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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » 6.3 Running fixes in Celestial Nav

   
Author Topic: 6.3 Running fixes in Celestial Nav
RTM


 - posted October 20, 2015 01:36 PM      Profile for RTM           Edit/Delete Post 
Refer to figure 6.3-1 Running fix between LAN latitude line and sun line.
Lan line log 556 intersects sun line log 605.
Q: Sun line LOP has a Zn = 282 degrees. Meaning the suns bearing relative to the vessels is 282 degrees true. The suns bearing at LAN should be 180 degrees true south. If I have correctly understood these bearings and the book references a 30 to 60 degree difference in the suns bearing when taking sights to be a optimal change in sun bearings, then obviously I'm missing the connection.
Q: page 78 paragraph 2, "...advance the 556 line to 605 line to take in account your motion between sights..." I'm trying to understand this concept. How can this latitude be accurate 49 nautical miles later?

From: Langley, B.C.
Capt Steve Miller


 - posted October 20, 2015 03:12 PM      Profile for Capt Steve Miller           Edit/Delete Post 
in answer to your first question - your statement that the Suns bearing relative to the vessel is 282°T is incorrect. your vessel is on a course of 065°T and the Zn is at 282°T which results in a relative bearing of 143°T (360° - 282° = 78°, then add the course of 065 = 143°T). Also remember a Zn of 282° produces a LOP along a 192°/012° line and a Zn for the LAN sight as you say is 180° or 000° depending on whether the Sun is N or S of you and produces a Latitude line which is at 090°/270°. The 2 LOPs are 270° - 192° = 78° which will produce a good Fix.

Question 2 - The latitude was good at the 556 log and does not change with your motion. The concept of the Running Fix has one 'carrying' a prior LOP along your course line at the distance you have traveled between the times of the two LOPs. Where the advanced(506 log to 605 log) LOP crosses the latest (605 log) LOP is your position Fix at the time of the 605 log.

From: Starpath
RTM


 - posted October 22, 2015 04:40 PM      Profile for RTM           Edit/Delete Post 
Steve
Thank you for your explanations. I believe I now have a better understanding of a running fix. Regarding my first question I think I understand your term "relative bearing of 143 degrees" with respect to the sun's bearing from the vessels. But I can't understand why it is 143 degrees True? It appears to me to be 143 degrees from the vessels course of 065 degrees T which would put the sun 282 degrees T = Zn.
Regarding question 2 I now understand the logic to "carrying" a prior LOP.

From: Langley, B.C.
Capt Steve Miller


 - posted October 22, 2015 07:32 PM      Profile for Capt Steve Miller           Edit/Delete Post 
You are quite right about the 143°T - it is a relative bearing. Its proper notation would be 143° relative to port.
Sorry about the confusion.

From: Starpath
David Burch


 - posted October 22, 2015 10:22 PM      Profile for David Burch           Edit/Delete Post 
I believe all has been answered here, but i want to clarify the terminology and notation.

this vessel is on course 065 T. the bearing to the sun is 282 T. There is not often call for using relative bearings in cel nav, but if one were to use a relative bearings for this sun they would always be relative to the bow of the boat and would have a label R.

Thus when on heading 065 T and a sun is observed at bearing 282 T, the bearing of the sun is 143 to the left of the bow, which makes the bearing 360 - 143 = 217 R.

Relative bearings are always given as 0 to 360 measured to the right of the bow.

Put another way, you can refer to an “angle on the bow” of 143 Port, but when using the term “relative bearings” this would have to be 217R.

The use of angle on the bow is best reserved for wind directions. For general navigation we must be careful with such terms as it could be interpreted as a vessel aspect, namely 143 Red, which is a different concept.

From: Starpath, Seattle, WA
RTM


 - posted October 23, 2015 11:59 PM      Profile for RTM           Edit/Delete Post 
Thank you kindly David and Steve. Very much appreciate your time and expertise. All good for now. I find celestial navigation fascinating looking forward to practising what I'm learning.
From: Langley, B.C.


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