Sandrena
Well-Known Member
What does a transit produce?
Astro - navigation
- Thread starter Sybarite
- Start date
alant
Well-Known Member
So I would assume, but having never been on the RYA course, I wouldn't know. I was using it at sea long before the RYA course (or GPS) came on the scene. And there did seem to be some confusion amongst some posters here about the fact that a sight just produces a position line - like almost any other technique in navigation, except a transit.
(Indeed, your own question to Sealeopard tended to suggest a slightly shaky understanding!)
My question was simply to confirm method.
laika
Well-Known Member
Well if this thread isn't long enough already the next question is:
How many people who've done Yachtmaster Ocean in the post-GPS era have chosen to do a qualifying passage entirely without GPS?
How many people who've done Yachtmaster Ocean in the post-GPS era have chosen to do a qualifying passage entirely without GPS?
lw395
Well-Known Member
Is a transit not a position line?..... the fact that a sight just produces a position line - like almost any other technique in navigation, except a transit.
(Indeed, your own question to Sealeopard tended to suggest a slightly shaky understanding!
snowleopard
Well-Known Member
In my days of non-electronic navigation I would frequently grab a position line from any available source - Bearing of a known object, depth contour, sun sight etc. I would subsequently cross it with another PL obtained by whatever means came to hand. It's only in the world of RYA examples that two position lines have to be from the same type of source.
Transferring a position line:
You start with a position line. Pick an arbitrary position anywhere along that line.
Sail a given distance and plot that from your arbitrary position.
Draw a new position line through your new point, parallel to the original PL. You are now somewhere on the new PL.
Transferring a position line:
You start with a position line. Pick an arbitrary position anywhere along that line.
Sail a given distance and plot that from your arbitrary position.
Draw a new position line through your new point, parallel to the original PL. You are now somewhere on the new PL.
snowleopard
Well-Known Member
A transit is of course just another position line!
There are a few techniques that produce a fix from a single object for example using the known height of a lighthouse. The one taught on nav courses gets distance off by vertical sextant angle. One I have used quite a few times at night is a dipping light. Not terribly precise but as it usually involves being many miles offshore, not too much of a problem.
Sandrena
Well-Known Member
According to the RYA Navigation handbook:
"...there is no rule that says just because you used a visual bearing for one position line you must use visual bearings for the other two. The most important considerations are whether the posion lines intersect at a suitable angle and whether they are sufficiently accurate..."
johnalison
Well-Known Member
Although really both those fixes are produced from the intersection of two lines; the compass bearing, and the circle of distance off.
Foolish Muse
Well-Known Member
If you want the absolute easiest, simplest and quickest guide to celestial navigation go to http://backbearing.com/ and download the Sextant Users Guide by Andrew Evans, mentioned in the 6th paragraph. It gives a complete yet simple description of all the stupid navigation terms that will confuse you (like "same name" and "assumed position") It also has the form you need to fill out and the plotting page that you can print off many copies. All of the other links you need are also on that page. I guarantee that you will completely understand using your sextant AND know how to find yourself within 2 hours - and its free!
Porthandbuoy
Well-Known Member
Thanks Foolish Muse. I've been away for a few days and have only just got round to catching up on the forum.
I got into astronavigation just after Gulf War One when the US Department of defense (sic) dicked around with the GPS satellites and the rig I was working on, in the Med, appeared to be doing 11kts across the Libyan desert!
This was in the days of Selective Availability when the civilian GPS accuracy was degraded.
As an amateur on a small boat I have never bothered with anything other than Sun-run-Sun and noon sights. It's all you really need to get you withing eyeball pilotage distance of your destination.
Next years almanac will include a table, or tables, giving the meridian passage of the Sun. Ideal for a quick and dirty noon longitude.
I got into astronavigation just after Gulf War One when the US Department of defense (sic) dicked around with the GPS satellites and the rig I was working on, in the Med, appeared to be doing 11kts across the Libyan desert!
This was in the days of Selective Availability when the civilian GPS accuracy was degraded.
As an amateur on a small boat I have never bothered with anything other than Sun-run-Sun and noon sights. It's all you really need to get you withing eyeball pilotage distance of your destination.
Next years almanac will include a table, or tables, giving the meridian passage of the Sun. Ideal for a quick and dirty noon longitude.
Uricanejack
Well-Known Member
The current one does as well. though there may be more info planned for next years.
lw395
Well-Known Member
Thank you for the link, I will have a look at that!
mikebryan
New Member
I have startend with Cunliffes Astro Navigation, unfortunatelly because of the rapid breakdown of the old grey matter I cannot get my head around LHA. I understand GHA, but LHA ???. If someone could explain to yet another Grandad, in terms of a previous Post ( spotlight over tennis court) would be very grateful.
Serin
Well-Known Member
Fair enough. I know astro can seem quite mysterious before people get to grips with it. That's one reason I liked the Admiralty manual so much. It gave (gives?) a wonderfully clear explanation of the theory behind the technique, which I, for one, found very useful when it came to understanding the technique.
And there is something magical about astro. A straightforward vertical angle of a pinpoint of light millions of miles away, some simple sums and bingo! A position line on the surface of the earth, with all the various uses that can be put to.
I have a lovely old ebony and ivory octant as well as two sextants (a beautiful vernier sextant that is probably at least 120 years old and a nice Plath micrometer) I have tried taking sights with it, but the mirror really needs restoration and the whole thing is way out of adjustment. But I would love to try it out against the Plath. The romance of it all!
Barnacle Bill
Well-Known Member
Or try: http://www.aztecsailing.co.uk/newaztec/ocean%20theory/ch1/astronav.html
It's free; but if you want a printed copy of much the same thing buy Reeds Ocean Handbook. This also has other Ocean stuff besides Astro (World Climate, Route Planning, Yacht Preparation, Heavy Weather and Storm Tactics, Communication, Passage Making etc).
It's free; but if you want a printed copy of much the same thing buy Reeds Ocean Handbook. This also has other Ocean stuff besides Astro (World Climate, Route Planning, Yacht Preparation, Heavy Weather and Storm Tactics, Communication, Passage Making etc).
Last edited:
Foolish Muse
Well-Known Member
Mikebryan: This is taken from my Sextant Users Guide, mentioned a few posts earlier: LHA, or Local Hour Angle, measures the angle of the sun West of your assumed position. LHA is always measured to your west, even if the sun is east of the AP. For example, if the sun is 10 deg west of your AP, then LHA is 10. But if the sun is 4 deg east of your AP, then the LHA is 356.
You calculate your LHA by subtracting your AP-Longitude from your GHA. Because your AP includes the minutes and tenths that are identical to the GHA, then the LHA is a nice even degrees only number, such as 356. Rather than 356 48.8'
I'm just assuming that you understand what "Assumed Position" is, but of course that might be a bad assumption.
Take a look at the Sextant Users Guide to understand all of this better. This is the exact reason why I wrote the darn thing, because I found the books to be totally incomprehensible!
You calculate your LHA by subtracting your AP-Longitude from your GHA. Because your AP includes the minutes and tenths that are identical to the GHA, then the LHA is a nice even degrees only number, such as 356. Rather than 356 48.8'
I'm just assuming that you understand what "Assumed Position" is, but of course that might be a bad assumption.
Take a look at the Sextant Users Guide to understand all of this better. This is the exact reason why I wrote the darn thing, because I found the books to be totally incomprehensible!
Last edited:
prv
Well-Known Member
I don't think it's even that - I haven't done much formal RYA training but I'm sure I've seen plenty of examples of transits crossed with bearings or bearings crossed with depth contours.
I think my go-to would be a visual bearing crossed with a radar range
Pete
alant
Well-Known Member
Not so, that rule is only good if your longitude is west of Greenwich.
If your longitude is east of Greenwich, you add this to the GHA to obtain LHA.
Basically, it is the angular distance between your longitude meridian and the meridian upon which the geographic position of the observed heavenly body sits.
Rules for calculating LHA:
Long East, LHA = GHA + LONG (- 360o as necessary)
Long West, LHA = GHA – LONG (+ 360o as necessary)
Last edited:
Foolish Muse
Well-Known Member
Alant, I'm happy to learn from you, and I know that your East/West formula has been long used, but I'm wondering if it is an added and unnecessary complication.
In astro navigation, there is no east or west longitude. All points are west of Greenwich. So if you are located on a map at 4 E. Then in actual fact you are at 356 W (in astro nav terms.) I believe that the E/W method that you mention is simply a method of turning map East into Astro West. Am I wrong here? I'm happy for your opinion.
In astro navigation, there is no east or west longitude. All points are west of Greenwich. So if you are located on a map at 4 E. Then in actual fact you are at 356 W (in astro nav terms.) I believe that the E/W method that you mention is simply a method of turning map East into Astro West. Am I wrong here? I'm happy for your opinion.
john_morris_uk
Well-Known Member
Interesting thread.
FWIW I was conducting an oral board for a candidate for YMO thus morning. I took my copies if AP3270 with me and the candidate said she had never seen the real ones. Only the pdf files version...
I didn't find Mary Blewitt's book as easy to understand as Tom Cunliffe. When people ask me I always say try them all. One might stick...
