Astro - navigation

[alant]

I don't know the place at all, but I read it as him trying to find the strait, not navigate within it.

Pete

That's the whole point.
Hitting the large green bits either side can be a pain.

PS if you ever visit the area, Iles de Cavallo are well worth a visit.

 

[Sinbad1951]

All a star or sun sight can do is provide a position line. You are somewhere on that line. At noon the Suns position line runs east/west so you at least have your specific latitude. If you have travelled for four hours at 6 knots in one direction it's quite easy to plot a good DR position from your last known position.
It is the intersection of more than one position line that gives your position. This can be done by either a round of star sights or by bringing forward various daily position lines from the sun.

 

[Uricanejack]

Can you do add-ups and take-aways? If so you can do astro. When I first started I had to use the Haversine formula using 5-figure log trig tables which was evil. Once I progressed to the Air Navigation tables it became relatively straightforward an with an app on a tablet or phone it is child's play.

Next question - who here has used astro in anger, i.e. when GPS was not available?

I think there are quite a few of us. Old codgers who predate GPS and other cheep electronics. Transit was available when I last used astro on regular basis. It was a long time ago but still would take an observation every day even with the transit system (little Kelvin Hughes Unit along with Kelvin Hughes Deca). have not been out of sight of land, often not even the Pub. for over 25 years.

 

[Uricanejack]

Dusted of the old sextant and revived some old brain cells last year. Had some youngsters with me who were interested in learning. Just going across the Hecate, You can see both sides on clear day. But I brought it anyway. Mine is a frieberger made by Zeiss. We also had a Davis 15.

Much to my surprise and pleasure they were keen to learn. They did and were very pleased with the achievement when they cam up with a reasonable position.

If anyone was interested in learning how it works, this might be as good a place as any to ask. Quite a few of us could possible help with any questions.
If anyone is interested.

 

[DownWest]

I don't know the place at all, but I read it as him trying to find the strait, not navigate within it.

Pete

Memo to self: Read the post more carefully )

 

[snowleopard]

What, exactly did you do, regarding "a bit of astro narrowed that down"?

I had a similar situation when navigating on the Cowes - St Malo race in pre-gps days. We had a total electrics failure from the start so the only working instrument was the compass. The weather was misty as we rounded the channel islands so we found ourselves somewhere west of Jersey, heading for a narrow rocky entrance. I took a sun position line which gave us a bearing towards St Malo and we found the finish line successfully. The key factor was the sun being in a suitable position, bearing SW so our position line ran SE-NW and we were able to transfer it to pass through our destination. The same trick wouldn't have worked in the morning.

 

[alant]

I had a similar situation when navigating on the Cowes - St Malo race in pre-gps days. We had a total electrics failure from the start so the only working instrument was the compass. The weather was misty as we rounded the channel islands so we found ourselves somewhere west of Jersey, heading for a narrow rocky entrance. I took a sun position line which gave us a bearing towards St Malo and we found the finish line successfully. The key factor was the sun being in a suitable position, bearing SW so our position line ran SE-NW and we were able to transfer it to pass through our destination. The same trick wouldn't have worked in the morning.

How did you know where you were on the position line, since this is the conundrum with a running fix.

 

[johnalison]

How did you know where you were on the position line, since this is the conundrum with a running fix.

I assumed that he didn't need to and that he just sailed down the line after covering the distance to the line.

 

[Minchsailor]

What, exactly did you do, regarding "a bit of astro narrowed that down"?

All I was trying to do was find the entrance to Bonifacio Straight which is about 10 miles wide between two considerable land masses (Corsica and Sardinia).

A noon sight gave latitude; a sight at dusk on Polaris conformed latitude, then dusk/dawn sights on Sirius and Rigel (I think, I usually used them as they are easy to find) together with running fixes reduced the uncertainty. Unfortunately, it was well into daylight by the time we sighted the coast, so we didn't have the benefits of lighthouses to get a better position and were a bit off. I seem to remember that we ended up about 15 miles S of the Straight, and that was after 350 miles of indifferent sailing. Of course, from there on it was eyeballing

 

[jwilson]

Next question - who here has used astro in anger, i.e. when GPS was not available?

I have used astro before GPS etc, also much more recently, on a modernish boat with all instruments off due to a major electrics problem - even the backup "waterproof" handheld GPS died as it became waterlogged (along with much of the interior of the boat). The handheld GPS eventually revived after dismantling, washing in fresh water and drying in the sun. It then confirmed my astro derived position to within about 2 miles.

These days people think a position 2 miles out is dreadful: it's actually 100% better than no position. In pre-GPS days in bad weather I have once not known where I was within 50 miles, which was beginning to slightly worrying as we were closing a coast.

 

[laika]

In pre-GPS days in bad weather I have once not known where I was within 50 miles

On the bright side, your log could have been amazingly accurate

 

[snowleopard]

How did you know where you were on the position line, since this is the conundrum with a running fix.

I didn't need to know. Having transferred the position line all I had to do was sail along it until I sighted the finish line. A bit like the way ships would sail along a line of latitude (which is just another position line) in the days before chronometers.

 

[alant]

I didn't need to know. Having transferred the position line all I had to do was sail along it until I sighted the finish line. A bit like the way ships would sail along a line of latitude (which is just another position line) in the days before chronometers.

Transferred the position line from where, to where?
I fully understand running down a latitude, that has some repeatable accuracy & simply easting or westing.

 

[Uricanejack]

Just a variation on a running fix.

You have a position line. you know you are on the line but not where on the line.
You need to find an entrance, island or some other fixed point.
Transfer your P/L to where you want to go.
Now you have 2 parallel lines. One you Know you are on. The Other you want to sail along.
The distance between 2 parallel lines is always the same.
Lay of you current heading or course steered, measure distance on course from current P/L to The one you want to sail along.
Time speed and distance run. To alter course for where you want to go.
You can reduce error by applying set and drift.

Good thinking just a wee bit out of the box.

 

[Uricanejack]

Some people think Astro is hard, full of complicated math's. I suppose it is hard if you don't know how. I failed O level math.
So to me math is hard particularly algebra. Always thought it was pointless BS when I was at school.
Then I chose to learn how to navigate. I hade to learn some geometry, trigonometry, vectors and worst of all algebra. I found I had to learn it for a whole bunch of other things to.
Applying them to the real world it made sense and was not hard. It was interesting. each is a little step up upon another.

Navigation is something we do every day in our ordinary life. we navigate without thinking about it every where we go. To the local shops to pick up a paper. To school, or work. when there is road works we go another way.
If we don't know the other way we follow signs. Other cars. Ask directions. We might even look at a map. Often we just guess. The next street over will go roughly the same way. We use all kinds of different clues to give us and idea where we are and where we are going. Some people are good at it others not so much.

My oldest son could spot the golden arches for a McDonalds from about 10 miles away in his car seat. And knew how to get to all of them. His brother even 20 years later is unlikely to know which continent he is on. One was always watching. The other always has something else on his mind.

Navigation is about situational awareness. Knowing where you are and what's around you and where you are going. Using all the clues available to you. They might be just visual, or sounds. might be having been there before, Or having been told what to expect. The wind the waves. Information from a chart or a book of directions. Compass. Drawing implements and plotting. Each clue helps you be aware of what's around, where you are and where you are going. Astro starts with knowing the sun rises in the east and sets in the west. Where is the north star and how to find it in the sky.
Today we have RADAR, GPS, AIS, Computers and plotters. They can ether help improve your situational awareness or replace it.
Do you need to know or understand all the aspects of ocean (astro) navigation? Lots of people get by without it almost all of the time.

 

[dom]

Great post Uricane -- you've really brought the key points alive. :encouragement:

 

[chwarae]

Very apt name for a star gazer, if you're Welsh.

 

[Serin]

Just a variation on a running fix.

You have a position line. you know you are on the line but not where on the line.
You need to find an entrance, island or some other fixed point.

Or simply a second sight. At dawn or dusk it's sometimes possible to find a second "heavenly body" in the right position to give a good "cross" with the first one. Or it can be a sight taken some time later, in which case it's just an ordinary running fix. A sight taken at dawn, followed in due course by a sun sight is quite a common approach.

Not as accurate as the same technique can be when used in coastal navigation, but you don't expect that with astro. And, of course, the sextant was also a useful instrument for vertical and horizontal angles (if you have a station pointer) when coasting.

Nostalgia

 

[alant]

Or simply a second sight. At dawn or dusk it's sometimes possible to find a second "heavenly body" in the right position to give a good "cross" with the first one. Or it can be a sight taken some time later, in which case it's just an ordinary running fix. A sight taken at dawn, followed in due course by a sun sight is quite a common approach.

Not as accurate as the same technique can be when used in coastal navigation, but you don't expect that with astro. And, of course, the sextant was also a useful instrument for vertical and horizontal angles (if you have a station pointer) when coasting.

Nostalgia

Not "common", more 'standard', its just the usual " sun, run, sun" approach, taught on the RYA course.

 

[Serin]

Not "common", more 'standard', its just the usual " sun, run, sun" approach, taught on the RYA course.

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! )