# Sights...Yet again!!



## Gaurav

Good day all.
I have a few questions that have been bothering me, regarding sight calculation,and I would to have some reviews/answers from you people.

1) Why is amplitude only taken during sunrise and sunset?
2) Why long by chorn is prefered for morning sight calculation?
3)Missing the Sun at mer pass., and also missing it within its ex-meridian limits, lets assume its almost 1400 Hrs. Can I take sight using long by Chron? ( i read somewhere,and heard too, that Intercept is prefered at this time). Can you please reason out?

Guess, these are enough as for now... 
Will come up with others later 
Thanks though.


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## Pompeyfan

On behalf of the 'SN Moderating Team', welcome aboard Gaurav.

Hopefully, someone will be able to help with the information you are seeking. Good luck (Thumb)


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## vic pitcher

*Noon Sights*

2- Hour angle too small


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## vasco

Bit rusty but here goes:

1) take bearing when about half a diameter above horizon to allow for refraction. May be less refraction, not sure about that. Formula much more simple to use.
2) Long by chron v Marc St Hilaire is a freindly battle between navigators. I think it is personal preference.
3) I have never had a problem missing the sun at noon and using the intercept method, not tried it long by chron. The whole deal about timing at noon was for using the ex-meridian tables for ease of calculation, calculators have superceded this.

Others on here may be able to argue the merits for me, Marc st Hilaire/intercept has always proved good.


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## 8575

Ex-Meridian: If taking your sight at 1400, then using the Ex-Mer method would be of little use; the time range between Meridian Altitude and the limit of Ex-Mer is normally assumed to be a maximum of 20 minutes (ie as near to Mer-Alt as possible). At 1400 you'd be better taking a standard afternoon sight and 'running' it back to Noon to cross that position line with the morning position line that you'd run up to Noon - hardly a fix at all really just crossed transferred position lines.

Within the time limits mentioned above the Ex-Mer sight gives a position line and bearing which when plotted can give a latitude as near to Mer-Alt as possible - it is not an absolute latitude.

By the way I started out using Long-by-Chron but for some reason switched to Marc St Hillaire and stayed with it - no idea why!


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## Nova Scotian

Gaurav said:


> Good day all.
> I have a few questions that have been bothering me, regarding sight calculation,and I would to have some reviews/answers from you people.
> 
> 1) Why is amplitude only taken during sunrise and sunset?
> 2) Why long by chorn is prefered for morning sight calculation?
> 3)Missing the Sun at mer pass., and also missing it within its ex-meridian limits, lets assume its almost 1400 Hrs. Can I take sight using long by Chron? ( i read somewhere,and heard too, that Intercept is prefered at this time). Can you please reason out?
> 
> Guess, these are enough as for now...
> Will come up with others later
> Thanks though.


(1) A quick method of checking compass error. Zenith distance is 90 degrees and allows the use of Napiers Rules to determine the bearing using observers latitude and declination of body.
(2) Always preferred MSH over LBC. It seems to me that MSH replaced LBC in the early sixties especially with the introduction of quick-method tables. When I went to pre-sea school the instructors were really pushing MSH, but I found most Mates were still using LBC when I first went to sea.
(3) LBC indentifies the longitude through which the position line cuts the dead reckoning latitude. Good when the body is on, or near,the prime vertical. When the sun is near the meridian it can give a longitude which appears to differ greatly from the dead reckoning longitude.


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## Gaurav

Greeting all. Thanks for the answers.

Vasco you got my qst. wrong. I wanted to know 'why'.

Waighty, Yes I cannot take ex-mer sight once crossed the ex-mer limits,but which standard method to prefer and why ( long by chron / intercept)? 

Yes I did a bit of research as well and the best answer I could find regarding amplitude sights in the morning is similar to Nova Scotian's,also his third answer ( missed the ex merpass one) sounds good.Even I somwhat felt this could be the reason,really appreciate it. Could you please elaborate the second half of this answer too, that is, how does the Sun's being near the meridian give a 'faulty' Obs lat using LBC.


Thanks again all


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## vasco

Gaurav,

Sorry I could not answer your questions. I do believe my response to 1) is similar to NS, i.e. easier to calculate. 

From my trusty Navigators Bible, (Basic Principles of Marine Navigation)
the formula is sine amplitude= sine dec x sec latitude.

For 2)Book states that the intercept method is gaining popularity with the more progressive navigator.

For 3) Long by Chron is only good when when the body is close to the Prime Vertical, ie close to due East/West, I distantly recall this may be something to do with the hour Angle. whereas Intercept method can be used for any bearing.

Sorry I cannot be more helpful and thanks for making me rattle my brain. 

Just a little note, especially for the more progressive navigator, my book has traveled well over a million miles and is a good 40yrs old!


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## Ankit

Hi gaurav
We use long by chron in the morning sight because at that time angle P in pzx trangle is nearly constant but our zenith distance changing very fast because at sunrise sun gaining its altitude. If we use intercept method we get error in our pl line due to change in the true zenith distamce.. but in long by chron we dont need zenith distance. I hope u satisfied with this answer.


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