Hc calculated position

[Jumbo123]

I'm trying to get my head around Astro Navigation. Can anyone help me find out what the letters Hc stands for? I know it means calculated altitude. SWMBO reckons they were first used by an East-End sailor who took the initials from Calculated Haltitude and reversed them. Any better offers?

 

[GHA]

I assumed or read somewhere it was height calculated (or maybe computed?)- often wrong though.....

 

[Jumbo123]

Yep, sounds good to me. I'll think "height calculated" from now onward.

 

[alant]

I'm trying to get my head around Astro Navigation. Can anyone help me find out what the letters Hc stands for? I know it means calculated altitude. SWMBO reckons they were first used by an East-End sailor who took the initials from Calculated Haltitude and reversed them. Any better offers?

Calculated height, taken from assumed position, use it to compare with Ho (observed height), basically your sextant angle of height Ho being compared to a theoretical sextant height Hc, if you were taking it from your assumed position.
Difference is toward or away, from observed position, to give a line of position. This is the Mark St Hillaire method ( also used by Sumner - an American).
Your assumed position is effectively a position which uses nice round numbers of latitude, to make entering into the tables easier.

 

[stephenh]

Hs (height sextant) is the sextant altitude; with Dip and error this gives
Ha ( height apparent) - apparent altitude ; with altitude corrections this gives
Ho ( height observed) or True altitude which compared with
Hc (height calculated) or Tabulated altitude (Tab. alt.)

and from there your intercept.

 

[Jumbo123]

Astro Nav is amazing. Before starting it I never imagined that Altitude could provide you your latitude, and an angle could be a time as well as a distance. I haven't got as far as finding my position yet, -not least because of the cost of a sextant- but the journey is fascinating. @stephenh your list of abbreviations is v. useful. Thanks. Would a cardboard sextant be any use in completing my course? (See http://www.astromediashop.co.uk/Special_Offers.html)

 

[alant]

Astro Nav is amazing. Before starting it I never imagined that Altitude could provide you your latitude, and an angle could be a time as well as a distance. I haven't got as far as finding my position yet, -not least because of the cost of a sextant- but the journey is fascinating. @stephenh your list of abbreviations is v. useful. Thanks. Would a cardboard sextant be any use in completing my course? (See http://www.astromediashop.co.uk/Special_Offers.html)

No.

 

[Skylark]

............................ I haven't got as far as finding my position yet, -not least because of the cost of a sextant- but the journey is fascinating.

Patience is a virtue! Good quality sextants are often seen in the For Sale forum at reasonable prices. Sometimes a bit of a risk but setting an ebay "search alert" can pay dividend. Many of the plastic sextants give perfectly adequate results, too. The reward of using one to derive a Position Line is well worth the effort although from your posts it seems that you've discovered this already.

Welcome to the forum!

 

[RobF]

Astro Nav is amazing. Before starting it I never imagined that Altitude could provide you your latitude, and an angle could be a time as well as a distance. I haven't got as far as finding my position yet, -not least because of the cost of a sextant- but the journey is fascinating. @stephenh your list of abbreviations is v. useful. Thanks. Would a cardboard sextant be any use in completing my course? (See http://www.astromediashop.co.uk/Special_Offers.html)

I bought my plastic EbbCo from a fellow forumite for £20. Used with a bowl of water, I can get my readings accurate to a minute (1NM) when practising my calcs (compared with this very useful site - www.satellite-calculations.com/Satellite/suncalc.htm) . Looking forward to using the sextant on my boat this summer.

 

[Jumbo123]

£20 sounds a good price. I asked whether a cardboard one would be useful in completing the course but got a summary NO! Actually I could go up to (say) £35 - £40 without incurring too much wrath from SWMBO. I've been looking at the Davis mark 3 but the thing is so much more expensive in the UK than in America. Also I would like to do some star sights and people think it would be no good for that because it has no 'scope.

 

[capnsensible]

£20 sounds a good price. I asked whether a cardboard one would be useful in completing the course but got a summary NO! Actually I could go up to (say) £35 - £40 without incurring too much wrath from SWMBO. I've been looking at the Davis mark 3 but the thing is so much more expensive in the UK than in America. Also I would like to do some star sights and people think it would be no good for that because it has no 'scope.

Man who pays peanuts gets monkey.........

 

[RobF]

Man who pays peanuts gets monkey.........

I don't disagree, but if you're learning AstroNav a £20 sextant is more than adequate. Clearly a Freiberger or C&P will get a more accurate reading (with practice), but will also cost about 20 times what I paid.

 

[alant]

I don't disagree, but if you're learning AstroNav a £20 sextant is more than adequate. Clearly a Freiberger or C&P will get a more accurate reading (with practice), but will also cost about 20 times what I paid.

Cardboard sextant, at sea? Bit damp for that iirc.

 

[Jumbo123]

Cardboard sextant, at sea? Bit damp for that iirc.

My immediate aim is to learn AstroNav. I was thinking a cardboard one may be sufficient for that purpose, not for using out of sight of land on the Bismark Sea!

 

[capnsensible]

I don't disagree, but if you're learning AstroNav a £20 sextant is more than adequate. Clearly a Freiberger or C&P will get a more accurate reading (with practice), but will also cost about 20 times what I paid.

I have never used a 20 quid sextant so I cant really say. However when Im teaching practical sight reduction, I use a Davies so that the students can have a good play round with routine calibration too. Sun, moon and planets no problem. Stars, as you say, with practice.

I keep my Celestaire for mostly me but its good to let people have a go with it to see what an improvement it makes!

 

[capnsensible]

I would put a cardboard sextant on a shelf with chocolate fireguards, motorbike ashtrays and Piers Morgan.

 

[<152587>]

As a schoolboy in the endeavour of inwardly digesting trig, we were tasked with making a theodolite a la Blue Peter, card board, sticky tape etc. We then surveyed the playground, some of the better constructed ones were surprisingly close to the "official" plans. If it costs a fiver go for it. The results may be spurious but you will have practiced and learnt.

 

[Jumbo123]

As a schoolboy in the endeavour of inwardly digesting trig, we were tasked with making a theodolite a la Blue Peter, card board, sticky tape etc. We then surveyed the playground, some of the better constructed ones were surprisingly close to the "official" plans. If it costs a fiver go for it. The results may be spurious but you will have practiced and learnt.

I think you make a very good point. I (for my sins) tried school teaching (in Brixton in the 1970s after leaving uni.)
The educational value of making and using cardboard 'models' is well understood in educational circles, but ybw.com is about proper sailing on really wet sea, so that viewpoint probably isn't going to get much street-cred here.

 

[jwilson]

I don't disagree, but if you're learning AstroNav a £20 sextant is more than adequate. Clearly a Freiberger or C&P will get a more accurate reading (with practice), but will also cost about 20 times what I paid.

It was a long time ago but my brand new Ebbco Special plastic sextant cost about £12 if I remember correctly. Used it for many years for real nav. As my eyes got older but pockets slightly deeper I bought an ex-US Navy WW2 vintage sextant to get a glass telescope instead of the plastic lenses of the Ebbco. Very little practical difference in accuracy of the sights, though the plastic one needed index error checked before and after each set of sights.

 

[RobF]

Cardboard sextant, at sea? Bit damp for that iirc.

I should stress that the £20 Sextant I bought was a second hand Ebbco. More than adequate for learning AstroNav and a bit more substantial (and water resistant) than a cardboard one