Celestial Navigation: Sextant Basics

hello sailors I'm captain Tom Searcy from the Maryland School of sailing and seamanship and I'd like to show you a few things about the Sexton which is used in ocean navigation the sextant is an optical instrument used to measure a vertical angle between the horizon and a celestial body such as the Sun the Moon planets and stars this vertical angle is then used in the calculations to help you determine a line of position and your location on earth on on the sea here's the section that I've used for over 60,000 miles of blue water ocean sailing it's a Freiburger yacht sexton manufactured in Germany - very high-quality German engineering and manufacturing standards it's aluminum lightweight compact and it's been very rugged reliable for all that time at sea with me it's imported in this country but a klaus and instrument company and let's look at the names of some of the parts first there's the frame and that's this structure here which everything is attached to and then there's the index arm which pivots at this point and it rotates along the ark here then there's the ark itself with the degree marks scribed on it from 0 to 120 and each line on there is a whole degree then we have this index mark which you use to line up with the degree marks because this part of the index arm moves then there's a trigger which releases or locks the index arm into position when you're when you leave go the trigger it locks when you depress it it unlocks here is an index mark on this micrometer and here's the micrometer with the micrometer scale and this scale has minutes on it each line is a minute that is a minute portion of a degree 60 minutes to a degree so there's 60 marks scribed around this drum of the micrometer and then there's an index mark here that you line up to make that reading of the number of minutes here's the index mirror which is rigidly attached to the index arm and it rotates with the index arm here are the index shades these are the shades that you use when you're shooting the Sun to obscure the brightness of the Sun but still allows you to see a crisp clear image of the Sun here's a horizon mirror and then and in this case this is a split horizon mirror and we'll talk more about that later here's the horizon shades again - to block out the brightness of the Sun and here's the scope that you look through and the scope is adjustable for focus and again the index arm which rotates at this point and attached to the index arm is the index mirror the trigger mechanism and the micrometer and all of these rotate with the index arm when you depress the trigger you can you can move them so those are the basic components of the sextant and here are the nachos the trigger engages in two so let me show you how this works if I depress the trigger I can move the index arm and when I release the trigger the trigger engages one of the notches in the bottom and stops the index mark on a fixed number of degrees on a whole degree and I can then rotate the micrometer with the minute scale on it in order to read the minutes so I have a number of degrees and I have a number of minutes that I can read notice that when I was rotating this index arm that the index mirror was rotating along with it because it's rigidly attached to it okay now here's the micrometer wheel with markings on it to go from zero to 60 and those are minutes and you notice the index mark on that micrometer meat wheel so there is 58 59 60 zero zero becomes 60 okay and then I can interpolate a decimal minute by just eyeball interpolation for example I would read this as three point five I would read that as three point seven now let's take a look at what you see when you use the Sexton when you look through the scope you're going to see images of the horizon the sky and also the celestial body in this case the Sun and these images are going to come through two different light paths the first light path is the image of the Sun is going to come into the index mirror bounce off of the index mirror pass through the index shades and bounce off of the horizon mirror and then pass into the scope and you're going to see from that the image of the Sun the other path is from the horizon itself the image of the horizon is going to go past the horizon mirror because in this case this is a split horizon mirror which is only half of the field of view through the scope I'll show you this in a second but that image of the horizon going to pass right by the horizon mirror and go into the scope the sextant will superimpose these two images in your field of view you're going to see both the image of the Sun and the image of the horizon superimposed and this is what you'll actually see through the scope now the circle is your the big circle is your field of vision through the scope the vertical line is the edge of the split horizon mirror in the left-hand field you'll see the water that is the sea water the sky and the horizon and in the right hand view you'll see the image of the Sun so keep this in mind the sun's image comes through this yellow light path that horizon comes through this blue light path and you end up seeing this through the scope when you initially look at the Sun through the scope in order to bring its image down to the horizon you'll start out with the index arm set at zero degrees and zero minutes in that case the light paths to the Sun will come through both two routes that we just discussed it will come down through this route to the index mirror horizon mirror and the scope it's one round it will also be coming directly through the straight end route here to your scope so therefore you have to have both sets of shades engaged in order to protect your eyes so the horizon of the index shades the upper shades have to be down and in front of the index mirror and the horizon shades have to be up and adjacent to the index Marron in this way both light paths will be protected with the shades and protect your eyes now the sextant is equipped with several different shades of different color intensities because you have different days where the Sun is bright some days and not as bright other days so you have a set of combinations that you have to select for today's lighting conditions to take today's atmospheric conditions in order to determine which to use you don't look through the scope in order to choose your shades you look through the shades directly now in this case in this section you can see there are three index shades and we have to look through those shades either singly or in combination with one another in order to choose the combination of shades that give you the best image of the Sun the resulting selection should be that the Sun is bright enough to be able to see well but not so bright as to hurt your eyes and you should really end up with a nice clean sharp disk of the Sun and in order to do that as I say you look through the shades themselves so what I usually do is take one shade look at the Sun okay that's not dark enough might take two shades get rid of that one might put another one on let's say I chose those two shades you put those down in front of the index mirror you do the same thing with the horizon shades you you look directly through them and you choose which combination is best and those are the ones that you engage now before you take any shots you have to adjust the mirrors this index mirror has to be adjusted so that it's perfectly perpendicular to the plane of the Ark and the horizon mirror has to be adjusted also to be perpendicular to the plane of the Ark plus it has to be adjusted it has to be tilted has to be tilted this way or this way in order to assure that it lines up with the zero mark on New York and for this then the index mirror has one adjusting screw in the back shown here and there's this little wrench that's used to make those adjustments with that adjusting screw and you see that the horizon mirror has two adjusting screws one for the perpendicularity and one for the zero adjustment I want to draw a diagram to show you what I mean with these adjustments in order to check the perpendicularity of the index mirror first take off the index shades set the arc at roughly 40 degrees okay and then look at the sextant from this direction I'm going to turn it so that I'm looking from the top of the sextant in other words you're actually you're actually seeing this view of the sextant okay and in this view you're going to see the arc reflected in the mirror you will also see the arc directly in this view and those two images of the arc have to be aligned if they're misaligned you just that screw in order to get into a line when they are aligned this mirror is then perpendicular to the plane of this arc now let me show you a sketch that will help to clarify this drawing is taken from Dutton's navigation and piloting by Elbert s Maloney 14th edition and it shows the view of the sextant from the top here is the index mirror and in the index mirror you see the reflected image of the arc actually it's it's the end of the arc way over on the right here that you're seeing in the mirror and you also see this portion of the arc directly not not reflected in the mirror and those two images need to line up in order for the index mirror to be perpendicular to the plane of the arc so when they're misaligned as shown here you adjust that screw on the back of the index mirror in order to get them into alignment next I have two adjustments to the horizon mirror the first is to get the mirror angled properly so that it lines up with the zero mark on the arc in order to do that I'm going to set the index arm exactly at zero degrees and zero minutes on the micrometer so a zero degrees here and zero minutes on the micrometer with that I'm going to look at the horizon through the through the scope and I want to see the horizon through two different light paths one coming in through the mirrors into the scope and one coming directly into the scope those should line up if my if my index if my horizon mirror is aligned properly so if I look and I see a step in the horizon then that means that the mirror is misaligned and I adjust this lower screw to change the angle of that mirror until the horizon lines up at that point I have a perfect zero and this is what you would see through the scope on the left hand side you see the direct view of the horizon on the right hand side you see the reflected view of the horizon that passes through the two mirrors and into the scope and since they are misaligned it means that you have an index error this would also be an index error where on the right hand side it is lower down but it's still misaligned and this is what you should see if you have no error no index error you should see both both light paths aligned next we have to check that the Eisen mirror is perpendicular to the plane of the ark to do that we site on the horizon and with the with the shades off the index arm set at zero degrees micrometer is zero minutes we look at the horizon and we tilt the sexton maybe 30 degrees when you do that if the horizon splits that means that your horizon mirror is not perpendicular to the plane so you take your wrench put it in in the in the in the upper adjusting screw look at the horizon tilt to Sexton again at 30 degrees make a change in this adjusting screw until that disappears until the horizon lines up straight check the other direction it should be the same okay take the wrench out look back at the horizon with the sextant vertical now and if you have a step at this point what you can do is get rid of that step by making a slight adjustment in the minutes on the micrometer if you can get rid of that step with it with a two or three minute change here in the micrometer that's fine because you can deal with that two or three minutes in the calculations but you have to have no step when the sextant is rocked when it's tilted in either direction now some people call this side error that is when the horizon mirror is not perpendicular to the plane of the arc it simply means that there is a horizontal displacement between the two images that you receive one through the mirror light path and the other through the direct light path so you can use other objects than the horizon to correct this error you can use a star you can use the Sun you can use the vertical side of a building and make adjustments so if you're looking at a star you may see two stars and you make an adjustment in this setting to bring those two stars into one star we're looking at the side of a building and you see two sides of a building you bring them together into one side of the building okay you can use those other methods as well but my own preference is to use the horizon let's see okay so now you're ready to take a shot let's shoot it let's shoot the Sun the first thing I recommend is that you do this from a sitting position with your feet locked maybe around the stands in a vertical stanchion on the boat and let your upper body free to swing don't lean on anything because as the boat moves and bounces over ways it's going to jerk your position and foul up your sight wear a harness clip in because you're going to be concentrating on taking your shot and you're not going to be thinking about as much what the boat is doing at that time so you're ready to go the first thing you want to do is check your index error and I recommend you check the index error every time you take a shot because it's temperature sensitive it will change throughout the day even on a metal sextant and if you just before you take your shots check check the index are so basically what this is just a refresh your memory if you set this on zero degrees zero minutes on the micrometer look at the horizon and if it's split if it's stepped adjust the micrometer until you get it to line up the horizon to line up take a reading on that because this this number that you see with a micrometer now you will use in your calculations to correct that for that index error okay now let's look at the details of the index error on the Sexton itself you can see up here towards the top of this picture that the index mark is lined up with this line which is actually the zero degree line although the zero degree doesn't show in the picture but that's the zero degree mark so that that mark is on there and you can see on the micrometer that the micrometer is set at zero minutes here's the index mark for that and you can see that over here is zero zero minutes here's five minutes here's ten minutes going around the scale and coming all the way around the other side here's 55 56 57 58 59 60 so zero would be 60 actually so that shows that the degree mark is lined up the minute mark is lined up on zero that would be a zero Degree zero minute reading let's look at the next picture and in this picture we have again the degree mark still lined up on zero degrees but on the micrometer we see that the micrometer is set at I would say one point four degrees for one point four minutes beg your pardon and therefore this degree mark or this index mark here is actually slightly higher than zero degrees it's actually one point four minutes higher or to the left of the zero Degree mark and this is called one point four minutes on the arc okay and let's look at the next diagram the next picture again the zero mark is lined up on the degree mark on the zero degree mark but the micrometer shows 57.6 here's fifty fifty-five remember this is 60 so it's 55 56 57 58 so therefore this is 57 and I would do an eyeball interpolation of 0.6 so fifty seven point six minutes it's reading therefore this mark this index mark here is slightly to the right of the zero Degree mark and we call that off the arc so it's two point four minutes off the arc or as if you count back from the zero it's one to two point four and that's how you read on the arc and off the arc leave that setting there to bring the Sun down put on the shades that you previously selected I went through how to do that so you put on those shades on the index shades and the horizon shades and here's a very important step you want to steady up your two hands use these fingers braced against your wrists your left fingers against your right wrist and use your index and thumb of your left hand on the trigger okay now when you initially look up at the Sun in this manner you're going to find the Sun in the right-hand portion of your field of view because you're seeing the Sun through this mirror light path so you look up at the Sun and you have to find it you may have to search around a little bit of look out to the side make sure you're in about the right ballpark you find the Sun you have these index and thumb on the trigger your other fingers braced on your wrist once you've cited the Sun in order to bring it down to the horizon you are not going to swing this index arm forward you do not do that what you do if you stop to think about this you want to end up with the scope horizontal looking at the horizon and you want to end up with the Sun sitting on horizon okay so what you're going to do to bring it down once I have the Sun sighted in the right-hand portion of my field of view then I'm going to depress the trigger and I'm going to rotate my right wrist the wrist that's holding the sextant okay let's light the Sun depress the trigger bring down nice so I think my scope is about horizontal and then I let go of the trigger now at this point I say I think it's horizontal because I cannot see the horizon because I have horizon shades on okay so I brought it down where I thought I was about horizontal with the scope I then take off the horizon shades I leave on the index shades and I look at the horizon and okay there's a horizon in my left hand field of view but also have to get the Sun in my right hand field of view and I may not find it initially and I might have to look around for vertically where the Sun in other words I may be looking here but the Sun is over here okay so I want to make sure that I'm looking at a spot which is below the Sun and I find the horizon now I may have to search a little bit and I see the brightness coming through and I find it in my right hand field and but it's well above the horizon or I might have even brought it down too far and it's down in the water okay to get it on the horizon I want to use the micrometer only do not use the trigger at this point because you'll never be able to hold things steady enough to do that so if I find it and I find this a couple of degrees above the horizon I simply adjust my one micrometer until I bring it down okay now I have the Sun sitting on the horizon approximately and this is what you'll actually see in your scope on the left hand side is the direct image of the water surface in the sky on the right hand is the color imposed by the son filters and then of course you see the bright Sun sitting there and as you see that you cannot actually with the split horizon mirror you do not actually see the horizon line all the way across your field of vision so when you swing the sextant you have to project the image of the horizon to kiss the bottom of the Sun and now I have to get an accurate placement of it on the horizon and at the moment that I have that accurate placement I have to get the time okay so the sequence here is to rock the Sexton back and forth while I'm looking at the Sun on the horizon when I rock like this what I will find is that the Sun swings like it's on a pendulum and what I'm looking for is when that Sun when that ball of the Sun just kisses the horizon okay and if it's above and it doesn't kiss the her eyes but still above I stop Rocking I make a little change here in the micrometer I go back in I rock again okay I might have to go back the other way and I I rock stop change the setting rock again change the setting but don't try to do them both together your the alternate lling and I'm looking for when it just kisses when I think that I have it just kissing the horizon I look at my watch and I pick off first the seconds and then I picked the hours in the minutes I'll put those into my tape recorder okay now here's one tip on this is that when I do this remember that in the morning the Sun is rising and in the afternoon is going down well the point is that what I'm doing this rock if it's in the morning I'll actually put the Sun where it's a little bit in the water the bottom of the Sun is slightly in the water and I just keep checking it it's only going to take a few seconds for it to come up and when it comes up to the right spot I checked my time in other words I don't try and make this adjustment precisely on the horizon I get it near the horizon morning it's coming up let it come up after noon it's going down let it go down and I checked my time when it hits but you only verify the kiss of the horizon while rocking one of the problems that people have with this with accuracy is that they think they've got it on horizon but they really have the sextant tilted like this so therefore you're getting an inaccurate measurement of that angle and the only way to do it accurately on a rocking rolling boat at sea is to do it dynamically with the Sun swinging on a pendulum and I keep rocking and the Sun is coming up its morning time and when it comes up to the right spot I check the time now that I have my time I then go back and I look at my degrees my minutes on the micrometer my tenths of minutes and I must also record my log reading my distance log reading on the boat because I'm going to use that when I want to plot running fixes and so forth but that's another whole story will sailors those are some of the basics of using the sextant for celestial navigation but to use it effectively there are a lot of steps beyond the bay six of the sex of itself you have to learn first of all of how to take accurate consistent shots in order to have confidence in your results you also have to learn the ocean navigation process where you utilize the information from the Sexton and integrated with the remainder of your navigating process of shore and this includes having a meaningful logbook as far as meaningful entries and also the discipline to make consistent and proper entries in that logbook also keeping a proper plotting sheet as far as setting it up properly and the practices that you are using employing that blogging sheet a lot of people I find make entries on scraps of paper and they haven't spread all over the boat you really need a consistent discipline procedure and practice in order to make effective use of this these techniques you have to learn dr dead reckoning plotting techniques you have to learn running fixes you have to learn how to apply write estimating positions current vector addition to your plots wind leeway the effects of weather use of the star finder for identifying the stars that you want out of the billions of stars that are up there in the sky so there are a lot of things that you have to learn and master in order to come at a real celestial navigator the sextant is only one part of that whole process now we do conduct online celestial Navis their full length classes the next one begins in the winter of 2014 and it will be a three day course conducted on Saturdays over a period of several weeks and also a conduct efficient training cruises we've conducted over 240 advance social ocean training cruises up between 400 and 1500 ocean miles each and so we have a lot of we have a lot of real-world experience that we can impart this to you if you join us on one of those cruises and get to actually learn how to apply the basics of classical navigation offshore and I think it's a process that you will enjoy once you once your undertaking so thank you for joining us

Channel: Maryland School of Sailing

Views: 211,347

Rating: 4.8950949 out of 5

Keywords: sextant, celestial navigation, blue water ocean sailing, maryland school of sailing, index error, index mirror, horizon mirror, split horizon mirror

Id: B_uEWNPnpiY

Length: 36min 49sec (2209 seconds)

Published: Sat Jun 15 2013