sun-sight-sextant

The World has evolved enormously and so has the technology.

Google has mapped the world so extensively that you can literally put the house number of a friend and google map will take you to entrance door of his house.

When the world has become so advanced it looks so outdated to talk about things like sextant.

But anything that works independant of any external resource will be valued for ages to come.

Sextant is one such equipment on board the ships.

And because it is independant of any external resource, you will still it there on board.

Since it is going there for long time, it is time that we know it better.

I have written a couple of articles on sextant and how it helps to get the ship’s position.

Article such as

In this post, I will discuss about getting ship’s position by Sun sight.

Let us begin.

1. Getting the DR position

The first thing that we must know is our DR position.

We cannot expect to suddenly land ourself in the middle of the sea.

We must be aware of our whereabout.

To know the DR position of the vessel, we just get the last known position of the ship and apply our course and speed to that position to get the present DR position.

This is not so difficult task.

Theoretically, we get this by calculating D’Lat and D’Long between the two positions.

ship-position-by-run

Practically, you can plot your last know position on the chart, draw the course and distance from this position and measure the arrived position from the chart.

Alternatively there are many softwares available to get the arrived position from a known position with known speed and course.

But if you are taking sun sight just for practicing your hands on sextant, you can simply take the present GPS position as DR position.

This will allow you to better check accuracy of your sight.

Let us say we have got the DR position as

  • 22 deg 00 min North
  • 119 deg 48 min East

2. Take the morning sight

In the morning when the sun is above the visible horizon, measure the sextant altitude of the Sun.

sun-sight-sextant

The accuracy of the sight results depend solely on the accuracy of measurement of sextant altitude.

And the accuracy of measuring sextant altitude solely depend upon the experience of using the sextant.

That is why it is important to practice taking sights when your GPS is still working.

Let us say we have taken the morning sight and we have measured the sextant altitude of the sun as 27 deg 22.1 minutes.

The time of the sight need to be noted down precisely to the second. So let us say that the time of the sight was 08:33:32 LT (00:33:32 GMT).

Next would be to calculate the observed longitude.

3. Calculate the observed longitude by Long by Chron method.

This is the part that we all have studied during our competency exams. We all know about the calculations.

And we know that for long-by-chron, we need to know

  • DR Position of the ship
  • Index error of Sextant
  • Height of eye
  • Measured sextant altitude of the Sun
  • Time of sight

For the calculation part, you can either do that manually or you can even use the Sight Calculator on this website.

Sight-Calculator

With the calculations, we will calculate the

  • Observed longitude
  • True Azimuth of the sun at the time of the sight

This is where most of the seafarers get confused. The observed longitude calculated by the Long by Chron method is not the longitude of the vessel.

With the Long-by-Chron calculations we will just get

  • One position line of the ship
  • Position through which to draw it

Azimuth is calculated to get the direction of position line.

For celestial observations, position line is always 90 degress from the azimuth of the body.

And the position through which we will draw the position line will be

  • DR Latitude of the vessel
  • Observed longitude of the vessel

Let us calculate these two things by using the Sight calculator of this website.

Go to Sight Calculator and then to Long-by-Chron. Next enter all the input values that you have.

And then click on “Calculate”.

sight calculation long by chron

This will calculate the

  • Observed longitude
  • Position line

observed long by sun sight

So with the morning sight, we have got one position line and we have the position through which to draw it (DR Latitude and observed longitude).

sight result position line by long by chron

As you can see, in this case the position line is almost parallel to the observed longitude.

2nd Position line by Mer-Pass

So with long by chron, we have got our first position line.

But to get the ship’s position we need a second position line.

We use the Sun’s Meridian passage to get the second position line.

In Mer-Pass, we measure the sextant altitude of the body when it is on the observer’s Meridian, i.e directly above the head of the observer.

To get the position line from Mer-pass, we would need

  • DR Position
  • The time when the Sun will be on our meridian or say directly above our head which is Mer-Pass time of the Sun for that day
  • Index error of Sextant
  • Height of eye
  • Measured sextant altitude of the Sun

Get the Mer-pass time of Sun

We know that the Sun is directly above our head around noon time. In reality it may be few minutes here and there from the noon time.

The Sun’s Mer-pass time for each day is given in the Nautical Almanac.

Mer-pass-time-Sun

But this time is the Local mean time. We need to get the time as per the ship’s clocks (and GMT time) for the Sun Mer-Pass.

And I am sure you know how do we convert LMT time to Ship’s time. It is by applying the LIT (longitude in time) to the LMT.

LMT-to-ship-time

And to get the longitude at the time of Mer-Pass, we need the Mer-Pass time so that we can apply the run to the observed longitude calculated by Long-by Chron.

It is a kind of chicken-egg situation.

But as the longitude itself will be DR, we can apply the run and calculate the longitude at 1200 Hrs LT.

Run-to-get-the-arrived-long-sun-sight

So we have the observed longitude at 08:33:32 LT and let us say the vessel was doing a course of 45 Deg with speed of 12 Knots between that time to noon time.

After applying the run, we get the arrived longitude as 120 deg 13.8 Mins EAST. I leave the calculation to you.

For 2nd May 2018, from the Almanac we have the LMT Merpass for Sun as 11:57.

Ship is maintaining the time as GMT+8 Hours. We convert the LMT Merpass time to ship’s time.

Calculate-Mer-pass-time-Sun

Now this is the time that the Sun would be directly above our head at our position.

Measure the sextant altitude at Mer-Pass time

At the Mer-Pass time that we have calculated (11:56:05 Ship’s time or 03:56:05 GMT), we need to measure the Sextant altitude of the Sun.

Around 2-3 minutes before this time, be ready with the sextant on the bridge wing.

Have one person stand by with an accurate clock.

Around 30 Seconds before the Mer-pass time, you should have brought the Sun to the horizon through the Sextant and keep on adjusting the Sun to keep it at the horizon.

When the person with the clock says stop to indicate the Mer-pass time, Read the sextant altitude from the sextant.

This will be the sextant altitude of the Sun at Mer-Pass.

It is important that the sextant altitude is measured accurately and exactly at the Mer-Pass time.

Calculate the observed Latitude and 2nd Position line

As the Sun was directly above above head at the time of mer-pass and at the time of sight, the position line will be in the east-west direction.

Or we can say that the latitude we will get from this sight will be our position line also as latitudes runs east-west direction.

To calculate the latitude by meridian passage, just head to the sight calculator and enter all the values and press calculate.

Latitude-by-merpass-sun-sight

Of course, you can do the calculation part manually or with any other software or excel sheets you may have. Just make sure to verify the calculations.

With this we will get the observed latitude of the ship at the time of the Meridian passage of the Sun.

Latitude-by-merpass-results-sun-sight

 

So we have our Latitude at the time of Meridian passage which was at 1156 Hrs.

Transfer first position line to Merpass time

To get the position of the ship at 1156 Hrs, we just need to transfer the first position line (that we got through long by Chron) to this time.

The first position line was at 0833 Hrs ship’s time. So the Run time to 1156 Hrs will be 3 Hours 23 Minutes.

As we agreed earlier our course during this time was 045 degrees and speed of 12 Knots.

By applying the run, we will get the first position line at the same time as the 2nd position line.

transfer-position-line-long-by-chron

We get the ship’s position

Now both the position lines are at same time. The position at which these two position lines would intersect will be the ship’s position at 1156 Hrs (Mer-pass time of Sun for that day).

ship-position-by-sun-sight

Since ages, maritime industry is obsessed with ship’s position at noon.

So if we need the ship’s position at noon, we just need to apply run for 4 minutes to the position at 1156 Hrs to get the noon position.

Conclusion

Getting the position by Sun sight is somewhat similar to getting the position by running fix in terrestrial navigation.

In both of these, there is only one object.

Like in running fix, for sun sight too we need to get the position line from the Sun at two different times.

One position line is then brought to the same time as the second position line.

The position at which the both position line (when brought to same times) intersect is the position of the ship.

For sun sight, we get first position line in the morning by measuring the sextant altitude and calculating the position line with Long-by-Chron.

2nd position line is by measuring the sextant altitude of the sun exactly at the time of its Mer-pass.

The morning position line is then brought to the same time as the position line at the time of Mer-pass.

The intersection of these two position lines gives us the position of the ship at the time of Mer-pass.

If we need to get the ship’s position at noon, we can just apply the run to get the ship’s position at noon.

Related Blogs

Follow us Today on
our social network

Capt Rajeev Jassal

About Capt Rajeev Jassal

Capt. Rajeev Jassal has sailed for over 19 years mainly on crude oil, product and chemical tankers. He holds MBA in shipping & Logistics degree from London. He has done extensive research on quantitatively measuring Safety culture onboard and safety climate ashore which he believes is the most important element for safer shipping.

Search Blog

26 Comments

Ivan
Ivan
Sep 10, 2018

Thank you very much sir for such a clear explanation. Waiting for more articles sir. Request: position by intercept sun and stars.

Rajeev Jassal
Rajeev Jassal
Oct 14, 2018

Thanks Ivan...

Rajiv Kumar Singh
Rajiv Kumar Singh
Jan 25, 2020

sir which method is most useful long by chron or intercept?

Kamil
Kamil
Sep 16, 2018

Okay, how about plotting celestial LOP and poaition on ECDIS ?

Rajeev Jassal
Rajeev Jassal
Oct 14, 2018

We have to calculate the position from the sight calculation and plot this position on ECDIS. No need of all those lines to be plotted on ECDIS.

Vimal Kumar
Vimal Kumar
Nov 9, 2018

Sir, Could you give me your mail Id or something as I have a personal doubt regarding my career and wanted to ask you a query regarding it.

Ajay Achuthan
Ajay Achuthan
Nov 10, 2018

Goid explanation Just a few corrections. Please contact me on email

Capt. Ahsan Bin Aslam
Capt. Ahsan Bin Aslam
Nov 11, 2018

A brilliant walk down the memory lane, the days of real navigation. Capt. Rajeev, keep up the good work.

Panagiotis Tasios
Panagiotis Tasios
Nov 15, 2018

Good Day Captain. I much appreciate your efforts to share in public such useful information. Thank you.

Prem Choudhary
Prem Choudhary
Dec 14, 2018

Why do use Long by chron in morning and Intercept in evening for sight calculation?

Alok Singh
Alok Singh
Jan 29, 2019

How I wish that books were written in such a comprehensive yet simple manner.Thanx a ton for all the blogs here sir.

Halifa
Halifa
Feb 20, 2019

I wonder if can we get positon without sun merd passage time ? Let say we get first line at morning sight 0833 then after 2 hours will get again a morning sight. Is it possible sir ?

Mohammed Ansari
Mohammed Ansari
Jan 3, 2020

What u ll do with two observed longitudes

Mayank
Mayank
Jun 4, 2019

Sir, what if we take sights of 3 stars in the morning/evening and solve it by long by chron by applying for time difference. Will we get a fix by that ? How will that fix be different than Intercept method ? Thank You

Rishab
Rishab
Jul 3, 2020

Wow, just the way you explained each step is mind-blowing. Very detailed, yet interesting. Extremely well written and presented. Very impressive.

Robin
Robin
Sep 2, 2020

It's really helpful sir.. thank you

Timothy Dale
Timothy Dale
Jun 20, 2021

PERFECT LOTTERY SPELL THAT WORK FAST WITHIN 24 HOURS WITH DR RAYPOWER LOTTERY SPELL POWERS I saw so many testimonies about Dr Raypower a great lottery spell caster that will help you cast a lottery spell and give you the rightful numbers to win the lottery, i didn't believe it, at first but as life got harder i decided to take a try,  I contacted him also and told him i want to win a lottery he cast a lottery spell for me which i use and i play and i won $3,000,000 (THREE MILLION DOLLARS). I am so grateful to this man Dr Raypower and I am making this known to everyone out there who have been trying all day to win the lottery, believe me this is the only way to win the lottery, this is the real secret we all have been searching for. Do not waste time contact him today for you to also be a winner contact info below. Email: Urgentspellcast@gmail.com or Urgentspellcast@yahoo.com WhatsApp: +1 (424) 330-8109 Website: https://urgentspellcast.wordpress.com/

micheal pan
micheal pan
Jun 22, 2021

BE SMART AND BECOME RICH IN LESS THAN 3DAYS....It all depends on how fast  you can be to get the new PROGRAMMED blank ATM card that is capable of hacking into any ATM machine,anywhere in the world. I got to know about  this BLANK ATM CARD when I was searching for job online about a month  ago..It has really changed my life for good and now I can say I'm rich and  I can never be poor again. The least money I get in a day with it is about  $50,000.(fifty thousand USD) Every now and then I keeping pumping money  into my account. Though is illegal,there is no risk of being caught  ,because it has been programmed in such a way that it is not traceable,it  also has a technique that makes it impossible for the CCTVs to detect  you..For details on how to get yours today, email the hackers on : ( atmmachinehackers1@gmail.com ). Tell your  loved once too, and start to live large. That's the simple testimony of how  my life changed for good...Love you all ...the email address again is ; atmmachinehackers1@gmail.com

Caillo Lisa
Caillo Lisa
Jun 22, 2021

This is another testimony on how Chief Dr Lucky cured my HIV disease. Do you need a cure for your HIV disease? Do you want to be cured from your cancer disease? Or you want to be free from any type of disease. Kindly visit his website https://chiefdrluckyherbaltherapy.wordpress.com/ . He just cured my HIV disease and I’m very grateful to him, he is the only herbalist that can cure you.   WhatsApp number : +2348132777335  Via Email : chiefdrlucky@gmail.com Thank you all for reading, God bless"

KETINA BARLEY
KETINA BARLEY
Jun 28, 2021

I want to say thanks once again to this great man called Dr Zuma and his spiritual way of helping people i lives in USA with my husband we love each other and also he care about me always look forward to make things easy for both of us 9 years after our wedding, we both work harder to make a family greatest surprise, we have a kid after some times again, we have another one so with this, we live in peace and he was so honest to me shortly, he started misbehaving that i don’t know what is going on then i asked him. Darling what is going on? you are so strange to me this few days hope i have not offended you? he said no. Not knowing he have an affair with one lady out side who promised him a car and apartment in one estate were i cannot see him also when he cannot see me i manage to stay with him pleading him he should forgive me if i have wrong him he started complaining he has no money that he has lost all his money in his business that he needs some money then i asked him how much is this money you are looking for? he did not know i can afford it. Then, he said $14,000USD i promised him i we give it to him just for him to care about his family. My greatest surprise, the next day, i went to work and our two kids were in school not knowing his going to leave the house before i come’s back i met some of his things outside i was waiting for him to come back he never come back i cried i miss him so much and he have taking all my money away i was only left with $800USD. One day, as i was ready a blog i saw a testifier made by someone in Australia called Jessica telling people about how this man call Dr Zuma helped her and the man’s contact email was there and his mobile number then i contacted him for a help and really, he brought back my husband now am so happy my brothers and sister if you are in such relationship problem kindly via Email spiritualherbalisthealing@gmail.com or Whatsapp +15068001647 Blog: https://spiritualherbal.blogspot.com he will help you solve all your problems.

tony cam
tony cam
Jul 10, 2021

My name is Tom cam!!! i am very grateful sharing this great testimonies with you all, The best thing that has ever happened in my life, is how i worn the Powerball lottery. I do believe that someday i will win the Powerball lottery. finally my dreams came through when i contacted Dr. OSE and tell him i needed the lottery winning special numbers cause i have come a long way spending money on ticket just to make sure i win. But i never knew that winning was so easy with the help of Dr. OSE, until the day i meant the spell caster testimony online, which a lot of people has talked about that he is very powerful and has great powers in casting lottery spell, so i decided to give it a try. I emailed Dr. OSE and he did a spell and gave me the winning lottery special numbers 62, and co-incidentally I have be playing this same number for the past 23years without any winning, But believe me when I play the special number 62 this time and the draws were out i was the mega winner because the special 62 matched all five white-ball numbers as well as the Powerball, in the April 4 drawing to win the $70 million jackpot prize...… Dr. OSE, truly you are the best, with Dr. OSE you can will millions of money through lottery. i am a living testimony and so very happy i meant him, and i will forever be grateful to him...… you can Email him for your own winning special lottery numbers now oseremenspelltemple@gmail.com OR WHATSAPP him +2348136482342

Jessica Smith
Jessica Smith
Jul 11, 2021

After being in relationship with Wilson for seven years,he broke up with me, I did everything possible to bring him back but all was in vain, I wanted him back so much because of the love I have for him, I begged him with everything, I made promises but he refused. I explained my problem to someone online and she suggested that I should contact a spell caster that could help me cast a spell to bring him back but I am the type that don’t believed in spell, I had no choice than to try it, I meant a spell caster called Dr Zuma zuk and I email him, and he told me there was no problem that everything will be okay before three days, that my ex will return to me before three days, he cast the spell and surprisingly in the second day, it was around 4pm. My ex called me, I was so surprised, I answered the call and all he said was that he was so sorry for everything that happened, that he wanted me to return to him, that he loves me so much. I was so happy and went to him, that was how we started living together happily again. Since then, I have made promise that anybody I know that have a relationship problem, I would be of help to such person by referring him or her to the only real and powerful spell caster who helped me with my own problem and who is different from all the fake ones out there. Anybody could need the help of the spell caster, his email: spiritualherbalisthealing@gmail.com or WhatsApp him +15068001647 blog: spiritualherbal.blogspot.com you can email him if you need his assistance in your relationship or anything. CONTACT HIM NOW FOR SOLUTION TO ALL YOUR PROBLEMS

jessica stephen
jessica stephen
Jul 15, 2021

I am not sure I could have done it without you. I contacted a real spell caster that helped me change my husband’s heart to love and want me again, and he really did and now we are very happy together. My husband left me for another woman, This was just 3 years of our marriage. The most painful thing is that I was pregnant with our second baby. I wanted him back. I did everything within my reach to bring him back but all was in vain, I wanted him back so badly because of the love I had for him, I begged him with everything, I made promises but he refused. I explained my problem to my friend and she suggested that I should rather contact a spell caster that could help me cast a spell to bring him back , I had no choice than to try it. I messaged the spell caster called dr unity, and he assured me there was no problem and that everything will be okay before 11 hours. He cast the spell and surprisingly 11 hours later my husband called me. I was so surprised, I answered the call and all he said was that he was so sorry for everything that had happened He wanted me to return to him. He also said he loved me so much. I was so happy and went to him that was how we started living together happily again.thanks to dr unity . if you are here and your Lover is turning you down, or your husband moved to another woman, do not cry anymore, contact Dr.Unity for help now.. Here his contact..WhatsApp him: +2348055361568 , Email him at: Unityspelltemple@gmail.com , Watch this YouTube channel to know how grateful I am https://youtube.com/shorts/rBPtBktMmtA?feature=share

Mrs. Santana Valdez
Mrs. Santana Valdez
Aug 3, 2021

i am giving this testimony cos l am happy My name is Mrs. Santana Valdez from Houston,taxes.i never believed in love spells or magic until i met this spell caster once. when i went to Canada in June 28th 2021 this year on a business summit. i meet a man called Dr Zuma zuk.He is powerful he could help you cast a spells to bring back my love's gone, misbehaving lover looking for some one to love you, bring back lost money and magic money spell or spell for a good job. i'm now happy & a living testimony cos the man i had wanted to marry left me 3 weeks before our wedding and my life was upside down cos our relationship has been on for 2 years. i really loved him, but his mother was against me and he had no good paying job. so when i met this spell caster, i told him what happened and explained the situation of things to him.. at first i was undecided, skeptical and doubtful, but i just gave it a try. and in 6 days when i returned to taxes, my boyfriend (is now my husband ) he called me by himself and came to me apologizing that everything had been settled with his mom and family and he got a new job interview so we should get married..i didn't believe it cos the spell caster only asked for my name and my boyfriends name and all i wanted him to do. well we are happily married now and we are expecting our little kid, and my husband also got a new job and our lives became much better. in case anyone needs the spell caster for some help, email address spiritualherbalisthealing@gmail.com Great Zuma zuk i thank you very much thank you in 1000000 times.. if not you i would have been losted and wasted thank you. Email Him Through his email address... spiritualherbalisthealing@gmail.com please make sure you contact him for any financial difficulties okay...What a powerful man such as Dr Zuma zuk.. he is so much powerful..\\ email him for any difficulties.. spiritualherbalisthealing@gmail.com His whatsapp +15068001647

LilyDawson
LilyDawson
Aug 17, 2021

My husband and I were going through a separation, after 9 years of marriage. I have seen that coming due to his reaction towards me. And I was about losing my home, then I had to contact Dr. Sunny, the love spell caster who cast powerful marriage blessings that reunited our marriage back together and also made our union stronger than as it were before. All thanks to Dr. Sunny for reuniting my home back we are living happily now contact Dr. Sunny for any marital problem's thanks to you Dr. Sunny you can contact Dr. Sunny via email solutioncentre911@gmail.com  call or  WhatsApp Dr. Sunny mobile number +2349056393169

Lisa oney
Lisa oney
Aug 23, 2021

Genuine and Amazing service on how to get back your ex. To whom it may concern, After spending hours searching the internet about how to get my lover back i was glad that i contacted Dr Akhigbe spell caster Without wasting much time i would like to write out the details of Dr Akhigbe whose details has done a lot of people a huge favor, you can contact him on drakhigbespellhome7@gmail.com. Or WhatsApp him on +2349021374574

Leave Comment