Sextant Education

[nigel1]

One important thing to do when using the sextant to measure the angle of a celestial body, is to slightly roll/rock the sextant in your hand as you have the body on the horizon. You'll ses the body describes an arc over the horizon, and you want it at the lowest part of the arc

[clearsea]

I've tried to use my sextant to measure distance off using the vertical angle from the top of a charted object with known height to the waterline or its base, but find that task particularly tricky. Not only is it hard to shoot the angle accurately to the required precision on a small boat, but the solution procedure is tricky to remember when not doing it routinely, and especially tricky when the base of the object is over the horizon. The description of how to get the latter solution in the current Bowditch using Table 15 is incomplete -- requires finding an old Bowditch from a few decades ago to sort it out (Table 9 in a 1975 edition). Furthermore, there have been errors in various verisons of the forumulae for that solution printed in some editions of Bowditch.

[Talbot]

Originally Posted by clearsea

I've tried to use my sextant to measure distance off using the vertical angle ....... but the solution procedure is tricky to remember when not doing it routinely.........

The solution is to use Norrie's Tables where the distance by vertical angle table requires no calculation, merely a bit of simple interpolation for full accuracy.

[clearsea]

Originally Posted by Talbot

The solution is to use Norrie's Tables where the distance by vertical angle table requires no calculation, merely a bit of simple interpolation for full accuracy.

When the object with known height is beyond the horizon?

[dacust]

Originally Posted by clearsea

When the object with known height is beyond the horizon?

I think that should read "An object with known height where the base is beyond the horizon".

-dan

[tartansailor]

[nigel1]

Many moons ago, we used Lecky's Tables, "The danger angle and offshore distance tables". This was pretty accurate, and in two parts, one for when the base of the object is above the horizon, and the other for when it was below the horizon. Dont think it is in print now, old copies can be found.

[Sam Plan B]

I would recommend two items. First, buy Celestial Navigation Simplified by William F. Buckley. He goes over the steps to compute a running fix using two sun sights. The DVD will cost about $30 and is available from Amazon. Second, buy Airborne, also by William F. Buckley. The book is long out of print but is available from Abebooks or ebay. Chapter 9 of the book is, for the most part, Buckley's spoken narrative from the DVD. You can follow along with the table excerpts he uses.

One should also keep in mind that celestial navigation is not as complicated as may be supposed. To find your longitude you need a reliable time piece (chronometer) set to GMT. At sea with your sextant, you must be able to track the sun to its apogee and find ship-time noon. With some very simple arithmetic, convert time shown on your chronometer at the instant of ship-time noon to angular measurement. That will be your longitude west (or east) of Greenwich. One hour of time = 15 degrees of longitude. (15 x 24 = 360) In the Atlantic, if ship-time noon is 3:00 PM you are 45 degrees west of Greenwich. This is called taking a noon sight.

[nigel1]

Originally Posted by Sam Plan B

One should also keep in mind that celestial navigation is not as complicated as may be supposed. To find your longitude you need a reliable time piece (chronometer) set to GMT. At sea with your sextant, you must be able to track the sun to its apogee and find ship-time noon. With some very simple arithmetic, convert time shown on your chronometer at the instant of ship-time noon to angular measurement. That will be your longitude west (or east) of Greenwich. One hour of time = 15 degrees of longitude. (15 x 24 = 360) In the Atlantic, if ship-time noon is 3:00 PM you are 45 degrees west of Greenwich. This is called taking a noon sight.

Not quite as simple as that as this assumes that sun when at its zenith on the Greenwich Meridian, the time is 1200 GMT/UTC, which it generally is not. The time difference between GMT and when the sun crosses the Greenwich Meridian is known as the Equation of time, and is tabulated in the almanac.
The traditional method of establishing longitude at sea is to take a series of sun sights in the morning, and then run them up to the noon sight.
A set of star sights will give both Lat and Long, and during the day, with the right conditions, a position can be had from position lines from the sun and Venus

[clearsea]

Originally Posted by nigel1

Not quite as simple as that as this assumes that sun when at its zenith on the Greenwich Meridian, the time is 1200 GMT/UTC, which it generally is not.

Time is one of the devils in learning CN. As one observer wrote in the Nav-L list a few years ago, "time is slithery stuff". It's like riding a bicycle though: once got it, CN makes sense. I still think a good course makes a big difference for most learners.

[nigel1]

Originally Posted by clearsea

Time is one of the devils in learning CN. As one observer wrote in the Nav-L list a few years ago, "time is slithery stuff". It's like riding a bicycle though: once got it, CN makes sense. I still think a good course makes a big difference for most learners.

When I sat my Second Mates ticket back in 1982, we had to sit an exam called Principles of Navigation, it had no real practical use, but was excellent way of understanding what it was all about, even to the extent of being able to explain the Retrograde Motion of Venus

[nigel1]

Found a copy of Bowditch on line, seems a better table, but I also found an aticle which disputes the accurary of Table 15, as there was a change in the equation between the earlier and later editions