Celestial Nav problem I can't solve

[RayCam]

Hi All. I'm currently studying Celestial Nav, and have a problem I'm hoping to have someone provide a little help with if you wouldn't mind.  http://www.seasources.net/youtube%20vide...uction.mp4. If you jump about 5mins 20 secs into the video, there is a Problem 3.  I don't get the same answer, and just can't see where I'm going wrong.  Thanks

[EdCa]

Ray,

What is meant by "the time diagram"? Perhaps they mean converting E 163° 51' to time difference from GMT. If that's so the difference is either 11 hours or 10 hours 56 minutes.

To begin, the corrected chronometer time is 09:09:50 before correcting it for GMT.

Using hyphens for a time figure is poor practice.

Once we get the correct GMT the rest will be easy.

Would you please let us know what the time diagram is?

Thanks,

Ed

[RayCam]

Hi Ed,
Thanks for pitching in.  I believe the Time Diagram is their way of working out GMT from chronometer time I believe.  They have a video on it.  If you move about 40 seconds into this video, they take about it http://www.seasources.net/youtube%20vide...ometer.mp4 

Thanks
Ray

[c_davidson]

Ray,

I don't know whether they're using Ho-249 or Ho-229 or if they're just using a calculator.  The resulting plots will all be close to each other.

Using the time diagram the specifics are as follows;

Date= January 16, 1981 (why must it always be "1981"?)

GMT= 21:09:50 (corrected for chronometer error)

Ho= 13° 42'   (13° 41.9')

GHA= 134° 58'

Sun's declination= S 20° 49.4'

LHA= 298°     (determined by 135° (GHA rounded up) 164° (DR Longitude rounded up)

Hc= 33° 49' (33° 48.7')

Zn= 121°

Intercept= 07' Away

Comment- The answer they provide is incorrect or they're being exceeding picky.  But......

Correct answer is 06.8' Away.  Round it up to 07' (seven minutes) 

Why?  Because they're using a LHA of 298° 49' and a DR Latitude of N 27° 09'.  There's no way in the world I'd do that- I'm not that smart and my pencil width, when plotting the LOP, will negate such accuracy.

Zn= 122° 30'     (Ho-249 gives a Zn of 121°)

In using the Hc and Z formulas you'll get the answer of 0° 06.8' (after comparing Ho and Hc)  and the Zn of 122° 30'.

In what you and I do, no one.....no one is going to argue about 1 nm. The boat's rolling, we might be half awake, the pencil broke, the boat needs tending at the most important moment of these calculations so some error shows that we're sailors and therefore human.  Once plotted there will be no difference in the LOP between their answer and mine.

Phew.  I'd like to see the reasoning for the answers they provide. Could I be wrong?  Of course.  But my answers are close enough to theirs.  My erasure is probably more worn out, though.

Ray, I hope this helps you.

Clen

[RayCam]

Hi Clen,

Thanks so much for taking the time to look at this. Well, you've fixed my first mistake about I handle time, so that gets me the correct Zn now at 122.7, but I'm a little out on the intercept distance.  My workings are

but I have have a different calc as follows

Hs          = 13° 39.4'
I.           =       +2.3
Dip         =       -7.6
Ha          = 13° 34.1'

Alt Corr   =     +12.4
Ho          =13° 46.5'  (You have a Ho of 13° 41.9' )

CCT =  21:09:50 on the 15th Jan (the day before)?  I think you've used 16th Jan

GHA             = 132° 35.6'
+min & Sec.  =    2° 27.5'
GHA             = 135° 03.1

DR Long       = 163°  51 E
Ass Long      =  163° 56.9 E

LHA              = 299°

Dec             = S  21°  02.2
d                 =           - 0.1
Dec             = S  21°  02.1

Ass Lat        = 27° N

Hc        = 13°  55.3
corr.     =        - 1.3
Hc        = 13°  54.0

Intercept (a) = 7.4 away. (question says it should be 8.3)

Zn = 122.7

If the Zn in the answer is the same, then I'd imagine there is no error in their calculations, so I've made an error somewhere.  I just can't work out where.

Maybe you can spot it?  We had calculated different Ho

Thanks

Ray

[Fred_B]

Ray,

Here's a quick comment which might initially help getting the correct Ho. The IE is +2.3' which means "it's on the arc" so you "take it off". That is- subtract that figure from Hs. In your calculations the Ho would be 13° 44.2' which is pretty close to Clen's.

I'll look at the remaining portion of above post tomorrow.

Fred

[RayCam]

I'm out on the next problem (question 4) in the same video. Quite depressing not knowing where the mistakes are. I'm getting Zn = 211.5 and intercept 31 away

[Fred_B]

Ray,

We've got to figure out the correct date- is it January 15 or January 16, 1981. The local date at E 163° is January 16, 1981. Greenwich date is January 15, 1981. At least I'm pretty sure it is.

If you're using the date of January 15, 1981 the Sun's declination at GMT 2100 is S 21° 00.8'

If you're using the date of January 16, 1981 the Sun's declination at GMT 2100 is S 20° 49.4'.

More to follow....must go.

Fred

[c_davidson]

Ray,

Ok, men.  Let's start over.  What's the date- the 15th or the 16th? 

My date of January 16, 1981 is incorrect.  Now I say it's the 15th as the Sun's just rising to your East meaning, using the time diagram, the Sun hasn't crossed the "lower meridian" of Greenwich (The IDL).

Can someone figure out the date?!

From there it'll be easy (just like CN is easy).

Clen

[RayCam]

Morning Clen & Fred.

Thanks for joining in.

The problem is listed here. http://www.seasources.net/youtube%20vide...uction.mp4

Scroll 5 mins into the video. It's problem 3.

Their notation for the chronometer is here - http://www.seasources.net/youtube%20vide...ometer.mp4 (apparently it always has to be set to GMT, but you just need to work out if it's am or pm)

Their notation on adding or subtracting index error is http://www.seasources.net/youtube%20vide...extant.mp4

From the examples at the back - It seems to me that if they say a corrections is -2.5 then you simply take 2.5 away from Hs.

Best
Ray

[CarlosPindle]

Ray, I think you're correct that the Index Correction they're speaking of is what must be added to the Hs.  They're calling it Index Correction and not Index Error.  

My complaints are the time format using a hyphen is never done.  The confusion of using Index correction is never done and should be stated as Index error.  These factors lead to a non-standard way of performing a sight reduction and aren't instructive.

That said, Ho= 13° 46.5'

Hs= 13° 39.4'
     + 0°  02.3'    (Index correction)This IS NOT the same as IE
     - 0°   07.6'    (Dip correction)
   +  0°   12.4'    (Altitude correction for SD, Parallax, Refraction)
--------------------------
       13° 46.5'  =  Ho


Hc= 13° 54'

Zn= 122.7° (call it 123°)

Intercept= 8.3' Away

Correct answer is "C"

The ONLY way to get the correct answer is to use an Hc of 13° 54.8' which you can only get using the Hc formula with the following particulars;

DR Latitude= 27°
Declination= S 21° 00.8'
LHA= 299°

Look at the 4 possible answers.  Zn is the same in all answers.  We know that B and D are wrong.  A & C?  From our laborious reduction we can tell that our Intercept is AWAY.  Even if we use an Ho of 13° 46.5' or 13° 47' or 13° 46' we notice these results point to 8.3' AWAY.

So "C" is the correct answer.

I think the question is more particularly suited for the plotting of an oil rig.  If I could get figures that accurate I must be doing NOTHING as the ocean is perfectly calm, no wind, perfect temperature, and I'm really good at taking a Sun sight!

This wont make anybody happy but the mysterious Ho-229 (alright...Pub. No. 229) is something I seen with my eyes, looked at, and then gave up any hope of wanting or trying to understand.  229 might give the answer they want but the added corrections are beyond me. I just want an LOP within a few nm of my location.  An Intercept 8.3 nm Away or Towards?!  Are you nuts?!  Just use 8 nm Away as we can't plot with that kind of accuracy.

Did this solve the question?  Hopefully.

Carlos

[RayCam]

Hi Carlos,

Thanks - that still means we've got an error in Hc as the answer is supposed to be 8.3 away (unless there is an error in the answer)

best
Ray

[LouisC]

Hello All and especially Ray....welcome aboard!

My guess is they're using Ho-229 to solve the problem.  Look at these pages from Pub. 229 Volume 2 page 339 to get Hc d and Z.  For interpolation of "d" see Pub. 229 Volume 2 page 2.  The pages are attached below.

As you'll see quickly that 229 is a bit closer to an accurate figure than Pub. 249.  You may find some difficulty in correcting "d" but look at the interpolation table and it'll be easy. 

Very quickly I got an Hc of 13° 54.9' which when compared to Ho of 13° 46.5'  produced an Intercept of 8.4 Away. The trick in getting that figure was correct use of 229's Interpolation Table.  I might be off by 0.1' as I'm not exactly sure how to use the "double second correction".

The Z figure (which of course will equal Zn) is 122.7°  (equal to the Zn in the problem)

Get the entire Pub. No. 229 here- Pub. No. 229


  Ho-229 Vol. 2 page 339.pdf (Size: 42.12 KB / Downloads: 398)
  Ho-229 Vol. 2 page 2 Interpolation.pdf (Size: 30.29 KB / Downloads: 412)

I  hope that clears things up for you.

Louis