanese Tamaya "Jupiter" and "Spica" sextants, beloved by the merchant marine around the world, are every bit as good in terms of manufacture, accuracy and usability.

The East German Freiberg sextants had greater errors along the tangent worm-screw in manufacture, but are also well thought-of as each came with a calibration certificate with the errors tabulated (as does any sextant worth keeping). They also came at a significantly reduced price and many mariners used them. British Hezzanith sextants are likewise extremely valued and accurate. There are Dutch Observator and French sextants, all of high quality. The Chinese have got in on the act and they produce a surprisingly good sextant (for the money) as the Astra IIIB, although the optics leave something to be desired when compared to German or Japanese sextants.

It is true to say all of these magnificent instruments are now in their twilight. But they will still work two hundred years from now...which cannot be said of any available GPS-receiver I know of. Modern electronics has a relatively short working-life. You only need to look at the long list of discontinued Garmin aviation GPS receivers of great popularity and usefulness to see the emphasis is now on frequent replacement instead of buying an instrument which will last your entire career.

In the middle of a vast ocean, a five-mile error in your fix is inconsequential. As you got nearer a coast and made landfall, visual bearings of landmarks took over, and a pilot (a man with local knowledge) was engaged to bring your vessel safely into harbour. Celestial navigation had served its needs for the mariner, and will continue to do so as long as there are mariners who continue to practice it.

However, in the aeronautical world it was a different mater. As aircraft speeds increased, and the need for extreme precision in navigation became paramount, the limitations of airborne celestial navigation became apparent, and drove the development of modern electronic navigation systems. This was inevitable, and as a result, airborne celestial navigation became a thing of the past - deservedly.

Those who choose to learn marine celestial navigation are keeping a connection to previous generations of mariners who went to sea in vessels of varying degrees of seaworthiness, manned with crews of varying competence, on voyages which had uncertain outcomes. One of the few certainties they had was the stars in the heavens, the navigational tables produced and checked by teams of mathematicians, and the easily-learned technique of measuring the height of a star above the horizon and from this (and the time of the observation), being able to draw a line on a chart and saying "we are somewhere on this line".


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