Noon sights by sextant

[kalanka]

Despite a boat with plenty of electronics I am attracted by the elegance of traditional methods. I thought I would start with a simple noon sun shot to determine latitude but am already uncertain of the process.

It is clear that what I want to measure is the maximum altitude of the sun but what is not clear is how this relates to local time. It cannot be the case that the sun is at maximum altitude over the whole of, say, the UK time zone at 1200 local time, so is there a correction from tables to tell me reasonably accurately when the maximum altitude will be at a particular point or estimated point?

Thanks

 

[KellysEye]

There is a relevant site here the home page is difficult to read but the links are clear. The best thing to do is buy a book, we bought Mary Blewitt's book on Astro, some books are gobbledegook but this is in plain English.

 

[ghostlymoron]

I'm a bit rusty on this despite having done a course 2 years ago. I suggest you do a search on google - there are plenty of free courses on astro nav.
Basically, you find the time when the sun is at is maximum height (altitude), note the time when this occurs and compare it with the time for the same at Greenwich. From this difference you can calculate your longitude. NB the sun is rarely directly overhead at Greenwich at noon. The sun rotates around the earth (for the purposes of astro nav) once a day that is 360 degrees in 24 hrs. So, for every hour difference in local noon (sun directly overhead and maximum elevation) you must be 15 degrees east or west of Greenwich.
BTW I found the RYA book easier to understand than most.

 

[john_morris_uk]

You need to find out when local noon is (approximately).

If you have a nautical Almanac and you have an idea of your longitude then the sum is very simple. (Or you look it up using the tables..!)

The process is that you start shooting the sun a few minutes before your 'predicted' local noon and measure as best you are able the highest altitude the sun achieves. The sums to find latitude are then very straightforward.


PS I find Tom Cunliffe's book on Astro much easier to understand than Mary Blewitts. Choose one that seems to make sense to you is probably best advice?

 

[John Barry]

You can't get longitude from a noon sight. A noon sight is to determine latitude.

You check the altitude of the Sun at local noon (when the altitude is at its maximum) and use this (corrected) and the Sun's declination (at approximately that time) to determine your latitude. The Sun's declination changes at most at about 1'.0 per hour (at the equinoxes) so you only need time to within a couple of minutes.

To get longitude you need accurate time (to the second), and more calculating (or looking up in tables).

I can give you the various formulae if you wish.

John

 

[Noahsdad]

The OP is only looking to find Latitude and to get a feel for astro.
No great leap to do a sun-run-sun sight after that for a position.
Found the Cunliffe book a good prompt to remind me of the procedure as getting rusty!

 

[ghostlymoron]

You can't get longitude from a noon sight. A noon sight is to determine latitude.

You check the altitude of the Sun at local noon (when the altitude is at its maximum) and use this (corrected) and the Sun's declination (at approximately that time) to determine your latitude. The Sun's declination changes at most at about 1'.0 per hour (at the equinoxes) so you only need time to within a couple of minutes.

To get longitude you need accurate time (to the second), and more calculating (or looking up in tables).

I can give you the various formulae if you wish.

John

Told you I was rusty!
Don't know about an accurate clock - Josh Slocum sailed with a one dollar alarm clock and he didn't get lost. (The clock was reduced from the usual 2$ because the minute hand was missing). These days any £3 quartz alarm is as accurate as the best mechanical chronometer. (Arguably)

 

[John Barry]

Slocum used the lunar distance method to get time.

Happy to comment on it but the OP said that he was just starting, and lunar distances are rather advanced (and the tables are no longer published)

John

 

[ghostlymoron]

Slocum used the lunar distance method to get time.

Happy to comment on it but the OP said that he was just starting, and lunar distances are rather advanced (and the tables are no longer published)

John

Interesting, he doesn't give details of his navigation methods in his book as far as I recall.

 

[Blue_mischief]

Buy several books ................

The best thing to do is buy a book,

As with any subject I've tried to learn from books, the best advice is to buy several books. Some maybe overall a poor buy, but every book will have a nugget of information, or an explanation, which helps in understanding the concepts in the other books. I have found this to be particularly true of boat radio, boat electrics and astro nav.

As an aside, one of my most useful books for nav including astro is DO Harvey's paperback, long out of print, "Yachting & Boating Book of Navigation". Very practical; I love his description of the cosmos:
"The size of the universe is infinite. The distance between each galaxy is unthinkable, the size of each galaxy is incredible, the distance between stars in a galaxy is unimaginable and the stars themselves colossal, some being more colossal than others, some less. I feel this is the only safe way to talk about these things. It is all rather frightening but there is nothing we can do about it and it has been going on for ages."

 

[Skylark]

This nautical almanac has a page per day Table (IIRC it's called Equation of Time) to show sun meridian passage. I believe this answers your question.

http://www.amazon.co.uk/2013-Nautic...8532369&sr=8-2&keywords=nautical+almanac+2013

It's nice to read that you are "attracted by the elegance of traditional methods". I was, too and I now own a beautiful sextant and have a library of wonderfully informative text books.

I guess it all depends upon how ones mind works but I can't agree that the Mary Blewitt book is of much value for learning the fundamentals of astro navigation. I found the Adlard Coles Ocean Yachtmaster to be a particularly good source of reference. I think that the Tom Cunliffe book is a good reminder of what you once knew and the Mary Blewitt book is good for last minute revision "before the exam", if you get my drift.

Be warned! You may well become adicted and sooner or later you'll be wanting to buy a copy of Nathaniel Bowditch American Practical Navigator.

 

[oldbilbo]

Be warned! You may well become adicted and sooner or later you'll be wanting to buy a copy of Nathaniel Bowditch American Practical Navigator.

....And to carry that many of us would need bigger boats!


( But of course, 'Bowditch' is available on the 'net... )

 

[Ex-SolentBoy]

You need to be able to understand all of the lines on this.

Any astro book will explain it.

This is for reference only. Do not copy or reproduce it. You can buy the cards from SeaChest.

[/URL]

 

[John Barry]

Interesting, he doesn't give details of his navigation methods in his book as far as I recall.

As far as I recall he owned up to using lunar distances quite late on in the book.

It was the standard method for determining time from 1802 when Bowditch published easy to use tables. They were printed up until 1912, when chronometers were so readily available (and so reliable) that the method was considered no longer relevant (accuracy is to about 30 seconds, depending on conditions; the best you can get with 0'.1 measurement accuracy is 12 seconds).

Certainly Slocum as a ships captain would have been well aware of it and well practiced. Chronometers were expensive, and the tables were cheap.


John

 

[Neil_Y]

You can't get longitude from a noon sight. A noon sight is to determine latitude.

I don't make a habit of disagreeing but the noon sight is the simplest way of getting Lat and long. With any Long you need accurate time so assuming you have an accurate watch set to a known time zone you can determine your local noon by tracking the sun sights up to its high point (zenith for those learning the sextant) No tables or formulae needed.

As the time needs to be accurate and the curve is quite flat at the top of the arc the sun makes, drawing a graph of heights and times before and after noon can help. You get a nice parabola of fixes from which you can draw a fairly accurate curve, from this pick a n angle either side of noon and note the time from the graph divide by two and you get your local time of noon (when the sun was exactly over head as far as you were concerned) Noon moves around the planet at 360deg every 24hrs so every minute is 0.25 of a degree of longitude.

In Plymouth at roughly 4deg west of London our noon is 16 minutes after Greenwich/London. So if you watched the sun you would see it reach it's highest point at 12.16

 

[John Barry]

I don't make a habit of disagreeing but the noon sight is the simplest way of getting Lat and long. With any Long you need accurate time so assuming you have an accurate watch set to a known time zone you can determine your local noon by tracking the sun sights up to its high point (zenith for those learning the sextant) No tables or formulae needed.

As the time needs to be accurate and the curve is quite flat at the top of the arc the sun makes, drawing a graph of heights and times before and after noon can help. You get a nice parabola of fixes from which you can draw a fairly accurate curve, from this pick a n angle either side of noon and note the time from the graph divide by two and you get your local time of noon (when the sun was exactly over head as far as you were concerned) Noon moves around the planet at 360deg every 24hrs so every minute is 0.25 of a degree of longitude.

In Plymouth at roughly 4deg west of London our noon is 16 minutes after Greenwich/London. So if you watched the sun you would see it reach it's highest point at 12.16

Well, as you've described, you can't get longitude from a noon sight.

You can get it from timed sights either side of noon (which is not the same thing at all). It's best if they are at least an hour before and after, otherwise accuracy is compromised. The simplest method is to check altitude (and time) an hour before expected noon, then check the time when the Sun is at the same altitude after noon. Halfway between the times is local noon which can be used to determine longitude as you describe, to an accuracy of 5-10 miles (you may need to correct for the Sun's change in declination).

The Noon sight itself is not sharply enough defined as a time to be of use. You won't be able to define the time of a noon maximum to better than 'within a couple of minutes', which would only give you a longitude to within 30 or 40 miles.

Possibly this is getting too advanced for the OPs 'easy starter' question


John

 

[oldbilbo]

Possibly this is getting too advanced for the OPs 'easy starter' question.

Aye, that's the trubl with astro. It quickly degenerates into gobblegook....

'KISS' is King.

A 'Noon Sight for Latitude' is one of the class of astro Lines of Position ( LOP ) called a 'Meridian Passage' sight ( MerPass ) and is close to the easiest means of getting an astro LOP. It is essentially not dependent on knowing accurate time - local or otherwise, but one needs to be aware of the APPROXIMATE time of local noon so one can be out there early, well-polished/salt-stained sextant in hand, ready to do the biz.

It was something of a ritual on blue-water merchant - and naval - vessels for a long time, for after the Noon Sight was taken by the Navigating Officer and all the cadets, compared, considered and resolved into a logged latitude, then the assembled miscreants would head into the wardroom for a 'swifter' then lunch when the steward called time. With not a lot else to do, it punctuated the day....

It is very handy for becoming used to handling the instrument and the associated bits and bobs, can be done readily on any harbour wall with a clear view of the southern ( ! ) horizon, then a pub lunch.

Arguably the next step would be a 'MerPass' shot of Polaris, which in some ways is even easier, for one doesn't need to wait for it climbing up to its zenith, for it is always 'there' - subject to a small and easy correction from the tables. All that's needed is a clear view of the star AND the horizon, so twilight is good ( morning or evening ). Should be no problem from the OP's Northern Scottish climes, given a suitable viewpoint and a break in the present summer storms.

Don't forget to factor in a correction for 'height of eye'.

 

[dharl]

You need to be able to understand all of the lines on this.

Any astro book will explain it.

This is for reference only. Do not copy or reproduce it. You can buy the cards from SeaChest.

[/URL]

had a pad of simular made up for me by a local printer when I was a young cadet and learning the ropes! Still dig out the Sextant from time to time, and over the weekend was camping in the New forrest and used a filled bowel of water as my horizon to do some star shots! My poistion was well off so need some more practice!

 

[dharl]

Aye, that's the trubl with astro. It quickly degenerates into gobblegook....

'KISS' is King.

A 'Noon Sight for Latitude' is one of the class of astro Lines of Position ( LOP ) called a 'Meridian Passage' sight ( MerPass ) and is close to the easiest means of getting an astro LOP. It is essentially not dependent on knowing accurate time - local or otherwise, but one needs to be aware of the APPROXIMATE time of local noon so one can be out there early, well-polished/salt-stained sextant in hand, ready to do the biz.

It was something of a ritual on blue-water merchant - and naval - vessels for a long time, for after the Noon Sight was taken by the Navigating Officer and all the cadets, compared, considered and resolved into a logged latitude, then the assembled miscreants would head into the wardroom for a 'swifter' then lunch when the steward called time. With not a lot else to do, it punctuated the day....

It is very handy for becoming used to handling the instrument and the associated bits and bobs, can be done readily on any harbour wall with a clear view of the southern ( ! ) horizon, then a pub lunch.

Arguably the next step would be a 'MerPass' shot of Polaris, which in some ways is even easier, for one doesn't need to wait for it climbing up to its zenith, for it is always 'there' - subject to a small and easy correction from the tables. All that's needed is a clear view of the star AND the horizon, so twilight is good ( morning or evening ). Should be no problem from the OP's Northern Scottish climes, given a suitable viewpoint and a break in the present summer storms.

Don't forget to factor in a correction for 'height of eye'.

was on a long passage accross the Pacific and all the Deck Officers entered into the competion of getting the best position lines. The hardest part was to make sure that you fiddled the figures so that your error was greater than the Old Mans. Closest to the Sun p/l bought the beers ;-)

 

[st599]

Despite a boat with plenty of electronics I am attracted by the elegance of traditional methods. I thought I would start with a simple noon sun shot to determine latitude but am already uncertain of the process.

It is clear that what I want to measure is the maximum altitude of the sun but what is not clear is how this relates to local time. It cannot be the case that the sun is at maximum altitude over the whole of, say, the UK time zone at 1200 local time, so is there a correction from tables to tell me reasonably accurately when the maximum altitude will be at a particular point or estimated point?

Thanks

Greenwich Mean Time - 12:00 is the mean average time that noon occurs at Greenwich.