Ships' Clock: Astro

That's a fascinating account of a "trade skill's brief flowering". Thanks for taking the time to write it. Those navigators (including you?) must have had a particular sense of job satisfaction forever denied to their modern counterparts.

I am a huge fan of technology and would hate to do without the electronic gizmos on board my boat, but it just isn't the same, is it?

I own a bubble sextant of WW2 vintage, which I bought years ago when I thought I might do some serious off road driving in the Saudi desert. I never did venture far enough to use the sextant, but I have played around with it occasionally and am not very impressed with its accuracy. This is probably the origin of my impression that astro in the air was always a bit rough'n'ready. You have cleared up that misconception.

Obviously my ancient bubble sextant is not in the same league as the kit you describe.
:D
 
Noon Sights

I was suprised that nobody mentioned the Longitude by Equal Altitudes method for finding longitude around noon.

Basically you took a sight at about 1100 hrs local time, recorded the altitude, noting the exact time by GMT using the chronometer. Then did the Lat by Mer Pass at local noon. Re- set the first altitude and wait until the sun is on the horizon again. Note the exact time GMT. Basically the two times are added together then divided by two and a whole raft of corrections are then applied to calculate the longitude at noon to link with the noon lat and give you the position you had been in at noon. Not too bad accuracy, checked it with transit (fore runner of GPS) when it was fitted to our vessel.

Good thread.
 
JayBee said:
Yes. It helps to be stationary and necessary to know GMT at meridian passage.

There are some observational difficulties in a moving craft, relating to the actual time of transit, which is why the method is not commonly used at sea. The transits of the sun and other celestial bodies were, until very recently, used to establish the longitude of points of land during hydrographic surveys.

Have you sorted out your confusion of zone time with local time yet?
:D
Click to expand...

"Have you sorted out your confusion of zone time with local time yet?
:D[/QUOTE]"

No confusion,
"As you progress, say E-W, every 15 degrees will put you in a different local hour,"
I know what I meant.:-)
 
[/QUOTE]"No confusion,
"As you progress, say E-W, every 15 degrees will put you in a different local hour,"
I know what I meant.:-)[/QUOTE]


Yes, but you seem to have totally missed the point....

When you are in a hole, stop digging.

You've won, I give up :D
 
"No confusion,
"As you progress, say E-W, every 15 degrees will put you in a different local hour,"
I know what I meant.:-)[/QUOTE]


Yes, but you seem to have totally missed the point....

When you are in a hole, stop digging.

You've won, I give up :D[/QUOTE]

Please see pm
 
I hope alant won't mind if I post a slightly edited version of my reply to his friendly pm:

At a particular instant, GMT (or UTC, I suppose we should call it) is the same wherever you are in the world. There is only one GMT of an astronomical "event" occurring on one particular meridian. As the earth rotates, each meridian "sees" the event taking place earlier or later than at Greenwich. If we know both the local mean time of the event and the Greenwich mean time of the event, we have the longitude.

Consider the noon sight. At the moment of the sun's maximum altitude, obtain the GMT of this observation.
At the same time, by definition, it is 1200 local apparent time, regardless of what the ship's clock, set to zone time, says. I think this is the root of your misunderstanding. Applying the equation of time to local apparent time, which you have just obtained by the observed mer pass, gives local mean time.

Back to practicalities. As you will know, it is very difficult to determine the exact moment when the sun is on the meridian because its altitude is changing slowly. It is necessary to take several sights either side of noon and graph the results against time. (see Mariner69's version of this)

I would agree with you that, in general, it is very difficult to obtain accurate longitude from a single noon shot.


Finally, we all make mistakes. That talented and expert navigator Francis Chichester, who, like you, taught the subject, was in some trouble on his RTW voyage when he found he could no longer remember the basic sight reduction formula. :)
 
Lady Campanula said:
Aw, bless..... :D

At the risk of getting just a little geeky about a 'trade skill' which had a brief flowering, then passed, let me just outline some of what the RAF - er, Number One Bomber Group - was capable of during the 70s.
Click to expand...

Thank you - that's *very* interesting.

These days, if I fire up ShipFinder on my iPhone, then:
(1) The ships have picked up a ~1575MHz signal from GPS sats
(2) They have broadcasted pos & cog, etc info on VHF 161 MHz
(3) A ground station has picked it up, and punts it into TCP/IP
<internet wavey lines>
(My, err "ground station" has done the 1575MHz stuff to work out where *it* is)
(4) I can pick it up on 3G (CDMA at, what, 2.1 GHz?) or wifi (2.4 GHz) and see pretty AIS pictures overlayed ont GoogleEarth images.

And then I can hit a button and <ting> the geotagged pictures are on Facebook!

Wow!
 
mcframe said:
Thank you - that's *very* interesting.

These days, if I fire up ShipFinder on my iPhone, then:
(1) The ships have picked up a ~1575MHz signal from GPS sats
(2) They have broadcasted pos & cog, etc info on VHF 161 MHz
(3) A ground station has picked it up, and punts it into TCP/IP
<internet wavey lines>
(My, err "ground station" has done the 1575MHz stuff to work out where *it* is)
(4) I can pick it up on 3G (CDMA at, what, 2.1 GHz?) or wifi (2.4 GHz) and see pretty AIS pictures overlayed ont GoogleEarth images.

And then I can hit a button and <ting> the geotagged pictures are on Facebook!

Wow!
Click to expand...

You'll be telling this tale to your grandchildren one day and they'll be looking at you.............:D
 
Two problems with the idea of finding longitude by chronometer time of noon sight:-

(1) At about the time of local noon, the altitude of the sun doesn't change very quickly. As a quick check look in the sight reduction tables and compare the tabulated altitude when the LHA is 0 and when it is 1, and you'll see that in the four minutes that it takes for the LHA to increase by 1 degree, the Hc changes by only 1 minute. In other words, the sun doesn't appear to rise, reach max altitude and then fall... it rises more and more slowly, then hangs, then start to fall. Trying to pick the exact moment of maximum altitude isn't easy.

(2) Probably doesn't concern the average yacht but ....
if a ship is steaming north or south at 15 knots, her own movement will cause the altitude of the sun to change by 15' per hour, or 1' every 4 minutes. That is the same as the rate at which the sun's altitude changes due to the rotation of the Earth, in the few minutes around local noon. So for a ship steaming south in the northern hemisphere (or north in the southern hemisphere) the sun will appear to continue rising for a few minutes after local noon, and will then "hang" for longer than if it were being observed from a stationary platform!
 
Very true, as we've all been saying you use local noon to determine longitude with accurate clocks but to find that time you use sights before and after noon. DRawing curves if you get a selection of altitudes or if you can accurately checking the time at fixed altitudes am and pm.

Maybe we should have said for complete beginners, don't expect to get noon altitude and time by a 10 or 15 minute period with you sextant. Every time you get it out to practice make a note of the altitude and time.
 
alant said:
So, you determined longitude How?
Click to expand...

I take it then you are unfamiliar with basic astro nav?

In the morning you take a sight and draw a position line. This will be roughly North-South but not in itself enough for a longitude.

At Noon you take another sight which gives a position line exactly East-West.

You then transfer the first position line forward using the difference in the log readings at the morning and noon sights plus estimated current effects. Where the transferred position line crosses the noon lat is your fix i.e. lat & long.

In the evening you take another position line and transfer the noon lat forward in the same way. Thus you get 2 fixes a day.

Of course there are inaccuracies introduced by use of DR to do the transfer but for ocean navigation purposes it is generally adequate.

A 3 or more star fix will be more accurate - if you can get one. Also when approaching land you can use position lines in other ways. e.g. when approaching your destination you can wait until the sun azimuth is at right angles to your destination then take a position line and sail down it.
 
timbartlett said:
Two problems with the idea of finding longitude by chronometer time of noon sight:-

(1) At about the time of local noon, the altitude of the sun doesn't change very quickly. As a quick check look in the sight reduction tables and compare the tabulated altitude when the LHA is 0 and when it is 1, and you'll see that in the four minutes that it takes for the LHA to increase by 1 degree, the Hc changes by only 1 minute. In other words, the sun doesn't appear to rise, reach max altitude and then fall... it rises more and more slowly, then hangs, then start to fall. Trying to pick the exact moment of maximum altitude isn't easy.

(2) Probably doesn't concern the average yacht but ....
if a ship is steaming north or south at 15 knots, her own movement will cause the altitude of the sun to change by 15' per hour, or 1' every 4 minutes. That is the same as the rate at which the sun's altitude changes due to the rotation of the Earth, in the few minutes around local noon. So for a ship steaming south in the northern hemisphere (or north in the southern hemisphere) the sun will appear to continue rising for a few minutes after local noon, and will then "hang" for longer than if it were being observed from a stationary platform!
Click to expand...

Pay attention at the back!

Hello Tim..... wondered where you were :)

1) has been covered, in a bit less detail, in earlier posts

2) this has also been referred to in a general sense ("it helps to be stationary").
 
timbartlett said:
....if a ship is steaming north or south at 15 knots, her own movement will cause the altitude of the sun to change by 15' per hour, or 1' every 4 minutes. That is the same as the rate at which the sun's altitude changes due to the rotation of the Earth, in the few minutes around local noon......
Click to expand...

Not strictly correct

Change of altitude in one minute of time = 15' cos lat sin az

Also in an earlier post


:D
 
Mrs Jaybee thinks I've got better things to do, mostly involving venturing outside,

...but I couldn't possibly agree with her.

Roll on Spring and launch day


:D
 
Final thought...

Most of us on this forum don’t teach astro nav, write books on the subject or make instructional videos.

If we did, we might have to consider ourselves fair game when making ill informed or misleading statements, presented as the fruit of expert knowledge.

So, before this interesting thread deteriorates further into Lounge type “banter”, I’m off to keep Mrs Jaybee happy, by chopping up some firewood or washing the car or something.


:D
 
JayBee said:
Originally Posted by timbartlett
....if a ship is steaming north or south at 15 knots, her own movement will cause the altitude of the sun to change by 15' per hour, or 1' every 4 minutes. That is the same as the rate at which the sun's altitude changes due to the rotation of the Earth, in the few minutes around local noon......
Click to expand...
Not strictly correct

Change of altitude in one minute of time = 15' cos lat sin az

Also in an earlier post
:D
Click to expand...
Sorry, I must have missed it.
But if you work out the change of altitude per minute of time using your formula, I think you'll find that for an average latitude (say, 45degrees, being as how that is the average of 0-90) and for Az=1degree it works out at about 0.2degrees per minute, or 0.8degrees per four minutes. And as I didn't specify a latitude or exactly how few was "a few minutes around local noon", I reckon 0.8 qualifies as near enough to 1 to make no difference.
The fundamental point is that you don't need to be moving at jet aircraft speeds for vessel movement to make a difference.
And as I think it was you who said "it helps if you're stationary" (or words to that effect) it seems that we agree.
So what the hell are we arguing about?
 
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