Might be time to buy a sextant

[Neil]

I bought an Astra when they were about third of the price they are now, just out of interest. I learned to do the moon, planets and stars as well as the sun from my back garden with the reflection in a dish of two stroke oil. The reflections of stars in the oil could prove a challenge. At least doing 'sun-run-sun-run-sun' from the same place meant there was no worries about log inaccuracies!

I've never ever used it on a boat. However, I can take pleasure in the instrument itself, knowing I know how to use it.

 

[[194224]]

I bought an Astra when they were about third of the price they are now, just out of interest. I learned to do the moon, planets and stars as well as the sun from my back garden with the reflection in a dish of two stroke oil. The reflections of stars in the oil could prove a challenge. At least doing 'sun-run-sun-run-sun' from the same place meant there was no worries about log inaccuracies!

I've never ever used it on a boat. However, I can take pleasure in the instrument itself, knowing I know how to use it.

That's an excellent way to practice although the altitude is limited, golden syrup was often touted as a good reflective medium but it attracts things! Also as you're not going anywhere the "run" bit of your sequence is irrelevant. Sun-sun-sun is what you are doing.

 

[srm]

That's an excellent way to practice

The first few courses I taught for Fishing Deck Officer certificates was in Lerwick, Shetland. There was a sea horizon to the south from Lerwick. Courses started in January but it was not until late February that the midday sun was more that 10 degrees above the horizon, and the normal refraction tables could be used. During the first course we went to the beach and took noon sights, then the position lines were calculated and plotted on a chart. We had an interesting range of latitudes but its difficult to estimate height of eye from a beach. The following years we were in a building with a window facing south so I took my theodolite in and measured the dip to the horizon, then worked back to give a height of eye. Results were a lot better with an accurate value for dip.

 

[[194224]]

January in Lerwick! I'm surprised you could hold the instrument steady enough

 

[srm]

January in Lerwick! I'm surprised you could hold the instrument steady enough

Na boy, we waited till March when the sun was high enough.

 

[[194224]]

Ah I see now, still impressive.

 

[boomerangben]

I've only read up to your comment (#21) so far so someone may have already pointed out that the US Navy requires their Officers to be proficient with celestial navigation.
The Scary, Practical Reason The US Navy Is Once Again Teaching Celestial Navigation

I am (almost) sure I read that airline pilots would have to become proficient too with celestial navigation. (Maybe I am wrong?)
The Reason Some Boeing 737 Cockpits Have Eyebrow Windows

Reading that article, it is TCAS that done most to render those windows obsolete. TCAS (traffic collision avoidance system) is a system that helps stops aircraft from flying into each other. It is more effective than a couple of pairs of mk1 eyeball, especially in clouds. It has nothing to do with navigation.
Oh well back to NDBs, compasses and stop watches.

 

[Buck Turgidson]

Reading that article, it is TCAS that done most to render those windows obsolete. TCAS (traffic collision avoidance system) is a system that helps stops aircraft from flying into each other. It is more effective than a couple of pairs of mk1 eyeball, especially in clouds. It has nothing to do with navigation.
Oh well back to NDBs, compasses and stop watches.

Nah. They are for spotting the runway when turning base.

 

[lustyd]

I'm in Hong Kong right now, and the accuracy of GPS when in the urban canyons of Hong Kong is pretty bad - hundreds of metres from where I actually am! Anywhere where than can be multiple paths caused by reflection or refraction from buildings, cliffs or whatever can degrade the accuracy quite severely.

Technically true of GPS I guess, although with your knowledge I'm surprised to see this come from you. The last two wrist watches I bought from Garmin both support duel frequency GNSS as well as all public GNSS systems which allows excellent urban canyon (and other difficult situations) fixes. Even my older watches could fix me within 10m in big urban canyons, and my "good" gps would happily do better than that consistently, and that's only a GPSMap 62s.

 

[AntarcticPilot]

Technically true of GPS I guess, although with your knowledge I'm surprised to see this come from you. The last two wrist watches I bought from Garmin both support duel frequency GNSS as well as all public GNSS systems which allows excellent urban canyon (and other difficult situations) fixes. Even my older watches could fix me within 10m in big urban canyons, and my "good" gps would happily do better than that consistently, and that's only a GPSMap 62s.

My experience is with a modern mobile phone, using WiFi and phone corrections. The accuracy - which is borne out by the common experience of others - is very bad in many parts of Hong Kong. My phone happily receives and uses all the GNSS systems. Not sure whether my phone supports dual frequency, but if it doesn't, then it is still a typical smartphone, and friends with the latest iPhone get similar results. Of course, Garmin are navigation specialists and I would expect their equipment to be better engineered, probably being better at rejecting bad solutions and determining which satellites are providing reliable data.

Car satnavs can, of course, constrain the solutions to be on a road, and use past data predictively to further constrain the result.

Multipath errors caused by reflections in urban canyons are unlikely to be mitigated by dual frequency working; the point of dual frequency is to assess and correct timing errors introduced by ionospheric conditions that change the speed of propagation. Different frequencies are retarded by different amounts, so the effect can be computed and used to correct the solution. Errors arising from reflections from buildings aren't frequency dependent, so they can't be corrected in the same way.

Hong Kong may be an extreme case - almost every part of the city is high rise, with relatively narrow streets and few open areas. The vast majority of buildings are 20+ stories high, with 50+ not being unusual; of course these are all steel framed buildings. The accuracy returns to normal in parks and away from built-up areas. Further, the built-up areas are surrounded by hills that are even higher. Few cities are as dense as Hong Kong, and I will admit that even there, the problem is confined to the most built-up areas such as Central and Kowloon.

I will note that I have read academic papers that have considered this issue in the context of "Smart City" systems. The problem is well known.

 

[jwfrary]

It is not unlikely for SA to be enabled again if things escalate ..... and take note - differential will not help if they do enable SA again this time ... I am sure the random offset will be such as to defeat correction.

Interestingly, I experiance a GPS denial event, specifically the differential corrections were being jammed. Pretty localised on this occasion, but a good reminder that it does exist and is used operationally.

The effect was to reduce the accuracy of the position to standard GPS plus minus 10 or so meters. Still not good for high accuracy survey operations but adequate for normal marine operations. Good reminder though!

 

[coopec]

My 14 year old son has just told me the world will end In 2 days.
After doing a google search it seems there is a chance of a huge solar storm knocking out satellites. I'm guessing gps could be wiped out temporarily.
I do have an interest in finding my way with the old ways and hope to buy one soon regardless .

There are a lot of fake/replica/collectable sextants made in India which are for sale on eBay. They are great for display purposes at home but they are useless for navigation.

If you are going to buy a working sextant get something like a Davis, Ebbco (el cheapos) But get advice from someone who is familiar with sextants.


Typical eBay description

NAUTICAL MARINE SHIP NAVIGATIONAL BRASS SEXTANT INSTRUMENT REPLICA

DESCRIPTION

Material : High Quality Solid Brass
Finish : Shiny Polish Brass(shown as picture)
Length : 4'' Inch Approx
Width : 3.1" Inch Approx
A Fine Quality Handmade Item
Perfect Piece For Your Office & Home Decor Collection
Great Nautical Collector Piece
Reproduction Item

Best sextants: 5 plastic options under £400 for taking a noon sight
Which Sextant Is Best For A Beginner? – Casual Navigation

 

[jordanbasset]

Did the world end on the 24th September and I missed it

 

[srm]

Did the world end on the 24th September and I missed it

No, it was just a reboot.

 

[lustyd]

They are great for display purposes at home but they are useless for navigation.

I see this said all of the time as if the sextant is some unbelievably sophisticated contraption. It's a protractor and a stick, sometimes with some lenses and shades. They were making them a thousand years ago, I'm pretty sure even a cheap one is adequate and quite straightforward to check with an actual protractor!

 

[jwfrary]

I see this said all of the time as if the sextant is some unbelievably sophisticated contraption. It's a protractor and a stick, sometimes with some lenses and shades. They were making them a thousand years ago, I'm pretty sure even a cheap one is adequate and quite straightforward to check with an actual protractor!

I suppose its all down to what kind of precision your after! 1 or two mininutes is several miles of error, it's going to be tricky to measure a 60th of a degree with a protractor!

In any case its fairly easy to use some vertical or horizontal sextant angle calculations to assertain if a sextant is reading correctly. All depends what yoye trying to do.

Mines a Davies,mk15 but I would love one of the 1950s variants that's solid no nonsense and not shiny brass, great optics as well alass there are other priorities afoot!

 

[Buck Turgidson]

I suppose its all down to what kind of precision your after! 1 or two mininutes is several miles of error, it's going to be tricky to measure a 60th of a degree with a protractor!

In any case its fairly easy to use some vertical or horizontal sextant angle calculations to assertain if a sextant is reading correctly. All depends what yoye trying to do.

Mines a Davies,mk15 but I would love one of the 1950s variants that's solid no nonsense and not shiny brass, great optics as well alass there are other priorities afoot!

Mine's a MK15 too. Last sun run sun I did was 2nm from my GPS position. More than good enough to navigate with should a massive solar flare take out GPS. (which is the only realistic reason to lose GPS for a prolonged period. Nobody is turning it off or downgrading accuracy of a Safety of Life service without prior notice and in a limited area despite what the doom mongers claim).

 

[srm]

I see this said all of the time as if the sextant is some unbelievably sophisticated contraption.

I think the post you quoted was referring to the ornamental/display varieties on ebay, not navigational instruments.

A sextant for navigation is a simple elegant instrument, but requires precise machining and even then may have systematic errors, hence the good/professional ones have an independent test certificate.

 

[zoidberg]

There is an awful lot of spurious information about GPS in this thread.
Might I suggest anyone who is interested look at the actual data rather than listen to this guff!
Real Gen
More real Gen
Truth about SA

What? Confuse the picture with 'fact, logic and reason'...?
Where d'you think you are? TikTok....?

 

[lustyd]

I think the post you quoted was referring to the ornamental/display varieties on ebay, not navigational instruments.

A sextant for navigation is a simple elegant instrument, but requires precise machining and even then may have systematic errors, hence the good ones have an independent test certificate.

Yes I know what they were talking about, and most of those would navigate just fine. The talk of precise machining comes from old men with a fetish, not from the genuine need. Sure, a highly engineered one is more accurate, but to say the others are useless is utterly ridiculous and calls for a visit to the national maritime museum to observe what has been used throughout history. If you're close enough to something that precision matters, switch to pilotage because your sextant isn't the appropriate instrument at that point.