Using a Sextant....anyone out there ??

M

Matata

Well-Known Member
Joined
5 Jan 2009
Messages
402
Visit site
I'm currently doing my RYA oceon theory course. Using a sextant is a major part of the course. Basically a sextant measures the angle between ,say, the suns lower limb and the horizon. All very well if you can see the horizon that is! At sea level the horizon is horizontal from the eye so why can you not use a spirit level to record the horizontal rather than actually being able to see the horizon itself? If you know what I mean !!!!
 
To get an accurate fix you need a very accurate angle. We are talking seconds of arc. Even if the boat was absolutely still, no spirit level would give you the same degree of accuracy as a sight to the horizon. Even the best levels can only be read within about one degree because you are looking at a small bubble which you have to centre between two lines by eye.
 
Thanks Guys for the speedy and very informative helpful replies. Oh what the youth of today are missing out on.!!!! PS Excellent forum TA
 
On doing some further research after recently doing my Ocean theory course - we'd discussed among ourselves the very question you ask....


Use a bucket or basin of water and bring the body down to meet its reflection in it. Half the angle and you get the height above the "horizon". Obviously it's only really effective if it's calm, but you should give it a bash. You might be pleasantly surprised.
/forums/images/graemlins/grin.gif
 
Idealy you should take five sights and average them with the corresponding times of course.A single sight is never to be trusted so the absolute minimum is three.And they have to be taken within very little time of each other.There is a table that will give you the maximum permissible time between sights,according to the boat's speed,if they are to be used for averaging .Not alway easy.
 
I never understood why all sextants weren't built like this. I had no problem taking sights through a periscope sextant - which is just a bubble sextant with a periscope on it so that it can be put through a small hole in the roof of a pressurised aircraft cabin but I really struggled with a normal half-mirror sextant even when the horizon is clearly visible and I was on terra firma. Apparently the Boeing 747 still has a hole in the roof for a periscope, though Boeing now call it a smoke removal hatch.

The aircraft ones had to have an artificial horizon as the horizon was in a different place depending on your height - were any ever made purely for use on the surface?
 
In answer to your original question, 'yes', in theory you could use a spirit level, but in practice it isn't anywhere near accurate enough.

A sextant with a built in spirit level is made and is usually called a Bubble sextant. They were/are produced for aircraft and are notoriously difficult to use on a small boat. In any sort of seaway they are a nightmare. After the Second world war, when lots of Bubble Sextants were available through the surplus market, there were articles published as to how a bubble sextant can be modified for use on a small boat (by removing the bubble part and allowing the sextant to 'see' the horizon). A bubble sextant works in an aircraft because it is usually a much more stable platform and the bubble doesn't dance about all over the place. They still correct for 'height' even though it might be several thousand feet!

If you want to practice taking sights in the middle of the land, then the standard way is to get a bowl of water and allow it to settle and use it as an artificial horizon. Line the reflection of the sun in the water up with the 'real sun' through your sextant and halve the angle you measure. If you overlap the suns then there is no need to make the correction for the semi diameter, and there is no height of eye correction using this method either. If you bring lower limb to touch upper limb, then you need to make the correction for one semi diameter.

Hope that this all helps - otherwise ask your course instructor. He/she should know...!

I know that people say you should have a minimum of three sights and possible five. In a small boat this isn't always practical and you need to gain a sense of what is a 'good sight' and when its a 'bad sight'. After you have taken a few, you learn to treat some sights with suspicion and your position at sea become suspect as well.

Sadly a whole art of 'suspicious navigation' is being lost with the confidence that we all have in our GPS's. I wouldn't sail without mine, but I know that having it has altered my skills as a navigator.
 
I have a US Army, bubble sextant (octant, actually, Bendix Aviation Corporation) that I used to use at sea (when lack of cloud cover allowed) after having removed the bubble. Unlike a mirror sextant both the horizon and celestial body are viewed through prisms, the one for the latter having an adjustable gearing with a very precise, fine movement. However, that principle, without a telescope to give a position to point at, made searching for an object extremely difficult and limited the use to sun and moon only.
 
I have a lovely old Tamaya, originally a surveying sextant according to my enquiries of Tamaya in Japan, but now fitted with shades. Beautiful instrument with no corrections needed. But the real problem I find is the weight of the sight reduction tables - lumping great books that weigh half a ton to carry around. I know you can get calculators with the tables in them, but somehow that just feels wrong.

There's a PC programme that works quite well, called Navigator (http://www.tecepe.com.br/nav/default.htm). This uses the intercept method, and can produce a really accurage fix. It copes with sun run sun fairly easily, and does stars equally well.

But again, as the point of carrying a sextant these days seems to be in case the electronics fail, I'm back to carrying the heavy reduction tables about with me as well. Hmmm
 
"But the real problem I find is the weight of the sight reduction tables"

Agreed - APITA - however, as of last year (?) Celestaire now do a 'commercial' edition of the Air Tables which are smaller than A4 and much easier to store / transport.

The typeface is a bit small but not a problem, even with my aging sight.
I am going to trade in my UK (v. large) tables and buy a new set.

A full set of NP 401 is good for ballast........!

Later edit - Vol 2 of the Celestaire commercial edition and the online edition has many errors that are currently being corrected - see end posts on this link :

http://www.ssca.org/DiscBoard/viewtopic.php?t=7139&sid=3a3c4fa9d895cc756f3876adb4be4c2b
 
[ QUOTE ]
But the real problem I find is the weight of the sight reduction tables - lumping great books that weigh half a ton to carry around. I know you can get calculators with the tables in them, but somehow that just feels wrong.


[/ QUOTE ]

When navigating commercially I used to use Burtons for the six figure log tables and the Admiralty Almanac together with a decent chronometer. Not too much weight there. I occasionally used a slide rule for some of the 'sailings' but not for the Marc St Hilaire part.

The wonderful old chronometers built in 1810 in St Petersburg were taken off the vessel by a chap who replaced the two mechanical clocks with one Seiko chronometer watch. I suspect he became very rich doing that on the some 70 ships.

Once the electronic calculator had become available in the mid 70's I used the calculator. It meant that five stars could be taken and calculated in less than 10 minutes. Then GPS came in and took the fun away.

The lower you went in a ship the better your horizon became particularly in foggy weather. Same applies to some extent in a yacht, the distances are somewhat smaller /forums/images/graemlins/smile.gif
 
I wouldn't be without mine but in practice I find it almost useless in a small boat.

You have to have the horizon, sun and the time all together.

Singlehanded makes getting the time a guess.

Visiblity is rarely good enough to see the horizon

The ocean swell makes it near impossible to hold a sextant as the boat rolls about and you only see the horizon when you are on top of a wave which makes height of eye also a guess and you are only on top of a wave for about one tenth of the time.

and how often can you see the sun at noon.

An artificial horizon sounds a great idea.

Can one be added to a normal sextant?
 
1 banana 2 banana 3 banana 4 banana ... read the chron.

Blimey memories !

Sight reduction tables ... you only needed 2 books of Air Reduction - one for Sun / single bodies and other for selected stars + an almanac and calculator. Reckon I could carry those ok even on my small boat.

Sadly my Brother in Florida pinched my Sextant from my MN days and took it to USA ... must collar him one day about that.
(Plath Mariner)
 
" Singlehanded makes getting the time a guess. "

I strap a (rated) Casio wrist watch to the sextant handle, rather than my wrist, say "now" to myself (outloud - to the amusement of bystanders..) and instantly read the secs. time.

I used to add 1 sec. ( on the advice of someone who knows more about these things than I do ), however, after a while you become so fast that this is not necessary. This was worked out by many various reductions.

As for the vis. and the horizon, sorry can't help....!
 
[ QUOTE ]
The ocean swell makes it near impossible to hold a sextant as the boat rolls about and you only see the horizon when you are on top of a wave which makes height of eye also a guess and you are only on top of a wave for about one tenth of the time.

[/ QUOTE ]

Ocean swell and height of eye is no problem. The bit of the horizon you are looking at will itself be the top of a swell; just make sure that you're at the top when you take the reading.
 
[ QUOTE ]
[ QUOTE ]
The ocean swell makes it near impossible to hold a sextant as the boat rolls about and you only see the horizon when you are on top of a wave which makes height of eye also a guess and you are only on top of a wave for about one tenth of the time.

[/ QUOTE ]

Ocean swell and height of eye is no problem. The bit of the horizon you are looking at will itself be the top of a swell; just make sure that you're at the top when you take the reading.

[/ QUOTE ]Bit of thread drift here, but here goes...

That's interesting as I use another method. I estimate height of swell from top to bottom and when I take a sight at the top of a swell, I add half the swell height to the height of eye. I can't be sure that the horizon that I see will be the top of a particular swell.
 
You must log in or register to reply here.

Share this page

Top