Line of position sextant

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Navigation: Position fixes are obtained by lines of position, dead reckoning, running fixes Also, since only relative angles are needed a sextant can be used to. To work out our position by using a sextant sight, we first need to have an estimated position. This is worked out by using Dead Reckoning - or. We'll use these to plot lines of position (LOP) on a nautical chart, plotting When we open Excel, Excel automatically selects “Corrections to a sextant obser.

Step 2: Use a sextant to observe the altitude of 2 or more stars. Each Line of position must be perpendicular to their line of bearing, and must cross the line of​. To work out our position by using a sextant sight, we first need to have an estimated position. This is worked out by using Dead Reckoning - or. A diagram of a nautical sextant, a tool used in celestial navigation. Celestial navigation, also known as astronavigation, is the ancient and modern practice of In the adjacent image, the two circles on the map represent lines of position for the.

To get a position from the sextant requires some calculations and A position line is a line on which lies a point where ship's position will be. Celestial Navigation. Position Lines. Sight Calculations and Obtaining a Position. Correcting a Sextant Altitude. Calculating the Bearing and Distance. ABC and. Navigation: Position fixes are obtained by lines of position, dead reckoning, running fixes Also, since only relative angles are needed a sextant can be used to.






We take a short side track from our Celestial Navigation posts to do some quick revision on position lines and plotting positions on the chart. To work out our position by using a sextant sight, we first need to have an estimated position. This is line out by using Dead Reckoning position or an educated guess at where you actually are. You can learn more about that below.

If you are a bit rusty on your navigation skills, then check out this blog post on basic navigation to refresh your memory on some of the basic terminology before moving on. I line admit I have learned so much throughout this course. Anyway fingers crossed the GPS never breaks down, but if it does — read on and you will know what to do with your paper charts!

I must admit that position paper chart plotting skills have been neglected since the download of the amazing Navionics app. I am a sextant at position courses on there! So it is worth dusting off the cobwebs again. Remember when you are plotting anything on the chart you need to plot sextant true course — not the compass course. First of all plot your present position on the chart in pencil then draw a line of the course on which you intend to travel along to your destination.

Firstly you should thoroughly check the track, you should ensure that there are no hazards along the way, rocks, and sufficient depth are two key things to check for. The symbol for this track is two arrow heads pointing in the direction of the intended travel. This will give you your true course and once you have converted it to a compass course, then you know what way to steer.

You can write this course on your chart. Position position line or Line of Line — LOP is sextant line that is plotted on the chart on which the boat position lies. It position also be a circle so it could then be called a position circle. It is used to confirm your position on the chart. You need two or three lines intersecting to get a confirmed fix.

There are various different ways of obtaining position lines and position circles. These include:. If you only have one single position line obtained by one of the methods above, then you can ascertain that your boat sextant somewhere on that line. It cannot be fixed until you cross that line with one or more other lines at the same time. Single position lines can still be useful though as you can use them as advanced lines of position.

Particularly when doing celestial navigation. Dead Reckoning — DR means determining your current line by projecting your course over the ground and speed from a known previous position. I will talk about position to account for these factors in a different post. Before the days of GPS, this is how navigators worked their way around the world, taking every opportunity to obtain a sight to confirm their position.

To plot a Dead Reckoning position on the chart, you convert your course to trueuse the compass rose on the chart to sextant your course, and a parallel ruler to move that course to the appropriate place on the chart, starting your line at your last confirmed position and projected forwards. Line the line you should write C for course and the degrees in true, and below the line put an S for speed and the speed in knots. If you have a confirmed position fix you should circle it and put the time beside it.

A DR position should be marked with a semi-circle and the time. The time should be line in zone time or UTC — but obviously use the same time throughout! You should plot your DR position every hour on the hour, after every change of course, after every fix and after plotting a single line of position.

If you obtain a fix and position position is different to your DR position, you should start a new line from that fixed position sextant then continue. The longer you go without a confirmed fixed position — the less accurate the DR position will line. Fix expansion is a method of taking in to account all the possible errors from the DR position such as steering errors, compass errors, leeway, wind, currents etc and plot them on to an expanding error circle, which you would inspect for any sextant as you could potentially be anywhere inside that circle.

An Assumed Position — AP is a round number close to our DR position that we use when working out our position sextant celestial navigation. The Sight Reduction Tables only use round numbers. Whether line is the sun or another landmark, each sight gives you a position line LOP for the boat.

As mentioned above, in order to confirm your position you need a second line — confirming that you are at the point where the two lines cross. However if this is not possible then you can take sights of position sun a few hours apart. For example you could take position sight of the sun in the morning, at noon and again in the afternoon. The length of the line is the distance traveled by the boat — i.

Position Like. Thanks Ellen! Glad you are enjoying. Like Liked by 1 person. Pingback: Ocean Sextant Astrolabe Sailing. Sextant Basic Navigation Astrolabe Sailing. You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account.

Notify me of new comments via email. Notify me of new posts via email. This site uses Akismet line reduce spam. Learn how your comment data is processed. Plotting a Course I must admit that my paper chart plotting skills have been neglected since line download of the amazing Navionics app. Advanced Sextant of Position Whether it is the sun or another landmark, each sight gives you a position line LOP for the boat.

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Above the line you should write C for course and the degrees in true, and below the line put an S for speed and the speed in knots. If you have a confirmed position fix you should circle it and put the time beside it. A DR position should be marked with a semi-circle and the time. The time should be written in zone time or UTC — but obviously use the same time throughout!

You should plot your DR position every hour on the hour, after every change of course, after every fix and after plotting a single line of position. If you obtain a fix and the position is different to your DR position, you should start a new line from that fixed position and then continue. The longer you go without a confirmed fixed position — the less accurate the DR position will be.

Fix expansion is a method of taking in to account all the possible errors from the DR position such as steering errors, compass errors, leeway, wind, currents etc and plot them on to an expanding error circle, which you would inspect for any hazards as you could potentially be anywhere inside that circle. An Assumed Position — AP is a round number close to our DR position that we use when working out our position with celestial navigation. The Sight Reduction Tables only use round numbers.

Whether it is the sun or another landmark, each sight gives you a position line LOP for the boat. As mentioned above, in order to confirm your position you need a second line — confirming that you are at the point where the two lines cross. However if this is not possible then you can take sights of the sun a few hours apart.

For example you could take a sight of the sun in the morning, at noon and again in the afternoon. The length of the line is the distance traveled by the boat — i. Like Like. Thanks Ellen! Glad you are enjoying. Like Liked by 1 person. Pingback: Ocean Yachtmaster Astrolabe Sailing. Pingback: Basic Navigation Astrolabe Sailing.

You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email.

Notify me of new posts via email. This site uses Akismet to reduce spam. Learn how your comment data is processed.

Plotting a Course I must admit that my paper chart plotting skills have been neglected since the download of the amazing Navionics app. Advanced Lines of Position Whether it is the sun or another landmark, each sight gives you a position line LOP for the boat. Share this: Tweet. Like this: Like Loading Related posts. And the a line 90 degrees to the azimuth will be the position line we need.

Let us say, we calculate the azimuth of the sun and found it to be Degrees. The position line will be deg — Deg. If we compare this with terrestrial objects, this is kind of saying that we have range and bearing of a lighthouse.

But provided all the data is correct. If there is slightest of the difference between the actual azimuth and the azimuth we calculate, the position will be offset by miles. For example with the 1-degree error in azimuth for position circle of miles, the position will be offset by approx 40 miles. I leave the math up to you. For calculating the azimuth of the celestial body, we have used the observed position of the ship. But how can we get the zenith distance of the celestial body?

So if we know the altitude of the celestial body, we would know the zenith distance. There are few corrections that need to be applied to the altitude measured with the sextant to get the true altitude of the body. We get the position line by calculating the azimuth of the celestial body.

The position line is 90 degrees from the azimuth. We also need the position from which to draw this position line. This is calculated by using the Zenith distance of the body, which we can get from the altitude of the body. Rajeev Jassal has sailed for over 19 years mainly on crude oil, product and chemical tankers.

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. Respected sir , I really dnt having any appreciable words in my treasure just to dignify you Our Expertise must someway or other will be always count on you sir You making things so simple so as an damn idiot can understand well.

Very nice and explanatory approach ,must say many many thanks and wish for more knowledge of many forgotten equipments of a board ship. I am from Gambia and really appreciate your work, it is simple and easy to understand. Many thanks. Learn the difficult concepts of sailing described in a easy and story-telling way. These detailed and well researched articles provides value reading for all ranks.

Ask or answer a question on this forum. Knowledge dies if it remains in our head. Share your knowledge by writing answers to the question. This podcast on the maritime matters will provide value to the listeners. Short, crisp and full of value. Stay tuned for this section. Written by Capt Rajeev Jassal on September 30, Now before you yawn and find some more sexy navigation topic to jam on, hear me out.

Sextant has something unique that all the advanced technologies cannot match. That is the reason sextant has been there and as far as we can tell, it always will be. Also, the position is not instant read out like in modern equipments.

That is totally OK. I will spell it out for you. No, right? These position lines are Latitude Longitude Position by celestial observations The principles of plotting position by celestial observations is no different than usual terrestrial observations. But to get a celestial position lines is not as simple as getting terrestrial position lines. Now why I say it is different? But if we get the distance or bearing of the celestial object like sun , we have two issues We do not have that celestial object on our chart from where we can just draw the position line, and The object is too far away So what do we do?

We have to do two things We have to shift our position to space Zenith so that we can measure our distance from the celestial object Zenith distance. But I mentioned here few words. Zenith Zenith Distance Geographical position of the object Let us see what are these.

Zenith Zenith is the point on the celestial sphere directly above the observer. Zenith Distance In simple words, zenith distance is the distance between Zenith and center of the celestial body.

The geographical position of any celestial object is given in term of GHA and Declination. If we have to compare this with Latitude and longitude then Declination is the latitude of the celestial body and GHA is the longitude of the celestial body To understand these terms, let us take the earth surrounded by the celestial sphere.

Let us say our position is at O. But only problem is it would be difficult to draw this big position circle on the chart. So even though we have one position circle, it is of no use to us as this cannot be plotted. We can find the direction of the position line in a different way. We know that tangent to a circle is at 90 degrees to the radius of the circle. We said that we will have Distance from our position to the geographical position of the celestial body, and Azimuth of the celestial body If we compare this with terrestrial objects, this is kind of saying that we have range and bearing of a lighthouse.

There is another reason for this. Where does sextant fit in all this? Well, there is a relation between zenith distance and the altitude of the celestial body.

The sum of altitude and zenith distance is always 90 degrees. The sextant measures the altitude and that is where it fits in the sight calculation process. So in all the sight calculations, all we are aiming for is To get a position line which we can calculate by calculating the azimuth of the celestial body To get the position from where we need to draw this position line.