Accuracy of Sextant

[JMB1943]

The posts that regularly ask about the loss of various ships prompts me to ask, in the context of rescue at the time or location of the wreck post-war, about the accuracy of a given position that might be radioed out.

Consider a vessel in mid-ocean which has had no sight of landfall in 4 or 5 days; if it founders,

 —in calm weather, on a day of clear skies and a sextant reading can be obtained by an experienced user

— the ship’s chronometer is accurate (to what typically ??)

what would be the size of the square or circle that would be expected to contain the position?

Regards,

JMB

 

 

[KizmeRD]

In ideal conditions and in the hands of a competent navigator, a sextant can be read to within 0.1 of a minute, i.e. accurate to one-tenth of a nautical mile - however fixing a ship’s position rarely needs to be as accurate as that, and after four or five days at sea on a trans-ocean voyage, even being able to fix the ship within a five mile radius would ordinarily be good enough. The problem doesn’t lie with navigational instruments, but much more on weather conditions and visibility. If you can’t see the sun or the stars (and the horizon) because of fog, cloud cover or sea spray, then it doesn’t matter how good your sextant or chronometer is, you will have to rely on dead reckoning.

The need for accurate navigation is far more acute in coastal waters where there are hazards such as shallow water and rocky outcrops to avoid.

MB

Edited 1 March by KizmeRD

[JMB1943]

Capt. RD,

Thank you for your answer, I did not realize that the sextant could be so accurate!

Regards,

JMB

 

[KizmeRD]

In expert hands, and with ideal environmental conditions, yes!

Some navigating officers would have been better at it than others, of course, but anyone qualified to the level of Master Mariner would have been very well acquainted with using it (and most Mates were qualified Master Mariners simply gaining experience and waiting for the opportunity to achieve their own command).

Out of sight of land the sextant would also have needed both an accurate time-piece and a set of nautical tables. In addition to providing celestial and solar fixes, sextants can also be used to take horizontal and vertical sextant angles off the land, however as stated previously, none of this is possible without good visibility and a stable platform, so in fog or during a storm, a sextant (even in the right hands) won’t help much.

MB