Slow walk through sextant position lines - where's the massive error?

[Durcott]

Hi all,

I've been taking a couple of sextant sun sights over this month at known locations, with a view to seeing how close my calculated position line comes to my known position.

Both times I see that I am far too far away for comfort. I've gone through my method line by line in three books, and I just cannot find the error.

Is anyone up for a slow walk through my figures - either by private message or within the thread?

Cheers!

Jeff

 

[westernman]

Hi all,

I've been taking a couple of sextant sun sights over this month at known locations, with a view to seeing how close my calculated position line comes to my known position.

Both times I see that I am far too far away for comfort. I've gone through my method line by line in three books, and I just cannot find the error.

Is anyone up for a slow walk through my figures - either by private message or within the thread?

Cheers!

Jeff

How far away is too far??

 

[Durcott]

40 miles in the wrong direction - in other words 80 miles. It's as if I've picked up a wrong sign somewhere, or read the sextant wrong by about 1 degree.

 

[westernman]

40 miles in the wrong direction - in other words 80 miles. It's as if I've picked up a wrong sign somewhere, or read the sextant wrong by about 1 degree.

Surely all directions are wrong??

Anyway, may be the issue really is that you are about 1 degree off in your sextant reading.
Have you calibrated it?

 

[Greemble]

...read the sextant wrong by about 1 degree.

Sounds to me that you've spotted your error

 

[AntarcticPilot]

It's too large for any reasonable index error on the instrument, but you can easily check that by sighting a distant object. Is it a micrometer sextant or a Vernier one? The latter can be a bit tricky to read, and done wrong could account for the error. I know how to read a Vernier, but can't describe it!

Otherwise, my immediate thought is the correction for refraction; are you applying it in the right sense? Refraction makes a celestial body appear higher in the sky than it is, so you subtract it. At this time of the year, with the sun low in the sky, it could account for your error.

 

[Durcott]

It's a pretty good sextant - and I checked it 'by the book'. It's the first time I've used this on in simulated anger though.

I just took a look on the t'Internet at the calculated declination and azimuth for my true and assumed positions to compare with my results.

I would say the sextant reading agrees with the prediction, but my calculated altitude is perhaps too high. So my calculations tell me to draw my position line away from the sun, whereas common sense says it ought to be towards. So it's possible I have an error in reading the reduction table, but I can't **** well see it. I could do with another pair of eyes to sanity check what I'm doing. I'll spell it out here if it won't bore people to death.

 

[Durcott]

Ok I'll give this a go.....

Hah - I've just written out the entire set of numbers, and near the end I found the error.

As I did, I noticed that the b**********d '-' sign in the 'd' column for the reduction tables isn't printed on every line. I missed it. There we go - problem solved, and the same error applies to both sets of sightings.

Thanks for the support - it made me check my working VERY carefully
Jeff

 

[Durcott]

That's better - my position line passes 0.5 miles from my true position. I'm happy with that!

I hope my boat will be as steady as that car park when the time comes ;-)

 

[westernman]

That's better - my position line passes 0.5 miles from my true position. I'm happy with that!

I hope my boat will be as steady as that car park when the time comes ;-)

That is a very good result.

 

[john_morris_uk]

Ok I'll give this a go.....

Hah - I've just written out the entire set of numbers, and near the end I found the error.

As I did, I noticed that the b**********d '-' sign in the 'd' column for the reduction tables isn't printed on every line. I missed it. There we go - problem solved, and the same error applies to both sets of sightings.

Thanks for the support - it made me check my working VERY carefully
Jeff

You can usually work out which way to correct using ‘d’ by inspection.

 

[Durcott]

You can usually work out which way to correct using ‘d’ by inspection.

Agreed - if I had my wits about me at the time.

 

[Skylark]

That's better - my position line passes 0.5 miles from my true position. I'm happy with that!

I hope my boat will be as steady as that car park when the time comes ;-)

That is incredibly accurate for a (first) position line………I don’t think that I’ve ever approached that level of accuracy ?

Probably a good idea to do a few more such that you feel comfortable with a more realistic result.

 

[john_morris_uk]

Agreed - if I had my wits about me at the time.

If it’s any consolation I was going through a YM Ocean candidates calculations during his oral exam last week and made a basic error in looking up the GHA of the sun. Skill fade is a real phenomenon.

 

[KAM]

There's an app called "GPS anti spoof" which gives you the altitude of celestial bodies at your position in real time. It's very handy for checking your readings when learning astro nav.