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GPS VS SEXTANT / ASTROLABE

 
Edited

?? Sextant vs GPS ???
?
Part 1 of 2
GPS is the most accurate around 40 inch or 1 meter.
?
The drawback list.? ?
?
  1. Satellite ? and other countries take out
  2. Satellite and space debre
  3. Sun spots?
  4. Thunder ?
  5. Power aka battery dead?
  6. About anything effecting electronics.? ? ? ?
??????
?
The Sextant?
?
While the theoretical maximum accuracy of a celestial fix is 0.1 miles, in reality you will probably never achieve closer than 1 mile.
?
?
?
How Accurate is a Celestial Position Fix?
The theoretical accuracy of a celestial position fix is based on the accuracy of a sextant. Most sextants let you measure angles to within 0.1 minutes of a degree. 0.1 minutes {6 seconds 0.12 miles 600 feet at equator } on the altitude of a celestial body translates to 0.1 nautical miles on the surface of the earth.
?
In reality, however, it is almost impossible to actually achieve an accuracy of 0.1 minutes with a sextant.
?
You are taking your measurements from a moving boat, which is floating on a moving ocean. The horizon needs to be clear, and the celestial body needs to be a perfect, bright speck.
?
It is far more realistic to get an accuracy of 1 minute with a sextant in day-to-day conditions.
?
An accuracy of 1 minute with the sextant implies a final position fix accuracy of 1 mile.?
?
With some reading in WW2 the Navey found it 1? with a good operator most was under 5 miles .
?
Of course, getting that sort of accuracy relies on minimising every other source of error. Unfortunately, in celestial navigation, there are plenty of potential sources of error: .
?
  1. Errors reading the measurement from the sextant
  2. Errors identifying a celestial body
  3. Potential inaccuracies in your chronometer
  4. Errors in identifying prevailing atmospheric conditions
  5. Errors in your calculations
  6. Rounding errors
  7. Errors plotting the final fix
  8. operator holding the sextant
?
On plus side is?
  1. no power need?
  2. No satellites?
  3. Works with Sun spots
  4. Works when electronics does not?
  5. No power need or batteries?
?
Fortunately though, practise and precision can be used to reduce or eliminate most of these errors.
?
We can therefore say that an experienced navigator can realistically expect an accuracy of around 1 mile when completing a celestial position fix.?
?
Beginners, on the other hand, will be lucky to achieve an accuracy of 10 miles the majority of the time.
?
Dave?
?
[img]https://th.bing.com/th/id/OIP._r0SgDl4wmCPBVt4pQVWSAHaFj?w=166&h=124&c=7&r=0&o=5&dpr=2&pid=1.7&PC=EMMX01[/img]?
 

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Need a good watch if you want latitude and longitude.?

On 02-Mar-25 20:25, davesmith1800 wrote:

?? Sextant vs GPS ???
?
Part 1 of 2
?
GPS is the most accurate around 40 inch or 1 meter.
?
The drawback list.? ?
?
  1. Satellite ? and other countries take out
  2. Satellite and space debre
  3. Sun spots?
  4. Thunder ?
  5. Power aka battery dead?
  6. About anything effecting electronics.? ? ? ?
??????
?
The Sextant?
?
While the theoretical maximum accuracy of a celestial fix is 0.1 miles, in reality you will probably never achieve closer than 1 mile.
?
?
?
How Accurate is a Celestial Position Fix?
The theoretical accuracy of a celestial position fix is based on the accuracy of a sextant. Most sextants let you measure angles to within 0.1 minutes of a degree. 0.1 minutes {6 seconds 0.12 miles 600 feet at equator } on the altitude of a celestial body translates to 0.1 nautical miles on the surface of the earth.
?
In reality, however, it is almost impossible to actually achieve an accuracy of 0.1 minutes with a sextant.
?
You are taking your measurements from a moving boat, which is floating on a moving ocean. The horizon needs to be clear, and the celestial body needs to be a perfect, bright speck.
?
It is far more realistic to get an accuracy of 1 minute with a sextant in day-to-day conditions.
?
An accuracy of 1 minute with the sextant implies a final position fix accuracy of 1 mile.?
?
With some reading in WW2 the Navey found it 1? with a good operator most was under 5 miles .
?
Of course, getting that sort of accuracy relies on minimising every other source of error. Unfortunately, in celestial navigation, there are plenty of potential sources of error: .
?
  1. Errors reading the measurement from the sextant
  2. Errors identifying a celestial body
  3. Potential inaccuracies in your chronometer
  4. Errors in identifying prevailing atmospheric conditions
  5. Errors in your calculations
  6. Rounding errors
  7. Errors plotting the final fix
  8. operator holding the sextant
?
On plus side is?
  1. no power need?
  2. No satellites?
  3. Works with Sun spots
  4. Works when electronics does not?
  5. No power need or batteries?
?
Fortunately though, practise and precision can be used to reduce or eliminate most of these errors.
?
?
?
We can therefore say that an experienced navigator can realistically expect an accuracy of around 1 mile when completing a celestial position fix.?
?
?
?
Beginners, on the other hand, will be lucky to achieve an accuracy of 10 miles the majority of the time.
?
?
?
Dave?
 

?*?How Accurate does a?*?
?*?Celestial Fix need to be?*?
This also includes GPS ?
Part 2 of 2 ?
Celestial navigation is used when you are far out at sea, when no other methods of position fixing are available.
?
If you are hundreds of miles from the nearest land, 10 miles of error in your position is not too significant.
?
Remember, in the middle of an ocean, you will be using a small scale chart. The chart probably covers most of the ocean.
?
Your position only needs to be accurate enough for you to make landfall in roughly the right place. After that, visual navigational techniques can take over.
Aka {LINE OF SIGHT} easly 10 miles or more.
 
Edited

I agree ??
Thank you for bringing the time up.?
?
It surprising what how far lust one minute off is at the equator.?
?
?Calculation 1??
Diameter at equator
(1/1) 24,500. ÷? ? ? ??
(1/2) 24. =? ? ? ??
One hour at equator is?
(1/3) 1,020.833 ÷?
(1/4) 60. =
One minute At the equator =
(1/5) 17.014. Miles,
?
One second at the equator = 1,497 feet. ?and 10 seconds is 1.7 miles
That is a long distance?
?
Dave?
?

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