Imperial vs. metric and navigation

[PeteH]

I am of that age group that still THINKS in "imperial". However I`ve had to work in both most of my Life. Bar`s, Kp/Hr, psi, temp in degC, and F. And was doing my Marine Qualification`s when the "official" change from Imperial to SI, was in effect, so I started in Imperial and had to quickly get to speed with SI and metrication.

[1969Mk3Spitfire]

3 minutes ago, JohnD said:

In the sense that one minute (1/60th) one degree of longitude (1/360th of the Earth's circumference) was defined as one nautical mile, and that the Earth turned around in 1 day, so it is unit-ish of time.  Trouble is, the Earth is NOT spherical, it's an  oblate spheroid, and does NOT turn at an exactly constant rate.  So now, a Nautical Mile is defined as exactly 1,852 metres.

Suck on that, Imperialists!

John

You’re quite right, I didn’t expect to find a pedantic celestial expert within this forum.

Equation of Time shows the adjustment required. The Local Mean Time of the sun meridian passage is shown in the picture. It is as late as 12h16 at Greenwich in Feb and as early as 11h44 in October.

A nautical almanac shows GHA of the sun at each second of each day.

Notwithstanding, charts show longitude and this is used for Time Zones. Each 1 hour time zone is centred from Greenwich meridian. UT (GMT or Z) is 7.5 deg east and west. Cartographers use 360 and 24 hour notation.
 

Fascinating subject to learn, I use it for celestial navigation.

 

[Chris A]

All this technical stuff just goes to prove time is an illusion and it is always appero time, somewhere. Cheers 🍹

[PeteH]

4 hours ago, 1969Mk3Spitfire said:

You’re quite right, I didn’t expect to find a pedantic celestial expert within this forum.

Equation of Time shows the adjustment required. The Local Mean Time of the sun meridian passage is shown in the picture. It is as late as 12h16 at Greenwich in Feb and as early as 11h44 in October.

A nautical almanac shows GHA of the sun at each second of each day.

Notwithstanding, charts show longitude and this is used for Time Zones. Each 1 hour time zone is centred from Greenwich meridian. UT (GMT or Z) is 7.5 deg east and west. Cartographers use 360 and 24 hour notation.
 

Fascinating subject to learn, I use it for celestial navigation.

 

This is the sort of chart, used to make my eye`s glaze over, The Nav; dept had books of similar. The Second Officer usually doubled as "Navigator", in the time before the universal introduction of "Sat Nav". It was his duty to check the course and position daily using "noon" sights, and plot the position of the ship as near as humanly possible. Being my "offwatch" I had many occasion to go and chat whilst he made that and then got authorisation from the Capt; to correct course. One such was brilliant at it, and Metorology too, I learned a lot talking to him. We "ginger beers" just had to make sure the damn thing kept going!!😁

Edited June 19, 2023 by PeteH

[1969Mk3Spitfire]

12 hours ago, PeteH said:

This is the sort of chart, used to make my eye`s glaze over, The Nav; dept had books of similar. The Second Officer usually doubled as "Navigator", in the time before the universal introduction of "Sat Nav". It was his duty to check the course and position daily using "noon" sights, and plot the position of the ship as near as humanly possible. Being my "offwatch" I had many occasion to go and chat whilst he made that and then got authorisation from the Capt; to correct course. One such was brilliant at it, and Metorology too, I learned a lot talking to him. We "ginger beers" just had to make sure the damn thing kept going!!😁

Fascinating story. A sextant is a wonderful tool to own, a piece of engineering excellence and a joy to use.

The sun is the easiest celestial body to use for navigation. Its meridian passage (midday, locally, wherever one is in the world), maximum sextant angle, gives a measure of latitude.

Leisure sailors on an ocean crossing usually take 2-3 sextant readings per day; forenoon, MP and afternoon. The resulting Position Lines are plotted and a Running Fix is maintained to improve the Dead Reckoning position.

Stars and planets can be used, too, but only at twilight when it’s dark enough to see the body but light enough to see the horizon. Polaris, the pole star, also gives a direct measure of latitude.

 

 

[JohnD]

Never used a sextant - would love to know how, but never sailed out of sight of land!

But proud of my Boy Scout trick of finding North from the Sun and a watch.    Point the hour hand at the Sun, halve the angle of that with the 12 position.   That direction is South, so North is the other way.  Simples!  But beware  British Summer Time!

John

[ahebron]

I told an actor mate that trick and when he went London he kept getting lost.
I told him how it works here in the Southern Hemisphere.

[1969Mk3Spitfire]

@JohnDUsing a sextant is pretty straight forward.  It simply measures the angle of a heavenly body against the horizon.  Navigators use the PZX spherical triangle (P=Pole, Z=observers zenith, X=body). Sight Reduction Tables are simply tables of trigonometry, same concept as sin, cos and tan for a linear triangle but using arguments of Lat, Declination, Local Hour Angle to give results of Altitude and Azimuth.  Once one has grasped the definition of the terms, its fairly easy.

The other key part of the process is an accurate measure of time (Harrisons chronometer etc).  Time is basis of navigation.

The Marcq St Hilaire method (intercept method) is probably the most used process of sight reduction. It compares altitude and azimuth of the body calculated from the time of the observation to the altitude measured by the sextant.  The difference between the two derives a position line.

By coincidence, it's the Longest Day tomorrow, the declination of the sun is 23.5 deg N, sitting on Tropic of Cancer, summer solstice.  Afterwhich, it will reduce to 23.5 deg S, sitting on Tropic of Capricorn, winter solstice, on 21st Dec.

Given that we are approx 50 deg N, the sun will always be due south of us as it passes overhead at local noon.  

 

Sorry for the drift to those who have fallen asleep.

[dannyb]

There is a good documentary on YouTube about the Harrison clocks. H1, 2, 3 and 4 to determine longitudes, which worked in poor visibility.

They still have these at the Greenwich Observatory 

Danny

Edited June 20, 2023 by dannyb

[JohnD]

And the book "Longitude" by Dava Sobel was all about Harrison and his chronometers.     A good read!

But the point of them was not that they worked in poor visibility, but they worked at all!      Ordinary clocks until then did not keep time accurately, and Longitude  - how far around the globe you are from your starting point - is determined by finding noon where you are with your sextant and comparing that with the time at home.     To be able to do so was the "weapon of mass destruction" of it's age and an enormous prize was offered to anyone who could achieve it, which Harrison eventually won (see the book!).     Although the secret of his clocks soon leaked out, they were an important aspect of the success of the British Navy and Empire.

John

We've got a long way from "Home-made tools, haven't we?   I'll ask for a new thread!

[PeteH]

Pedant warning. The primary navigation time instrument is. Not a clock, they are far more accurate are technically chronometers. Yawn😁!. In my day the ships radio operator would put the Greenwich time signal on the tannoy system at noon. For a check, IF he could find a signal.

edit I see John got ahead of me😄

Pete

Edited June 20, 2023 by PeteH

[1969Mk3Spitfire]

More thread drift!

HF radio propagation is a good subject but probably best left for another day.

Ships clock is usually moved by an hour with each passage of 15 degrees of Longitude. Most important for the chef to ensure crew welfare and fat bellies.

Ships chronometer remains set to UT and used for navigation.

I wear a mechanical timepiece with “superlative chronometer officially certified” engraved on its face but I wouldn’t use it without being able to check it against a calibration signal.

For leisure sailors, it’s normal practice to note accuracy against a time signal each week for a month prior to making a passage, hence making a calibration curve. A £10 Casio is normally very accurate, although they can drift due to higher ambient temperature associated with longer passages. Carrying two cheap watches only confuses, which one is wrong! On my last Atlantic crossing I had 3 cheap digital watches and, fortunately, they were within a second of each other after 18 days at sea.

 

[Wagger]

If you are going to set your watch by a time signal, do not use a DAB radio or freeview. It is about two seconds behind the fm/am radio.

Looking at earlier posts all that I can say is that the metric system is a compromise. It should never have been decimal, a third of ten is 3.333 recurring in decimal. Duo-decimal would have been far better. There is evidence somewhere that some humans counted on their knuckles, twelve on their fingers, two on the thumb. The fingers used as units, the thumbs as multipliers. Note that I use words, not numbers. 10 is sixteen in hexadecimal, eight in octal, etc. It was very difficult for me to teach binary and Hex to some students at adult education classes. Some could not understand that.

We slipped way down the league as mathmaticians when we adopted the metric system. It is convenient and far from perfect.

Maybe we should have used 'Pi' as a base, then all numbers would appear to be irrational. Maybe even the Naperian logarithm 'e'.

Rant over. Must be feeling better.

Lloyd.

 

[PeteH]

6 hours ago, 1969Mk3Spitfire said:

More thread drift!

HF radio propagation is a good subject but probably best left for another day.

Ships clock is usually moved by an hour with each passage of 15 degrees of Longitude. Most important for the chef to ensure crew welfare and fat bellies.

Ships chronometer remains set to UT and used for navigation.

I wear a mechanical timepiece with “superlative chronometer officially certified” engraved on its face but I wouldn’t use it without being able to check it against a calibration signal.

For leisure sailors, it’s normal practice to note accuracy against a time signal each week for a month prior to making a passage, hence making a calibration curve. A £10 Casio is normally very accurate, although they can drift due to higher ambient temperature associated with longer passages. Carrying two cheap watches only confuses, which one is wrong! On my last Atlantic crossing I had 3 cheap digital watches and, fortunately, they were within a second of each other after 18 days at sea.

 

Lots of "Drift" when you`ve lost your main propulsion, and for "N" hrs your at mercy of whatever the wind sea and currents decide, Even more so when the "lee shore" is less than 2 Mile away, Brown underwear is the order of the day!

Pete

[Dick Twitchen]

Sure the seafarers among us have nautical tales that few would understand/believe and in a language /dialect beloved of those of us who lived that life.  'Swinging the lamp' a pastime much enjoyed over a tot or two.

12 hours ago, JohnD said:

Never used a sextant - would love to know how

John, if I remember and have room I'll pop mine in the back of the GT6 and you can have a practice at Tetre Rouge.

Dick

[JohnD]

Surely, "Swinging the lead", Dick?

And thank you for the offer, especially the tot or two!    I won't be at Tertres Rouge, I'm based at Houx with the Talbot Team, but if time permits, I shall 'swing by'!     Houx may be conveneient to the paddocks, but cannot be compared to the best campsite at Le Mans, embellished by the TSSC organisation!

John

[Chris A]

15 minutes ago, JohnD said:

"Swinging the lead",

That phrase I do know. The first thing I recognise since this thread got onto naughty nautical matters

[Dick Twitchen]

'Swinging the lead' infers loafing about/shirking work, whereas 'Swinging the lamp' is story telling (true/false/somewhere in between).

Getting withdrawl symptoms here, but despite having left 'active' service 16 years ago, and duties as an Admiralty Trials Master about 9 years ago I still appear to be in demand mentoring senior warfare courses; for free but they do buy me a drink and lunch.

Dick 

[Colin Lindsay]

50 minutes ago, Dick Twitchen said:

'Swinging the lead' infers loafing about/shirking work, whereas 'Swinging the lamp' is story telling (true/false/somewhere in between).

The equivalent of 'pull up a sandbag'...

[JohnD]

Sorry, Dick, I shouldn't  try to correct an Ancient Mariner on Jackspeak!

[Peter Truman]

Question we here in eastern Aus based on the International Date Line (Longitude) are around 10 hrs ahead of you in the UK yet our winter solstice ie shortest day is today June 22nd, whereas your summer solstice ie longest day was yesterday June 21st, as the solstices are based on the Earth's orbit is this 1 day difference just co-incidental and at what point in the world ie latitude and longitude would the date for the southern winter and northern summer solstice be the same.

The difference in Longitude btwn UK and Aus is notionally 137 degrees (3 degrees W to 134 degrees E) not 180, and Latitude (N to S) 81 degrees, NZ which is further south still has it's winter solstice on the 22nd June. 

I know "a little knowledge (or nil) is a dangerous thing"!

The reason for the Q was SWMBO birthday is today the 22nd & she commented she didn't know she was born on the summer solstice (she's English) I told her she wasn't as the UK solstice is the 21st, she then said why are the UK/Aus solstices a day apart!

 

Edited June 21, 2023 by Peter Truman

[ahebron]

Check when winter solstice is on the Antipodes Islands. That’s the nearest land mass if you can them a mass to the antipode of London. You can’t call them up as they are uninhabited and are a NZ territory. 
 

[1969Mk3Spitfire]

The 2023 Nautical Almanac shows that the declination of the sun reaches its max north, 23 degrees, 26.3 minutes at 03h00 UT on 21st June. It stays at this same value until 04h00 the following day. I don’t have it with greater accuracy that one decimal minute of arc but it’s probably safe to assume a peak at 15h00?

Similarly, winter solstice, 23 degrees, 26.3 minutes south, is first reached at 17h00 UT on 21st December and stays until 15h00 on 22nd. Assumed peak at 03h00?

For example, if the longitude of Sydney is approx 150 deg E.

Longitude is another way of expressing time: 360 degrees is same as 24 hours.

Hence 150 degrees is same as 10 hours.

Using the formula LMT = GMT +E long (or -W Long).

Summer solstice happens precisely (?), say 15h00 on 21st June at Greenwich. What time is it in Sydney when it’s 15h00 at Greenwich?

15h00 + 10h00 is 01h00 on 22nd June.

Locally, when it’s 03h00 on 22nd Dec at Greenwich it will be 13h00 in Sydney.

[Wagger]

The 'Leap' year messes with this as the solstices drift by a quarter of a day per year. Plus, our orbit is slightly elliptical so the the earliest sunset and latest sunrise do not occur on the same day, Also applicable to earliest sunset and latest sunrise.

In the uk, the earlist sunset is about 13th December, latest sunrise 31st December and solstice about 21st Dec.

True noon varies between 11 45 and 12 15 GMT. It is, actually, why we change the clocks GMT/BST.

Not a lot of people know that.

Once upon a time we used sundials. On a cloudy day they are as useful as luminous versions.

[dougbgt6]

My dad was desperate for a sat nav, despite being 86 and his only ever journey to Morrisons down the road. I did however buy him an atomic watch, it takes it's signal on long wave from a transmitter attached to an atomic clock. He was ecstatic, it also spoke to him, just as well as his eyesight wasn't good.  We were with him when he peacefully died. The  the watch right on cue told us it was 22:15.

Doug   

Edited June 22, 2023 by dougbgt6
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