Celestial navigation practices


Scott Mills

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Sorry Mike, but the 21st century is here to stay. These days you can use even use a cellphone to get your lat/lon coordinates from the GPS satellites, and then you just have to plot it on a chart. There are some more expensive marine GPS units that also could use the GLONASS system as a backup, which is the Russian version of GPS, in the remote case of the US maintained GPS system failure.
Sam,

Agreed! And it is not just NAVSTAR and GLONASS satellites anymore. China continues to build its Beidou satellite global positioning system, the European Union has Galileo (which will be fully deployed by the end of the year), and the Indian government is working to expand its IRNSS system.

So, in the next decade, you will have not one, not two, but at least 5 global positioning satellite systems with full coverage of the globe; and as you say, these things are here to stay--unless there is a nuclear war, or some apocalypse level solar activity.

And of course, you're right about GPS system breakdown. A modern smart phone has GPS built in, which works regardless of whether or not you have paid your bill! Combine that with the now ubiquitous portable solar chargers on the market, there is hardly any scenario I can imagine where someone could be stuck at sea and not have access to GPS coordinates--at least not a scenario where they could also take readings from a sextant, then do the calculations they would need.
 
Nov 14, 2005
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Sorry Mike, but the 21st century is here to stay. These days you can use even use a cellphone to get your lat/lon coordinates from the GPS satellites, and then you just have to plot it on a chart. There are some more expensive marine GPS units that also could use the GLONASS system as a backup, which is the Russian version of GPS, in the remote case of the US maintained GPS system failure.
You won't even have to do that. With the low orbit sat system they are putting up you'll be able to connect to the internet anywhere on the planet. Just hit your "where am I" tab on your phone.
 
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Jim Currie

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The way the world is going, there won't be any need to know where you are...you will be wherever your smart phone is and you don't need to move to use it.

However, what will you all do when Cyber warfare knocks out each country's smart bits of hardware in orbit? Or you all become vegetarians using fresh air for fuel ;)
 

Jim Currie

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I thought you would have used an electronic Astrolabe, Georges.:cool:
PS that really was a picture of the first sextant I bought for £12. Not the first one I had though. It came from the loft of the James Watt Nautical College. I found it when clearing out the l;oft of all bits and pieces. It really was old...had a little wooden handle an circular frames supports. Wish I had it now...it'd be worth a bob or two.
 

Georges Guay

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My Cassens & Plath have a AA battery in the handle and a light in the index bar for night vision! That’s what we call astronomical technology …
 

Georges Guay

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Samuel book p.476-477 ;

Thomas Hubbard Sumner was a captain in the American merchant navy, born in 1807. In 1837, he was Captain of the Cabot, a 115 feet, 330 tons square rigged barque which was on a voyage between Charleston, South Carolina and Greenock near Glasgow, Scotland. We were in December at the end of autumn and the weather was so bad that no celestial sights had been possible for several days. Captain Sumner was a very knowledgeable and experienced seaman who knew how to hold dead reckoning navigation. However, at the entrance of St. George’s Canal, between England and Ireland, he needed a more accurate position. Between two clouds in the morning, he managed to take only one sight of the sun by sextant. He had to make the most of this meager celestial information to determine his position as accurately as possible.

Here is the problem from is book published in 1845; Finding a ship’s position at sea by projection on Mercator Chart when Latitude, Longitude and ship’s apparent time are uncertain, with only one altitude of the sun.

On 17th December, 1837, sea account, a ship having run between 600 and 700 miles without any observation and being near the land, the latitude by dead reckoning was 51° 37' N but liable to error of 10 miles on either side, N or S. The altitude of the sun's lower limb, was 12° 02’ at about 10.12am, the eye of the observer being 17 feet above the sea; the mean time at Greenwich, by chronometer, was 10h 47m 13s am.

Nautical Almanac on 17th December 1837
- Sun Declination: 23° 23’S
- Equation of Time 03m 37s

Require:

i) The true bearing of the land (Line of Position),
ii) The sun's true azimuth,
iii)The error in longitude the vessel was subject for the uncertainty of the latitude


Any courageous enthusiast’s willing to answer? :)
 

David Allison

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Not me George. I wouldn't have a clue. The one thing I will say is the only time they were on track, is when they were passing through track. It reminds me of the MacDonald Douglas DC-8 aircraft in the 1960's and early 1970's which carried navigators. Four on the flight deck, Captain, Co-pilot, Flight Engineer and Navigator. A small dome in the ceiling of the flight deck allowed the navigator to take celestial shots with a sextant. When they made landfall after a transatlantic crossing the navigator would get a free beer on the layover, if he was within 5 mile of track. I say 'he' as there were virtually no female navigators in those years.
 
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May 3, 2005
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Didn't they include dawn and evening star sights? That's when the horizon is most clear and star altitudes can be measured most accurately. As I recall while serving on a small naval ship, the navigator stood no watches - he was up at all hours for star and sun sights, as well reducing sightings for a fix, and taking and plotting bearings when near land. He had one primary assistant, a senior quartermaster. (In the army, a quartermaster stores and issues equipment and supplies, but in the navy, quartermasters help navigate.)
[QUOTE="Doug Criner, post: 416836, member: 13811

This is just a comment on the above in the difference of terms in Army and Navy terms as mentioned in the difference between Quartermasters in the Army and the Navy.

in the Army, a Division is a large group, such as the Second Division, and in the Navy, a Division is a small group, such as the O-E Division was a small group of the specialty rating of Electronic Technicians (at least it was when I was in the USN during the Dark Ages of the 20th
Century).

I suppose I have been in both the Army (in High School and College ROTC) and in the Navy (active duty in the USN) so I am familiar at least with these two terms.
 
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Added- Too late to edit previous post :
A Division in the Army may contain as many as 10,000 to 25,000 soldiers.
A Division in the Navy may contain as little as 12 to 15 persons in that Specialty Rating , as was the O-E Division on the ship on which I served.
 

Georges Guay

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Not me George. I wouldn't have a clue. The one thing I will say is the only time they were on track, is when they were passing through track. It reminds me of the MacDonald Douglas DC-8 aircraft in the 1960's and early 1970's which carried navigators. Four on the flight deck, Captain, Co-pilot, Flight Engineer and Navigator. A small dome in the ceiling of the flight deck allowed the navigator to take celestial shots with a sextant. When they made landfall after a transatlantic crossing the navigator would get a free beer on the layover, if he was within 5 mile of track. I say 'he' as there were virtually no female navigators in those years.
Let say that the navigator was using a General Chart of a scale of 1:250,000. A pencil sharpened by teeth would make a line of about 1mm. Such a line on that General Chart meant 250,000mm on the ground or 5.4 nautical miles! Therefore, knowing where you are at a precision of the thickness of a pencil line was certainly worth a pint!

If the MacDonald Douglas DC-8 was flying due west at 204 knots and by so, the sun appeared to remain steady abeam in the port window, what was the latitude of the plane?

The speed of the plane must have been the same as the earth speed rotation, no? At the equator, the speed of an observer into space is 360° x 60 miles per degree = 21,600 miles in 24 hours, thus 900 knots. But at the poles, the speed of the observer is nada zero knot since he spins around himself as the meridians converge to the poles. We know that:

- Cosines 0° (equator) = 1 ... 900 knots x 1 = 900 knots
- Cosines 90° (pole) = 0 ... 900 knots x 0 = 0 knot

Consequently, the speed or the distance run due east or west on any other latitude, as to be offset by the Cosines of the latitude or Cos lat°.

Speed plane = Speed earth
204 = 900 Cos lat°
Cos lat°= 204 ÷ 900 (cosˉ¹ 0.2267)
Latitude = 76° 53’ 56’’ N

Furthermore, if the plane was flying due west and the sun remained abeam in the port window, thus due south or on the meridian of the pilot himself, the tabulated nautical almanac Greenwich Hour Angle of the sun matching the plane chronometer Greenwich Mean Time, would give his Longitude!

I’d bet he get two pints for that! :)
 
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Georges Guay

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My own Captain Sumner quandary is ready! ;)

Samuel, at p.479 you write; «This transferred LOP (from morning sight) is then crossed with the LOP taken at noon, which for noontime sun would be a horizontal line on the chart at ship’s noontime latitude. Where the two LOPs cross is the ship’s noontime.»

Captain Sumner idea was to simulate sun meridian passages at two different latitudes to estimate two corresponding longitudes by chronometer. He knew that the time of sun meridian passage was exactly 12:00 LAT (Local Apparent Time), that the difference between LAT against LMT (Local Mean Time) was the Equation of Time and that the difference between LMT and GMT (Greenwich Mean Time) would give the longitude. He also knew that at sun meridian passage the celestial body would bear due south, that the angle at pole (P) from PZX spherical triangle was 0° and thus, the GHA (Greenwich Hour Angle) of the sun equated the longitude.

Since Titanic navigators would also knew that, they would calculate the sun meridian passage time for an estimated longitude derived from course & speed. They might have had a pelorus or an alidade lined up to due south as well. Earlier than the sun meridian passage time, they would start to shoot the sun altitude. Soon, the micrometer drum would stop to increase, steady, shout «mark» and record instantly and precisely the chronometer, starting by the seconds hands. The ATS (Apparent Time Ship) or LAT (Local Apparent Time) would then be exactly noon, the master clock could be fine tune adjusted to 12:00.

From the nautical almanac, they would determine the sun declination and the GHA. After correcting the sextant altitude from index error, dip, refraction, semi diameter and parallax, they would obtain the sun true altitude and from 90°, deduced the zenith distance (ZX). A very precise latitude could then be easily determined by simple deduction. (see picture)

So now they had a precise latitude at 12:00 ATS. The sun azimuth exactly north or south true thus the angle at pole (P) within the spherical celestial triangle PZX was 0°; the GHA of the sun would equal the longitude of the observer. Knowing that the difference between the ATS and LMT is the Equation of Time and that the difference between LMT and GMT is the time longitude, they only had to convert that time longitude into arc degrees, minutes and seconds’ longitude.

ATS ~ Equation of Time = LMT
LMT ~ GMT = Longitude

They now had an accurate fix at noon ATS.

Running the morning LOP to apparent noon latitude LOP’s, would certainly induced an error in the position fixing since they only knew the distance run through the water, by patent log and magnetic compass. Working out the longitude by chronometer at noon local apparent time, gave a position so accurate that they could deduce the set & drift and filling up the intended box in the log book for future reference.

Note: You can take a sun lower limb sight at about 10:00 ATS, record the sextant altitude and exact chrono time. At about 14:00 ATS, you pre-adjust your sextant at the same altitude as 10:00 and wait for the sun lower limb to touch the horizon and again, record the exact chrono. Then, you can determine the exact time of meridian passage between the two sights and compare it with the half difference of the two chrono. Any disparity found in time is the chrono error! The method is called the chronometer error by same altitude of a celestial body or so … :)
 

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David Allison

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George,

Yes, a pencil width is worth a pint.

You have great questions which require thought. I get the same answers as you, without using trigonometric functions, so yes, I agree with your calculation of 77°N and yes the speed of the plane is same as the earth’s rotation. Speed on the DC-8 a touch low by about 300 knots. Now I’m sadly lacking in understanding your Sumner Quandary but Its ok, I’m just thankful I was never a nav.

So Captain Sumner first takes a fix and at that time knows his position. Then the Cabot, a 115 foot square rigged barque, pitching and rolling with a magnetic compass swinging back and forth with overcast skies in heavy weather for several days with someone at the wheel probably more worried about how to handle the next oncoming wave than maintaining a heading, approaches landfall. After these several days his run distance is 600 to 700 nautical miles. 650 ± 50 miles according to taffrail log. This is likely a record of a zig zag trail distance for the reason of tacking back and forth. I would say if he makes landfall before seeing the sun, his best option to send someone ashore and ask where they are. Perhaps not necessary as United Kingdom should be on the right and Ireland on the left.

Anyway, Captain Sumner gets lucky before making landfall with one brief peak at the sun. How many miles do you think the Cabot would be off track in those conditions? Just a guess is fine.
 

Georges Guay

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I would say if he makes landfall before seeing the sun, his best option to send someone ashore and ask where they are.
The barque time was about 10.12am. Shortly after his famous deduction, The Smalls lighthouse at the entrance of St-George’s Channel was sighted right ahead.

The approach of St-George’s Channel was therefore made in daylight time. The weather could’ve been overcast and rough, but the visibility good. In the old Pilot Books, you could consult handmade drawings of landscape (see picture). I am pretty sure that these drawings were available for St-George’s Channel approaches. If you were not able to match these illustrations with what was observed, you better had to look for other options as hoving to along with seaman sounding the sea bed and anchor(s) ready to let go!

Yo, ho, ho and another bottle of Whiskey!
 

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David Allison

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The Smalls lighthouse at the entrance of St-George’s Channel was sighted right ahead.
Very interesting George! Quite an amazing feat, wouldn't you agree? I would say Captain Sumner deserves a gold medal for being on approach to St-Georges channel after several days in stormy weather not able to obtain a fix. No doubt they had 'rules of thumb' long since lost, to help them. It's a bit like throwing darts, though.

Personally I would give Captain Sumner a gold star for a cross track error of up 30 nautical miles, a sliver star up to 60 nautical miles. I certainly wouldn't make him walk the plank or return him to navigation school if he exceeded 60 miles given those conditions. I'm quite serious they could question whether a landfall sighting was Ireland or United Kingdom. Even with GPS it is sometimes not easy to immediately identify what you are looking at.

In the world of celestial navigation how long does it reasonably take you to obtain a position? How about clear skies with the wind howling and the ship rock'n rolling, is it tough to take an accurate position?

Wonderful prints attached btw, thank you!
 

David Allison

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The weather could’ve been overcast and rough, but the visibility good.
Visibility is so very important. Ships in the middle of the Atlantic in 1912 on nights with no moon and overcast skies would in my opinion have no visibility in front of the ship. Take for example the Olympic. What speed would you run in those conditions?
 

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