How accurate is a sextant?

Question

Each degree is 60 minuites of arc about 67 miles on the earth's surface. As the sun is 30 minuites of arc across how many minuites can you line the sextant up to? Only 1 minuite out is over a mile. So how did they know for sure the exact position of the poles for the first explorers. Maybe they were in the wrong place!

Answer 1

well we do know it's more reliable than the astrolabe, but there are several factors that may effect its accuracy ... collimation error, perpendicularity errors and so on. still, since measurements are made relative to the horizon, it's a fairly accurate instrument. one benefit is that even on a ship, the horizon and the celestial object remain steady.

thus, it would appear that even though these measurements may not have been exact and barring anything that might have effected it negatively, they were probably accurate enough to place the poles in roughly the right spots.

a question i never would have even considered.

Answer 2

Most sextant alidades are graduated in 10" intervals, and generally can be interpolated to the nearest 5". This corresponds to about 500 feet in latitude.

When approaching the South Pole, Roald Amundsen and his party spent an entire day making hourly sextant observations of the Sun to determine the distance and direction from their first pole camp to the actual pole. (Their first pole camp turned out to be about half a mile away from the pole.) Scott made similar measurements with a theodolite, which is even more accurate than a sextant.

Most historians today doubt that Cook, Peary or Byrd actually reached the North Pole at all.

Answer 3

Bris Sextant

Answer 4

I asked a marine surveyor once how they fixed positions of oil rigs etc to a few metres, before the GPS system was developed. He reckoned that with a good sextant and atomic clock time signals you could do it, but it took several days of repeated readings from a stable platform. Hand-held on a moving ship, half a nautical mile was pretty good going with a sextant.

The Admiralty Longitude Competition in the 18th C, which led to John Harrison inventing his chronometer, specified an accuracy of half a degree, which is 30 nau. mi. That may seem pretty rough, but for the time it was amazing. Note, though, that they could fix latitude at that time far more accurately; it was the time element, not the angle reading, that caused the difficulty.

Answer 5

I've just been teaching myself to use a sextant. I was told sextants can generally read out to one-fifth or one-tenth of a minute and each minute of angular measurement represents a distance of one nautical mile, so it's not too bad. You go with the bottom of the solar disc touching the horizon line, and I find that pretty easy to judge.

Answer 6

There is a good programme on uk TV right now which is recreating the race to the south pole. The two teams are using old fashioned methods to pin point their location. On occasion, they have been out with their estimates of location by miles which apparently was a problem encountered in the origional race to the pole.

Answer 7

I was a ship's navigator for years, we mainly took star "sights" at dusk or dawn. We chose several stars seperated by about 120 degrees, and plotted the position lines on a piece of paper. They frequently produced a "cocked hat" and we assumed that we were in the middle of the triangle so produced. In fact this was how it was done before the introduction of satellite navigation., Apparently they still take "Sights" each day in the U.S. Navy for practice.

Answer 8

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Answer 9

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Answer 10

It can measure an angle with precision to the nearest ten seconds. (A degree is divided into 60 minutes; a minute is divided into 60 seconds.

And U do have a good point! :-p~~~