Ships' Clock: Astro

[JayBee]

Timing sights

For timing sights a cheap stopwatch is a good buy. It gives you time to park your sextant somewhere safe, then consult the deckwatch, before applying the elapsed time to Chronometer/Casio time and noting the obs altitude.

It beat the traditional walk from the bridge wing to the chronometer in the chartroom, muttering: "one and two and three and four......."

 

[Richard10002]

When I was a deck cadet it always amazed me that there was a big discussion about where to put the Noon position. 2 or 3 cadets, 3rd officer, and 2nd officer, did sights and, barring silly errors, we were always within something like a 5 mile circle, and never in the same spot. Mostly the 2nd officer's position was considered correct, (I wonder why).

When I reached the dizzy heights of 3rd officer, it was amazing how my sights became "more accurate" and how, every now and then, it was my position that was chosen to grace the chart

In real terms, none of us knew for sure who was correct, (until we got one of those new fangled big GPS screens to give us our actual position ),and what did it matter? We were well out of sight of land, no danger of going aground so, "roughly here", seemed fine to me.

A GPS time, coupled with a digital watch/clock, would be fine, and what is there to worry about if they are a few seconds out?

That's my opinion, as an ex professional

 

[Neil_Y]

Actually I do use the time when doing a noon site, if your watch is quite accurate (which casios are) then you can get a reasonable fix.

Compare local noon to gmt and you have longitude.

The simple answer to the question is two casios, your errors will be in your sights not in your clocks. I have an ssb receiver which gives time signals sometimes. For my navigation I'm happy with +-15-20miles. The longer the voyage the better it gets!

 

[nonitoo]

Actually I do use the time when doing a noon site, if your watch is quite accurate (which casios are) then you can get a reasonable fix.

Compare local noon to gmt and you have longitude.

I won't disagree with you but the problem really is, when is local noon ?

Especially in higher latitudes it is not easy to observe the exact time of max meridian altitude. Yes, the max altitude is easy to observe but the exact time...?

Tom

 

[alant]

Actually I do use the time when doing a noon site, if your watch is quite accurate (which casios are) then you can get a reasonable fix.

Compare local noon to gmt and you have longitude.

The simple answer to the question is two casios, your errors will be in your sights not in your clocks. I have an ssb receiver which gives time signals sometimes. For my navigation I'm happy with +-15-20miles. The longer the voyage the better it gets!

"Compare local noon to gmt and you have longitude."

So, why is a noon sight only used for a latitude position line, if its that easy to obtain longitude?

Everyone would be using their meridian passage sight to get an 'accurate' fix, rather than using the noon LAT as a horizontal transferred position line, to do so later in the day, which is the usual method.

Local Noon, is at the same 'local time' wherever it occurs, so how do you compare with GMT? Noon off the Scilly's, if at 1203 hrs, would be the same 'local 1203' time 1000 Nm out into the Atlantic, so which GMT would you use?

 

[capnsensible]

Try this.

Just before local noon, note your sextant reading and time. When you reckon you have nailed noon, re-set the sextant to the earlier reading. Keep up your oservatins until the sun hits your sextant angle and note the time again.

Half the time difference and add it to your first time notation. With practice and the conversion of arc to time table, you can get a good Longtitude.

This is a method recommended in the accompanying pamphlet with Davies Sextants! Has done it for me.

 

[capnsensible]

Don't forget to correct for the time difference of the exact Meridian Passage at Greenwich, bottom right hand boxes in the Almanac. Equation of Time.

 

[capnsensible]

alant,

Indeed lots of people do just take that one sight (sorry old and bold pro's!) and it can be surprisingly good. Personally though, I wouldn't like to rely on the one and prefer to do the traditional forenoon and afternoon sights, plus whatever else presents itsef. How handy is that daytime moon!

I have entertained myself-and crew- by getting not very good starsights for hours too.

Lets face it, on a long offshore trip, you can go comatose with boredom so there is nothing like testing your temper with sights! Love it.

 

[nonitoo]

Don't forget to correct for the time difference of the exact Meridian Passage at Greenwich, bottom right hand boxes in the Almanac. Equation of Time.

Dredging back to my Principles of Navigation Examinations that can be up to +/-15minutes.

As a theory it is sound but in practice I have my doubts ! Depends, I suppose, on the accuracy you are looking for.

Tom

 

[capnsensible]

As with everything, its all down to practice and should get you within say, 5ish miles.

May take half an Ocean to get it though!!

 

[Neil_Y]

All I can say is I taught myself astro so I don't know what the preferred or accepted methods are and this method works, but...and here's the problem.

Trying to judge the exact time when the sun reaches it's highest point is critical, but with the use of several sights before and after you can construct a curve, or (which also works) take an accurate local time of a set angle am and pm and then you can calculate the mid point or local noon. Allowing of course for your transferred position, distance run east or west.

Noon at 0deg longitude you know for the date you are sailing, your noon will be a few hours later mid atlantic, so if you know the time you know your longitude.

Like I said better than 15miles is OK for ocean passages as you resort to eyeball and lead line after this.

 

[JayBee]

Local Noon, is at the same 'local time' wherever it occurs, so how do you compare with GMT? Noon off the Scilly's, if at 1203 hrs, would be the same 'local 1203' time 1000 Nm out into the Atlantic, so which GMT would you use?

When it's 1203 LMT, 1000 miles due west (of Greenwich), in 50 degrees north, the GMT will be 1347.

(Divide 1000 miles of departure by cos Lat to get difference of longitude).

Subtract 1203 from GMT to get 1 hour 44 minutes. Convert this time difference to arc and get 26 degrees (west).

These figures are not exact - rounded off for simplicity.

 

[Skylark]

Many thanks to all of you who replied to my OP.

I did the RYA Ocean theory course several years ago but never practiced.

I recently bought the Adlard Coles' Ocean Yachtmaster course book and I'm systematically going through it again, chapter at a time.

At the boat show, I invested in all three volumes of AP3270 together with an almanac. I already own a C&P metal sextant.

I'm still at least a few years away from joining the ranks of those of you who've done an ocean crossings but, well, it's never too early to start the preparation.

I wouldn't dream of long distance sailing without at least trying to navigate by sextant. If for no other reason as an acknowledgement, appreciation and thanks to our forefathers for figuring it out in the first place!

Looks like the ubiquitous Casio wins the day. I wear a posh Swiss mechanical watch normally, but as anyone else who owns one will know, they are rubbish timekeepers!

Sounds like a good idea to have 2 and check their calibration versus Terry Wogan's analogue pips (sadly, no more) a few weeks before cast-off.

As an aside, I note that most posts refer to the sun, meridian passage et al. How about twilight three point fix from the stars. Is this "common practice" on a recreational yacht? Perhaps start a new thread on the practicalities, common use and techniques for astro...any takers?

David

 

[Richard10002]

As an aside, I note that most posts refer to the sun, meridian passage et al. How about twilight three point fix from the stars. Is this "common practice" on a recreational yacht? Perhaps start a new thread on the practicalities, common use and techniques for astro...any takers?

David

If you are doing Astro for "fun", (or just in case...), on a passage, I'm not sure about common practice, but it would make sense to do a star site at dawn and dusk. Much more difficult so, presumably, much more satisfying than a noon site.

I also guess that, if you are in the "routine", it would stand you in good stead if the various GPS systems on the boat failed.

 

[nonitoo]

"Stars" at dawn and dusk are likely to be more accurate than the noon fix.

All you have to do is pre-calculate the approximate alt. and az. of the most convenient stars based on your DR position. I used to use a WWII Rude Star Identifier (cost me 5/- in 1964, still have it at home). Surely there must be a modern equivalent nowadays.

One like mine is here - good price too
http://cgi.ebay.co.uk/Star-Identifi...ItemQQimsxq20100118?IMSfp=TL100118221004r7082

Preset the approx alt on the sextant, look at the azimuth and there it should be.

Don't forget to grab the stars in the darker part of the sky first, working round to the lighter as the light goes.

I never used Air Nav tables, preferring to calculate 6 separate Marq st Hilaire sights - it would take about 20 minutes start to finish. I preferred the smaller intercepts giving me a better idea of whether the sight was 'good' or not prior to plotting.

Tom

 

[JayBee]

As an aside, I note that most posts refer to the sun, meridian passage et al. How about twilight three point fix from the stars. Is this "common practice" on a recreational yacht? Perhaps start a new thread on the practicalities, common use and techniques for astro...any takers?

David

Don't know about "common practice" on a recreational yacht, but I suspect not.

Volume 1 of AP 3270, Selected Stars, makes planning and reduction of star sights a doddle, although the actual observations can sometimes be a little more demanding than sun sights. The main problem with AP 3270 is its sheer bulk and the fact that Vol 1 is only good for the current epoch.

It is possible to buy AP 3270 or the US equivalent, HO 249, in reduced format, but the print is so small that it is a bit difficult to read. You can also download just the (full size) pages you want, for free.

If these bulky volumes are difficult to stow on most boats, there is always the calculator option, freely available software for your laptop or my personal no-battery-required choice:

The Complete On-board Celestial Navigator by George Bennett.*


This slim, spiral bound volume is a compact and simple to use set of tables and long term almanac. Unfortunately the long term almanac in the current edition is only valid to 2011, so perhaps a bad buy at the moment.

* no connection, commercial or otherwise!

 

[alant]

alant,

Indeed lots of people do just take that one sight (sorry old and bold pro's!) and it can be surprisingly good. Personally though, I wouldn't like to rely on the one and prefer to do the traditional forenoon and afternoon sights, plus whatever else presents itsef. How handy is that daytime moon!

I have entertained myself-and crew- by getting not very good starsights for hours too.

Lets face it, on a long offshore trip, you can go comatose with boredom so there is nothing like testing your temper with sights! Love it.

"How handy is that daytime moon!"


Useful in theory, but when asked a few years ago, Tom Cunliffe admitted that it had never/rarely worked for him.

 

[alant]

When it's 1203 LMT, 1000 miles due west (of Greenwich), in 50 degrees north, the GMT will be 1347.

(Divide 1000 miles of departure by cos Lat to get difference of longitude).

Subtract 1203 from GMT to get 1 hour 44 minutes. Convert this time difference to arc and get 26 degrees (west).

These figures are not exact - rounded off for simplicity.

"Subtract 1203 from GMT to get 1 hour 44 minutes. Convert this time difference to arc and get 26 degrees (west)."

I'm always happy to learn, but I do not think you can 'assume' longitude this easily, since 'local time' is not exact.

As you progress, say E-W, every 15 degrees will put you in a different local hour, adjusting 'ships time accordingly', so you have at least this error to account for.

As previously mentioned, if it was that easy, ships would have been plotting exact fixes since John Harrison's Chronometer was invented.

I'm no expert, just teach the RYA course, so enlighten me please.

 

[JayBee]

"How handy is that daytime moon!"


Useful in theory, but when asked a few years ago, Tom Cunliffe admitted that it had never/rarely worked for him.

Well, when it's clearly visible up there in the daytime sky, above a sharp horizon, it works as well as any other celestial body. It just needs a couple more steps in the calculation.

Tom Cunliffe has written a book focussing largely on astro nav, but he isn't infallible.

He currently features in a video on errors of the sextant in the Yachting Monthly section of this website, which is basically a load of old cobblers.

 

[alant]

If you are doing Astro for "fun", (or just in case...), on a passage, I'm not sure about common practice, but it would make sense to do a star site at dawn and dusk. Much more difficult so, presumably, much more satisfying than a noon site.

I also guess that, if you are in the "routine", it would stand you in good stead if the various GPS systems on the boat failed.

This works.

However, you must plan your sights/stars in advance.

Had a guy on last crossing, who knew little about astro, but had bought a good sextant for the trip. With some practice + help to work through the reductions etc, plus Cunliffes book, got some good results ( 42' yacht).