Talk:Sextant

message: I have a question:
If one is to measure the height of the horizon above the imagined sea level, can you do that with a sextant?
I imagine it should be possible with something on the sextant to keep it totally horizontal.
Sverdrup 13:06, 21 Nov 2003 (UTC)
replies: text: If you can't see the horizon, an artificial horizon can used.  You can also do this if you are far above the horizon.  For aircraft, bubble sextants have been used.  A traditional artificial horizon is a small box with a glass cover (looks like a little house with a glass roof) and has a pool of mercury in it.  The mercury acts as a mirror and twice the angle to a star (or sun) is measured; the angle, being reflected, is twice the distance from the horizon.  Michael Daly 04:59, 10 September 2007 (UTC)

text: I should add that if you don't know your height above the horizon, there is no general means of measuring the altitude with a sextant that I've ever heard of.  An altimeter or independent means of determining altitude is required. Michael Daly 05:06, 11 September 2007 (UTC)
thread_title: A question

message: This article has a link to octant, which re-directs here. Anything on what to do with the link?? 66.245.121.70 02:18, 1 Dec 2004 (UTC)
thread_title:  Octant link 

message: A sextant is not exclusively used to measure angles between a celestial object and the horizon. As many sailors know, it is also used to measure angles between two landmarks by placing it horizontaly. This can be very helpful in determining one's position at sea in coastal navigation, if one has a Chart, of course. --Jprats
You can also use it (the normal way up) to measure distance-off by measuring the angular height of an object like a lighthouse (if the real height is known and corrections are made for the current height of the tide).  I've been wondering whether to put these ideas into here or into Pilotage.  We can do something here about these present day, modern uses of the sextant in pilotage, as well as as a backup for the day when the GPS system or your own GPS receiver(s) pack up while you're at sea.  It's not just a historical novelty.  --Nigelj 11:26, 6 August 2005 (UTC)
thread_title:  Pilotage, horizontal angles etc 

message: The reason I changed "precise location" to "approximate location" is that the use of a Line of Position (LOP) from a sextant reading is approximate, and the LOP from dead reckoning is likewise approximate. Stating "precise position" is implying something a little more specific. Generally, if you can establish your position within 1-3 miles of actual position you're doing pretty good. (unsigned comment by User:Quartermaster)
----
I changed it to just plain "fix".  All fixes are approximate.  A sextant fix might be good to a couple of miles, a GPS fix to a few yards, but they're both approximate.  Calling something "precise" is almost meaningless unless you specify the numeric uncertainty.  -- RoySmith (talk) 19:08, 21 May 2006 (UTC)
thread_title:  Approximate vs. Precise

message: I think that this article could use some diagramatic illustrations elxplaining the layout and principle of a sextants operation. ike9898 04:10, 28 October 2006 (UTC)
thread_title:  illustrations 

message: This article seems to be saying that Newton developed the sextant. Is it not true that Newton was born 1643 and Tycho Brahe was using sextants in the mid 1500s? ::Obstructio 02:19, 23 March 2007 (UTC)
thread_title:  Inventor? 

message: There are two different instruments referred to as sextants.  One is a reflecting instrument, the other is not.
The sextant referred to in the intro as being invented in 995 is a mural sextant.  This is not a reflecting instrument and is not anything like the sort of thing discussed in the majority of the article.  Ditto Brahe's instrument - that was a direct reading sextant - a span of 60° gave a reading range of 60°.  These things should have a separate section or a separate article.
Reflecting sextants derive from the octant - aka reflecting quadrant - and are the sort of instrument most think of today when they hear the word sextant.  Reflecting instruments read up to 120° from an arc that spans 60°
Michael Daly 05:06, 10 September 2007 (UTC)
replies: text: I've created a new page for the non-reflecting sextant and removed from this page information on non-reflectig instruments.  I have added a note to the top directing readers to the other page if they are interested in the other topic. Michael Daly 16:45, 13 September 2007 (UTC)
thread_title:  Confusion over sextants - two types - direct and reflecting 

message: The reason I created a new page for the astronomical sextant is that it is a completely different instrument.  The astronomical sextant is not a precursor.  The precursor to the sextant was the octant, and before that the reflecting quadrant.  The astronomical sextant is just an angle measuring instrument with a single arc that happens to be 1/6 of a circle.  Michael Daly 03:03, 20 September 2007 (UTC)
thread_title:  Astronomical Sextant is not a precursor to the Navigator's sextant. 

message: Since the Bris sextant is not a true sextant, I wonder if it should be on this page or on it's own page as a navigation instrument.  A link from this page under "see also" would be appropriate.  Michael Daly 22:58, 27 September 2007 (UTC)
replies: text: The Bris sextant is indeed a type of sextant, in the modern sense where the word no longer specifically requires a measuring range up to 120 degrees. See the discussion page at Bris sextant for more.24.148.18.151 (talk) 01:48, 26 October 2010 (UTC)
thread_title:  Bris sextant is not a sextant 

message: sir give me the practical information about sextant . what type I can do.  —Preceding unsigned comment added by 220.227.98.62 (talk) 05:26, 21 June 2008 (UTC)
thread_title:  About sextant practical 

message: Could somebody possibly write-up a section on how a Sextant is used, how it works?  The animation demonstrates this, but from just looking at it one wouldn't know that it is an animation, and it requires one to travel to a different page to find out how it works.  Many thanks, 82UK (talk) 17:05, 23 August 2010 (UTC)
replies: text: Well, I never noticed that it wasn't there! I have just added a subsection called 'Taking a sight', based on the book I have on the subject. I know about WP:NOTHOWTO and have tried to avoid that. If I have not quite succeeded (due to the very how-to nature of my source material!), please help me to tweak the wording. I also have not included any wikilinks in the new material yet, but there are plenty of possible links to add. Thanks for the suggestion. --Nigelj (talk) 21:03, 23 August 2010 (UTC)
thread_title:  Use of a Sextant 

message: The discussion of errors is a little weak. It provides no information on the scale of the errors. It (previously) included "side error" at the same level of importance as the other errors, yet side error is inconsequential for observations and does not need to be removed (text in the article modified). The item on index error should be re-written to emphasize that it does not need to be eliminated, only observed and recorded as the I.C. or "index correction". The item on telescope collimation mentions only the old method of testing this and doesn't describe simple tabletop methods of collimation. Also it should be mentioned that many/most? modern sextants have no provision for adjusting telescope collimation and the telescope may be presumed collimated by construction.24.148.18.151 (talk) 01:46, 26 October 2010 (UTC)
thread_title:  Errors discussion 

message: The phrase "scale of a sextant" appears in the introduction to the overall entry. Is"the scale of a sextant" a part of the sextant, or part of its use, or ...? At first, I thought that it might be the "arc" but that is wrong. Please explain what "a turn" is, and how it relates to the 1/6th on the name "sextant."97.114.90.169 (talk) 00:17, 14 November 2011 (UTC)
thread_title:  What is "the scale of a sextant?" 

message: (dark glass is not safe enough, see Arc welding lenses, see Telescope solar lenses)
was added as correction
Please either say nothing, refer to manufacturer instructions, or correctly say it is a special solar lense.  Dark glass alone will cause blindness over time.
This is also true for cheap* sunglasses but don't tell anyone!
(Cheap sunglasses use dark material and the eye feels safe but is actually being damaged at wavelengths that do not cause pain.  A doctor would suggest squinting eye lids instead of relying on cheap sunglasses for eye pain situations)
Never look at arc welding using dark sunglasses.
72.219.202.186 (talk) 14:22, 9 April 2014 (UTC)
thread_title:  in Section: Taking a Sight 

message: At the very beginning, how about a paragraph like...A sextant is a device for finding your position on the Earth while at sea.  It measures the vertical angle between the horizontal and a heavenly body.  Given these angle measurements and an accurate time piece you can calculate your longitude and latitude at sea.  It is a version of the simple protractor that measures vertical angles on land, but designed to be used specifically on the ocean.  The sextant's design makes use of the ocean's horizon line as the horizontal reference and is designed to be used while standing on a rocking surface, yet giving a high degree of accuracy.   
The current intro starts out too technical. Overall a good article!  Pb8bije6a7b6a3w (talk) 15:30, 29 October 2017 (UTC)
thread_title:  intro: more basic! 

message: Hello fellow Wikipedians,
I have just modified one external link on Sextant. Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:
*Added archive https://web.archive.org/web/20070624193729/http://www.irbs.com/bowditch/ to http://www.irbs.com/bowditch/
When you have finished reviewing my changes, you may follow the instructions on the template below to fix any issues with the URLs.
Cheers.—InternetArchiveBot (Report bug) 00:45, 22 January 2018 (UTC)
thread_title:  External links modified (January 2018) 

message: As a lot of work has evidently gone into the article, it may seem churlish to complain, but neither the diagram nor the description of the instrument are very clear. The text refers repeatedly to the 'index arm', but this is not marked on the diagram (as at 29 March 2018), so it is impossible to be sure what the description is referring to.  I *guess* that it is the broad bar projecting down from the circle holding the index mirror.  If so, it should be marked.  The caption does refer to an 'index bar', but this is not referred to in the text until the section on 'taking a sight'.  Again, I *guess* that 'index bar' is just another term for the 'index arm', but if so the terminology should be harmonized.  Also, the diagram shows a clamp, but it is not clear how this works.  What is it clamped *to*?  And how does the 'fine adjustment' screw work if the index bar is clamped?  It is stated that  'By setting the index bar to zero, the sun can be viewed through the telescope', but this does not seem correct unless the sun is very close to the horizon.  In the animated diagram, when the index bar is set to zero the sun is *not* visible through the telescope.  Finally, it is not clear how the horizon can still be seen when the index arm (or bar) has been rotated.  From other accounts, I think the view in the mirrors only covers *half* the field of view through the telescope, the other half being seen directly through the telescope.  But this is not consistent with the animated diagram, which shows the image of the sun as a complete circle overlapping the mid-line, then, when the caption reads 'swing to verify', the image of the sun moves away from the mid-line entirely, which seems impossible if the mid-line demarcates the half of the scene viewed indirectly through the mirrors from the half viewed directly through the telescope.  I hope that someone who understands the sextant better than I do can review the text and diagram and clarify it where necessary.86.179.155.102 (talk) 20:24, 29 March 2018 (UTC)
thread_title:  Diagram and explanation 

message: Suggested additional section.
I don't know how to edit or I'd make the changes myself.
Once a reading is made it should be corrected for a number of issues.
What follows are 3 of the corrections that need to be taken.
1) The horizon is below horizontal, that dip is dependent on the height above sea level.
1.77 x sqrt(height in meters) gives the correction in arc-minutes.
There are tables so you don't have to do the math.
2) If the horizon is shoreline a different correction must be taken.
https://davidburchnavigation.blogspot.com/2018/06/dip-short-distance-off-and-running-fix.html
3) If celestial object being sighted (i.e. either the Sun or the Moon) is close parallax must be taken into account.  Celestial navigation assumes readings are taken in line with the center of the Earth, so any reading other than directly overhead must be corrected.
The first link is an archive.org scan of a book written in the early 1800s teaching how to navigate it covers these corrections and others.
https://archive.org/details/newcompleteepito00nori/page/n13/mode/2up
These go over the same corrections.
https://astrolabesailing.com/2016/10/10/celestial-navigation-sextant-angle-corrections/
https://oceannavigator.com/celestial-navigation-series-part-5/ Rimorphis (talk) 21:05, 31 March 2024 (UTC)
thread_title:  Corrections to readings.