Nutation & Precession - a question for the elders

[Skylark]

Now into winter boating hibernation so out come the reference books for armchair, fireside reading.

Thumbing through my AP3270 Vol 1, I started to consider the small adjustments required for nutation and precession.

May I pose to question to the grey beard professional navigators on here that so willing give up their information to help us wannabies; are these adjustments significant within small boat navigation? If / when / how would they be applied in practice?

Awaiting with great interest

Many thanks

David

 

[jimbaerselman]

Ahh, the days when large spinning gyroscopes kept in their own accommodation were used to detect changes in lat and long. After correcting for changes in the vertical caused by the earth's rotation, of course.

Replaced now by these nifty little things.

 

[jdc]

I'm not an elder nor a professional navigator, indeed entirely self taught, so treat my answer with some suspicion. However...

They relate to position fixing using stars. They correct for an error of typically a mile or two. With still conditions and a good sextant you should get a fix to within a mile, hence they are significant I think. The tables themselves say that the corrections are significant for surface navigation but not for aircraft.

The error being corrected for grows as a function of time from the epoch the table is published (I suspect you have epoch 2010, at least I do), and is also a function of hour-angle and observer's latitude.

That said, I don't use them (the air-tables). I buy a new book of ephemerides each year, eg Reeds' one, and although much less convenient in that they don't have the pre-computations which make the air-tables so well liked by others, I don't have to bother with any such time variant correction.

 

[25931]

Now into winter boating hibernation so out come the reference books for armchair, fireside reading.

Thumbing through my AP3270 Vol 1, I started to consider the small adjustments required for nutation and precession.

May I pose to question to the grey beard professional navigators on here that so willing give up their information to help us wannabies; are these adjustments significant within small boat navigation? If / when / how would they be applied in practice?

Awaiting with great interest

Many thanks

David

They would be applied if you wanted a very precise position to be found by using a sextant, which you youngsters might wish to try , but I think that anyone needing to ask the question might be safer using a chart plotter.

 

[richardbayle]

Oh, I can't wait for this one.

This string will run and run as all the old salts come out of the woodwork. Me? I'm an electronics man, chart plotter, weather stations, navtex, iPad, internet, etc.

But I'm sure it will be educational.

Oh really nurse is it that time already?

 

[Simes]

I think that the Nutation and Precession tables do provide a valuable and useful refinement to the position lines obtained by the use of Volume 1. Errors in the region of 4 Miles are not unusual nor are errors in direction of 100º unusual.

Nutation and Precession is given in the back of Vol 1 and is applied to the position line obtained from a STAR sight. The value given is to be applied to the STAR's position line.
The value provided is usually given as ' and º which are Miles and Direction.
So a Nutation and Precession value of 3' 70º would mean that you should move your STAR Position line 3 Miles in the direction of 070º.

Instructions for use are provided in Vol 1.

Have fun Now,

Simes

 

[Skylark]

They would be applied if you wanted a very precise position to be found by using a sextant, which you youngsters might wish to try , but I think that anyone needing to ask the question might be safer using a chart plotter.

Ouch! but thank you for calling me a youngster.

I'd like to think that I'm as competent as any recreational sailor with using a sextant and navigating an ocean passage.

Nutation and precession don't get much of an airing so I thought I'd ask the question. My point is to establish if, in practical terms, it is worthwhile to adjust the position line to take account of N&P?

I like to read others' astro threads, there's almost always something to learn. I admit to having a gap in my knowledge (one of many), hence my question.

Yes, the the majority of my working week is spent in an office but I'd hoped to get a bit more encouragement for wanting to learn more about this great art than to receive a somewhat patronising rebuttal

One of my favourite reads is Bowditch American Practical Navigator and it was on these fine forums that I first heard of such a great resource.

To jdc, yes, you're absoluetly correct, my AP3270 Vol 1 is epoch 2010 and that is precisely the root of my question.

 

[AntarcticPilot]

If I've understood correctly, Nutation and precession corrections only apply to fixed celestial objects - that is, to stars. The ephemeris of Sun, Moon and Planets surely takes care of this correction?

Am I right?

 

[25931]

Ouch! but thank you for calling me a youngster.

I'd like to think that I'm as competent as any recreational sailor with using a sextant and navigating an ocean passage.

Nutation and precession don't get much of an airing so I thought I'd ask the question. My point is to establish if, in practical terms, it is worthwhile to adjust the position line to take account of N&P?

I like to read others' astro threads, there's almost always something to learn. I admit to having a gap in my knowledge (one of many), hence my question.

Yes, the the majority of my working week is spent in an office but I'd hoped to get a bit more encouragement for wanting to learn more about this great art than to receive a somewhat patronising rebuttal

One of my favourite reads is Bowditch American Practical Navigator and it was on these fine forums that I first heard of such a great resource.

To jdc, yes, you're absoluetly correct, my AP3270 Vol 1 is epoch 2010 and that is precisely the root of my question.

Sorry, it was meant to be humorous not patronising.

 

[jimbaerselman]

Groan! And I thought we were talking about gyroscopic nutation and precession errors in early inertial navigation systems.

While you lot are talking about earth axis nodding and wobble. How quaint!

 

[jdc]

...nor are errors in direction of 100º unusual...

At the risk of us both being taken as figures of fun for being interested in astro (see posts above;-) I'm not sure I get this. I thought that what one does from the tables of correction is move the PL or derived fix by the distance and bearing given in the correction table. This translation can be in any direction, but the PL itself is not rotated. So I don't understand what you meant by "errors in direction".

 

[Skylark]

Perhaps I can answer my own query by referring to the accompanying notes to the tables.

Table 5 allows for the change in the position of the stars from the epoch of the tabulation to that of the observation. Thus, for strict accuracy, it is necessary to apply the correction to a PL or fix deduced from these tables.

Their omission will not give rise to a positional error greater than 1`.6 in the years 2008-2011. The corrections are only applicable to sights reduced within this volume of tables (vol 1, selected stars).

My conclusion is that celestial navigation is remarkably similar to Ikea self assembly furniture - when all else fails, read the bl**dy manual.

To 25931, I missed your humour, sorry

 

[PeterR]

This thread has got me thinking. I haven't taken a star sight for 30 years. In fact not since since I submitted a five star fix for my Ocean Yachtmaster. I haven't even reduced a sun sight manually since Reeds stopped printing the Empherises in the standard almanac. Instead I rely on an old Sharp calculator with the Weems and Plath Petrel program loaded. This program will reduce star sights and the manual has a list of star corrections to apply but these are based on 1988. The calculator takes 1988 from the year of the sight and multiplies the correction by the answer to get the present day correction for precesion and nutation but does this correction hold true over such a long period?

The answer is only of academic interest because I would need a current copy of Vol 1 of AP 3270 in order to precompute altitude and azimuth to find a star. My star identification was never good enough to just select a star and pull it down to the horizon.

 

[jdc]

If I've understood correctly, Nutation and precession corrections only apply to fixed celestial objects - that is, to stars. The ephemeris of Sun, Moon and Planets surely takes care of this correction?

Am I right?

I suspect you know as much as me (and we could always compare notes in the Eagle or the Old Spring for instance!) but I understand that it's not so much that that the earth's general precession doesn't apply to observations of bodies within our own solar system, ie sun, moon or planets, but that they are tabulated individually and they vary so much due to complicated orbits and parallax that the tables are released anew every year, so corrections for precession are absorbed into the those for all other factors.

Stars on the other hand being so very much further away all vary (i) in the same way and to the same degree, and (ii) so much slower, that it's reasonable to print one (set of) table(s) which lasts for a decade, and just apply the same small correction to all of them, a correction which is good for the whole year.

Does this make sense?

 

[AntarcticPilot]

I suspect you know as much as me (and we could always compare notes in the Eagle or the Old Spring for instance!) but I understand that it's not so much that that the earth's general precession doesn't apply to observations of bodies within our own solar system, ie sun, moon or planets, but that they are tabulated individually and they vary so much due to complicated orbits and parallax that the tables are released anew every year, so corrections for precession are absorbed into the those for all other factors.

Stars on the other hand being so very much further away all vary (i) in the same way and to the same degree, and (ii) so much slower, that it's reasonable to print one (set of) table(s) which lasts for a decade, and just apply the same small correction to all of them, a correction which is good for the whole year.

Does this make sense?

Yes, that's pretty much what I thought. Thanks!

 

[JayBee]

..... I would need a current copy of Vol 1 of AP 3270 in order to precompute altitude and azimuth to find a star. My star identification was never good enough to just select a star and pull it down to the horizon.

You don't need tables, just an inexpensive 2102-D star finder, which is good for many years. Set it up with LHA Aries and your approximate latitude, then read off the azimuths and altitudes of as many stars you would like to observe. Pre-set the sextant and there they will be, in your horizon mirror, on the right relative bearing. No knowledge of constellations necessary and it works in twilight, when you can hardly see any stars with the naked eye.

Simples!