lustyd
Well-known member
It isn't, noon at Greenwich changes since our time system is standardised.
Sextant skills
- Thread starter zoidberg
- Start date
capnsensible
Well-known member
That's got nothing to do with it. The equation of time tells us when the sun is exactly at its highest at Greenwich for the meridian passage. It has been doing this well before watches were invented.
Equation of time - Wikipedia
In an earlier post I pointed out where to find the time of the meridian passage from the Nautical Almanac. I also posted a link to the.....Nautical Almanac. From this you can estimate your longtitude.
I suppose you will argue about this, but like the sun, I'm not bovvered. ??
Blueboatman
Well-known member
If I can arrive 10miles upwind and or uptide of my destination , I am a happy sun loving sailor
Edit : Obv, not Newtown IOW
Edit : Obv, not Newtown IOW
lustyd
Well-known member
Yes, and that time changes every day. The ball is raised at 1pm UTC which has nothing whatsoever to do with the height of the sun in 2021. We don't measure time by the sun anymore - update your skills
Skylark
Well-known member
Is this text book linked to the use of Reeds Astro Navigation Tables publication?
I only have a 2007 version of the Tables (so my comments maybe out of date). I do like the concise nature of only having to carry a single publication but, perhaps, many/most people would have learned the sight reduction process via the RYA course. Reeds uses a different variation of the intercept method requiring versine for altitude and ABC tables for azimuth. In itself, this requires new learning.
It would be nice if RYA and Reeds used the same sight reduction process as, potentially, we wouldn’t then have need for 3 volumes of the air tables.
capnsensible
Well-known member
capnsensible
Well-known member
There is an alternative method in the Nautical Almanac that doesn't require air navigation tables but errors are easy to make. Well I did despite persisting.
I've never found carrying three volumes particularly onerous unless I've been flying somewhere to join a yacht. ?
Don't forget the epoch thing for yer stars unless you want to do more sums ?
lustyd
Well-known member
Deleted, shouldn't feed the troll
capnsensible
Well-known member
The whole thread is about sextant skills. ?
You even posted a link to a video about Astro navigation. So you had best take your own advice. ??
lustyd
Well-known member
So you didn't read it then! Tolling as ever
Barnacle Bill
Active member
No, all you need to use this method is The Nautical Almanac, which contains its own Sight Reduction Table. (It also contains instructions on how to use it, but Reeds Ocean Handbook is considerably easier to follow.)
So all you need to carry is a watch, sextant, The Nautical Almanac and maybe a couple of copies of the form (download it from http://www.aztecsailing.co.uk ) which guides you through the process once you' ve done it a few times. Oh, and you'll need a pencil!
Buck Turgidson
Well-known member
That website was one of my best finds when I was learning and also where I get my almanac from. ???
zoidberg
Well-known member
Aw, herrnit! You've went and gorn and dunnit!
Dragging my reluctant friend Anna Lemmon into this, wot with her ''eq. of time" stuff. I could never work out which way to apply the correction - needing a simplistic mnemonic.
I can see the attractions of GPS, oh yus. Sure, the 'time' is usually out by a margin - but the sun is out by a whole lot more!!
That'd be why the stars are so much more reliable. No 'Lemmons' to bother about, innit.
Buck Turgidson
Well-known member
The Practical Navigator
This guy is excellent and his YouTube channel is also a great way to learn not just Astro but all aspects of Navigation. If you're not too anti America that is.
capnsensible
Well-known member
Yeah well I just look at the time of mer pass at Grenwich! ?Aw, herrnit! You've went and gorn and dunnit!
Dragging my reluctant friend Anna Lemmon into this, wot with her ''eq. of time" stuff. I could never work out which way to apply the correction - needing a simplistic mnemonic.
I can see the attractions of GPS, oh yus. Sure, the 'time' is usually out by a margin - but the sun is out by a whole lot more!!
That'd be why the stars are so much more reliable. No 'Lemmons' to bother about, innit.
zoidberg
Well-known member
I earned my crust for a while 'back in the day' using the Air Almanac, Sight Reduction Tables For Air Navigation and Selected Stars various. That system worked well enough - 'close enough for government work'. Most of the guys could do a 2000nm astro trip around the North Atlantic, once a month, and bring the 4-jet back towards a Radar Bomb Plot team in Lincolnshire by coming down a lane-in-the-sky half a mile wide within 30 seconds of pre-planned time. The better guys regularly turned in 'quarter-mile tracking and better than 10 seconds TOT'. These were the V-force stars crews who regularly beat the best of SAC and others in NATO Bombing Competitions.
Around that time, the Circular Error Probability ( CEP ) of our Polaris missiles was no better - 'or so I heard'.
Skylark
Well-known member
Thanks for the response. I teach the RYA Ocean Shorebased course. I’m sure that you are aware of the subtle differences between the two methods. I can see an advantage in having the one reference but unless the versine and ABC method is more widely adopted I believe that most students will stick to the method taught within the RYA course (and by many other notable authorities). Ocean navigation is not for everyone. Many students have to simply learn how to use a template. Few students grasp the concept well enough to be able to reduce a sight starting with a plain sheet of paper. Having finally mastered the art of following a template, I can’t see the majority being willing to learn an alternative process. Just my thoughts, it wouldn’t be the first time that I’m completely wrong ?
I get your points. One ones own boat, having tables aboard isn’t particularly onerous (but St Greta may have an opinion ?). On someone else’s boat, carrying a sextant, 3 volumes of air tables, including Vol 1 for the little buggers, takes up ones entire hand baggage allowance ?
Humblebee
Well-known member
Agreed, he explains things clearly and in a logical sequence.
Chris
Skylark
Well-known member
I’m still in awe of your claims in post 60. I can manage just fine doing 2, sometimes 3 LoP per day at 6kts but find it incomprehensible that you could update fixes, when pushed, every 10-12 minutes while flying at several hundred knots. Respect ?I earned my crust for a while 'back in the day' using the Air Almanac, Sight Reduction Tables For Air Navigation and Selected Stars various. That system worked well enough - 'close enough for government work'. Most of the guys could do a 2000nm astro trip around the North Atlantic, once a month, and bring the 4-jet back towards a Radar Bomb Plot team in Lincolnshire by coming down a lane-in-the-sky half a mile wide within 30 seconds of pre-planned time. The better guys regularly turned in 'quarter-mile tracking and better than 10 seconds TOT'. These were the V-force stars crews who regularly beat the best of SAC and others in NATO Bombing Competitions.
Around that time, the Circular Error Probability ( CEP ) of our Polaris missiles was no better - 'or so I heard'.
How did you maintain the DR? Plane sailing (or should that be Plane Flying ?)?
Skylark
Well-known member
The American Practical Navigator…….isn’t that Nathaniel Bowditch? 1773-1838. I didn’t realise that he was making YouTube videos 200 years ago ?
Thread drift factoid: when the Nantucket Whaler, the Essex, was sunk by a whale (the fact behind the Moby Dick novel), 3 lifeboats carried the crew but they only managed to salvage 2 copies of the American Practical Navigator from the sinking ship. The lifeboat without the navigation resource was lost.