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navigational sextant


abqmommyof4
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Hubby is a bit of a geek. Has anyone ever brought a sextant on board with them? Im thinking it would be fun, I checked the list of prohibited items and its not listed...

 

 

 

 

He sounds like my kind of geek :D I too would think that it should be OK with both port and ship security as there's nothing in a sextant that could be considered troublesome, just optics and a lot of benign metals. If he does take it along, please post a picture.

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On an eastbound TA on another line a passenger had brought along a sextant. He requested and was granted a notice in the daily activities sheet of when he would be on deck demonstrating it. Unfortunately either he or the cruise staff thought it correct to take a reading at 12 Noon ship's time every day (even as the clock moved forward most every night) rather than 1200 UTC as the readings precisely 24 hours apart are the only ones with any meaning.

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On an eastbound TA on another line a passenger had brought along a sextant. He requested and was granted a notice in the daily activities sheet of when he would be on deck demonstrating it. Unfortunately either he or the cruise staff thought it correct to take a reading at 12 Noon ship's time every day (even as the clock moved forward most every night) rather than 1200 UTC as the readings precisely 24 hours apart are the only ones with any meaning.

 

Not sure what you mean by "readings precisely 24 hours apart are the only ones with any meaning."

 

The sextant is used for two different types of navigational fixes. One is the "noon sight", which is used to determine local noon, not 1200 Z. This is done by measuring the time of the highest point of the sun's travel. This is used to determine longitude, comparing the time of local noon with the chronometer reading of GMT.

 

The other fixes are star fixes, which are done at "nautical twilight" when the stars and the horizon are both visible. A round of three star sightings gives the ship's position at that moment.

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A real brass sextant in it's box weighs over 4 kg. A no no if travelling by air to your cruise.

 

Ships using celestrial navigation in days past regulated the ship's time zone to have the sun near the zenith at around midday.

This enabled the sun to be shot at it's highest angle around noon & from this work out the ship's latitude & a good estimate of longitude.

What you see in films where they use a sextant is just a load of nonsense. You also need a chronometer, almanac & tables to work it out manually & it is not a few minutes job.

 

A sextants use could be demonstrated onboard by someone who knew how to use it, but it would be too technical an explanation for most.

Practising it's use could be another reason to take one on a cruise.

John - retired master mariner & sextant user for ship navigation - 1960's & 70's.

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I was taught to use a sextant by the U.S. Coast Guard, but on board a modern cruise ship, I am happy to rely on the ship's officers and crew to keep track of our position. I do study the "charts" far more diligently than most of the passengers, however, just because I am more comfortable knowing our position.

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Not sure what you mean by "readings precisely 24 hours apart are the only ones with any meaning."

 

The sextant is used for two different types of navigational fixes. One is the "noon sight", which is used to determine local noon, not 1200 Z. This is done by measuring the time of the highest point of the sun's travel. This is used to determine longitude, comparing the time of local noon with the chronometer reading of GMT.

 

The other fixes are star fixes, which are done at "nautical twilight" when the stars and the horizon are both visible. A round of three star sightings gives the ship's position at that moment.

 

Doesn't a noon sight give you latitude instead of longitude?

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I was taught to use a sextant by the U.S. Coast Guard, but on board a modern cruise ship, I am happy to rely on the ship's officers and crew to keep track of our position. I do study the "charts" far more diligently than most of the passengers, however, just because I am more comfortable knowing our position.

 

Which they do using GPS signals which give the ship's position with six-foot accuracy.

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Doesn't a noon sight give you latitude instead of longitude?

 

As sea-dog says, you can get both latitude and longitude from a noon sight. Longitude using the timing of local apparent noon, and then latitude using the angle of the sun at that time of local noon. Also as sea-dog says, it takes a few minutes to "reduce" a set of sextant readings to a position on a chart, even with today's navigational tables, and celestial navigation was always referred to as "the art of knowing where you were".

 

And despite the accuracy of GPS, and the requirement to have two systems of GPS onboard, SOLAS requires backup methods of navigation, which is why celestial navigation is still taught at maritime universities.

 

I recently was on a ship entering the port of Novorossisk, Russia, in the Black Sea. About an hour before arrival, both our GPS systems went haywire, and nothing we did could bring them back. As the pilot was climbing the pilot ladder, suddenly the GPS came back to normal. Coincidence? Natural factors? Signal jamming is the more likely explanation, and shows why GPS cannot be relied on completely.

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  • 11 months later...

I would like to share my experiences taking and reducing sights while on two cruises.  I’m going to assume you already know how to take and reduce sights.

                My kit for doing celestial navigation consists of my sextant, a precisely set quartz watch, my cellphone with a GPS app, a bound copy of the Nautical Almanac (download versions are available), and my laptop computer.  My laptop computer contains an Excel spreadsheet that does the sight reduction calculations via the Law of Cosines method; the spreadsheet is fast and doesn’t make arithmetic errors.  If you prefer a table method for reducing sights, H.O. 229 is available as *.pdf files for download.  I also bring a plotting pad and plotting instruments, but in practice I hardly ever do any plotting.

                My sextant case takes up a lot of room in my suitcase.

                You will need to get the Universal Coordinated Time (UTC) and your sight height above the water.  Unfortunately, the folks at the Guest Services desk generally have no idea what these are or how to get them.  UTC can be gotten off the Internet (https://www.time.gov), and some pre-cruise research will give you the height above the water (a.k.a. air draft)or overall length for your ship.  Given one of these, you can scale off your sight height above water from the ship profile drawing that came with your cruise ship orientation package.   Beware of the “elevation” feature on cell phone GPS apps – the GPS app I use has sometimes claimed the Lido deck to be below sea level.

                Should you not have Internet at sea, you can set an inexpensive quartz watch to UTC before you go on your cruise.  Of course, you should check your watch’s accuracy before you go on the cruise.   Beware of “UTC” cellphone apps- they either will not work out of land cellphone range or the time they give is based on the internal clock in your cellphone and cellphone clocks can have significant error.

                Now, on to taking sights:

I try to take my sights from an open deck high in the ship, but I have gotten surprisingly good results taking sights through window glass.  I get my sight heights by scaling off the ship profile drawing.  Don’t sweat the sight height – if you take your sights from high in the ship the resulting dip correction will be insensitive to small changes in sight elevation.

I get my actual position from my cellphone GPS app.  Bear in mind the ship will travel several nautical miles during your sight taking session – you will have to take that travel into account when you reduce your sights.

My Excel spreadsheet takes my inputs and does the sight reduction calculations; the final result being the intercept and the azimuth based on my entered actual position.  The smaller the intercept, the more accurate your sights are.

When you do take your sights, be prepared to attract a small crowd of onlookers.  I delight in showing them my sextant and explaining how celestial navigation works.

Have fun.

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1 hour ago, wgmiller said:

I would like to share my experiences taking and reducing sights while on two cruises.  I’m going to assume you already know how to take and reduce sights.

             

 

Welcome to Cruise Critic and thank you for sharing!

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On 9/25/2018 at 10:15 PM, caribill said:

 

Which they do using GPS signals which give the ship's position with six-foot accuracy.

 

I'm reminded of an instructor's rejoinder when a fellow student made a similar comment: "But the guy who drew that chart didn't have GPS.  That channel is likely more than six feet from where the chart says it is."

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Long time ago I use to navigate Navy patrol planes and sometimes the only navigation tool that was available over the vastness of the Pacific and Indian Ocean was the sextant.  While I could relate several good stories of my experiences with sextants my favorite sextant story is as follows:

 

In a boating related magazine there was a story about how an employee was retiring and because he was going to do a lot of sailing in his retirement his fellow employees decided to purchase him a really nice sextant as a going away gift.  The person, a male, tasked with procuring the sextant was having a hard time locating one so decided to go try their luck at a large sporting goods store.  Upon entering the sporting goods store they went to customer service and asked the female service representative if their store sold sextants.  The service rep starting blushing but managed to respond with “We sell all sorts of tents.  What you do in them is your business!”

CaptainR

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1 hour ago, CaptainR said:

 

Long time ago I use to navigate Navy patrol planes and sometimes the only navigation tool that was available over the vastness of the Pacific and Indian Ocean was the sextant.  While I could relate several good stories of my experiences with sextants my favorite sextant story is as follows:

 

In a boating related magazine there was a story about how an employee was retiring and because he was going to do a lot of sailing in his retirement his fellow employees decided to purchase him a really nice sextant as a going away gift.  The person, a male, tasked with procuring the sextant was having a hard time locating one so decided to go try their luck at a large sporting goods store.  Upon entering the sporting goods store they went to customer service and asked the female service representative if their store sold sextants.  The service rep starting blushing but managed to respond with “We sell all sorts of tents.  What you do in them is your business!”

CaptainR

 

Groan....

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