Calibrating and Using A Sextant

hi I'm Jim cook and one of the instructors for e learn to sail and we're going to do some Sexton shots this is our workshop on how to use a sextant how to take a shot and how to set up an artificial horizon and take a shot using the artificial horizon in order to take an accurate sextant shot you need to have either a true horizon or an artificial horizon a true horizon is a horizon that you cannot see any land in the direction you are taking the shot since this workshop is being held inland and we do not have an ocean or a lake that is big enough to have a true horizon we're going to use an artificial horizon which we will create using a pan of water this is for practice only you would never use an artificial horizon for actual navigational work if you're in an area that you have a true horizon you would want to use that true horizon in this video we will cover taking a shot using both a true horizon and an artificial horizon there are two types of sextants available metal and plastic metal sextants are heavier and more expensive but they are more accurate keep their settings longer and they're a lot nicer to look at a metal sextant will give you accuracy within a mile or so but this is greatly dependent on the weather in today's world with GPS readily and inexpensively available you'll be using the sextant as an emergency navigational tool only unless I was given a metal second as a gift I personally would use a plastic sextant plastic sexton's are accurate enough to be an emergency navigational tool typically able to be within four miles of your location you will need to check the mirror settings every time you use it before we start setting up and using the sextant let's make sure we're all using the same terminology and label the parts of the sexton we have the frame the scale measured in degrees the telescope the index arm the index arm lock the index arm micrometer used to measure minutes and tenths of a minutes the index mirror the index mirror filters horizon mirror and the horizon mirror filters the first thing we're going to cover is how to properly pick up a sextant you always want to reach underneath the sextant and lift it out of the box holding the frame of the sextant you do not want to lift the sextant using the index arm we have two sextants here at Davis 15 and a Davis 25 and I want to show you the difference between those two sextants first on the Davis 15 the horizon mirror which is this mirror right here is made up of 50% glass which is this side and 50% mirror which is this side this compares to the Davis 25 that has a one-piece horizon mirror the glass has a coating on it that will both allow the ray of light from the horizon to come through the glass and reflect the ray of light coming from the index mirror that makes this sextant easier to use but it also makes it more expensive before you get started using the sextant there are three adjustments that you have to make sure are correct on a metal sextant these three Corrections will probably remain the same all the time and you won't have to do any fiddling with them but on a plastic sextant you'll probably have to make the corrections every time you pick up the sextant the first of those Corrections is making sure that the index mirror is perpendicular to the frame of the sextant the thing we want to make sure is that we can see this part of the frame as we look past the index mirror and we can see this part of the frame by looking at the reflection through the index mirror we need to adjust this screw to get them to form a straight line this is an image of what you would see as you were looking along the sextant frame they're out of sync now and I adjust the index screw until they are in sync the next adjustment we need to make is for the horizon mirror there are two adjustment screws on the horizon mirror one on the outside and one on the inside of the mirror to make these adjustments we set the sextant at zero degrees zero minutes and we're going to be looking at the Sun because you want to pick something as far away as possible first thing you have to do is set the filters properly you don't want to burn out your eyeball there's a there's absolutely a reason why the old-time ships captains had a patch over one eye they didn't have the proper filters and they actually burned out their eyeballs so the first thing you want to do is take the sextant raise it up with all the filters in place you want to drop some filters in and other filters out until you find the combination of filters that gives you the best reading or the best image of the Sun this will take some practice when you're first doing it but after you've gotten used to doing it it becomes almost automatic so I will use these two filters for the horizon mirror and I'll use these two filters for the index mirror again it depends very much on the given day which set of filters you're going to use notice that we have a somewhat cloudy day today even if there's some cloud cover and there is not a crisp image of the Sun the proper set of filters may pull the image of the Sun out of the cloud cover the point is that if you get a clear image of either the top or the bottom of the Sun you can take the shot if you're using the bottom of the Sun we call that shooting the lower limb if you use the top you are shooting the upper limb I have the sexton set at zero degrees zero minutes I look up at the Sun and I can see that there are two suns that are up there I adjust the outside screw to get the Suns to line up vertically and I adjust the inside screw to align the Suns so they line up horizontally to actually take the shot I start with the index arm set to zero degrees zero minutes and with the filters in place look directly at the Sun be careful the filters must be fully in place and you must look directly through the center of the telescope to be sure you do not hurt your eye since you should be seeing an image from both the index mirror and the horizon mirror there should be two images of the celestial body in this case the Sun the coloring of these images will totally depend on the filters being used at the time if you have a perfectly calibrated sextant it will appear as if you are seeing only one image try turning the micrometer drum to be sure there is a second image behind the front one if there are any errors in the sextant settings you'll see there are two images of the Sun up there this is very typical for a plastic sexton he point the sextant at the Sun you squeeze the release levers of the index arm and you follow the Sun that is moving down you follow it down until you get close to the horizon then you have to move the horizon shades out of the way so you can see both the celestial body and the horizon you want to adjust the micro drum until you have the lower limb of the celestial body just kissing the horizon as you're adjusting the micrometer you want to pivot the sextant so that the Sun is creating a pendulum action against the horizon you do this to make sure you've got the correct shot of the the sextant is perpendicular to the horizon when you've got it when you have the spot where the Sun is just at just kissing the horizon you say mark and the person who's keeping track of your time writes down the time of the shot the previous graphics show what the experience would look like with a sextant that has a full horizon mirror the following graphic will show the experience if your Sexton has a split horizon mirror 50% clear glass and 50% mirror as with the other sextant as you start taking the shot the index arm starts at 0 degrees 0 minutes and again if you have a perfectly calibrated sextant it would look like you were only viewing one image but in most cases you will see that there are two images as you release the index armlock and start to move the index arm forward into the arc you will now have two images of the celestial body this is where it gets more complicated it is important to keep the image of the celestial body on the split between the clear glass and the mirror as you move the index arm further forward you need to follow the image of the celestial body that is moving down towards the horizon this is the tricky part the rate you are lowering the sextant and the rate you are moving the index arm forward need to be synchronized so that you keep the image of the Sun in the view of the telescope after you get close to level with the sextant release the index arm lock and use that hand to reach forward and lower the shades on the horizon mirror so that you can see their horizon now that you can see the horizon arc the sextant back and forth to get the pendulum motion shown using the micrometer drum to adjust the index arm so you get the limb of the celestial body just touching the horizon now that you've learned how to do one shot you're going to have to do nine more you need to take ten shots and then plot them on a piece of graph paper with time versus height of Sexton that takes care of the fact that you might be taking a shot while you're in a trough or on the crest or the the celestial body may be in offer on a crest and bouncing around the spin of the earth is a constant so the change in height of sextant should be a straight line when you plot those ten shots on a piece of graph paper using height of sex versus time on the two axes then you should have a straight line you do the plot you take a straightedge and you find the shot that is best for you to solve you then go to the tables and solve the shot if you don't have a true horizon then you're going to have to set up an artificial horizon you can buy artificial horizons but they're very small and I don't I don't like them a whole lot I just use a turkey basting pan from your local grocery store I take the pan fill it about two-thirds full of water and then set down a flat surface I then have these two pieces of Plexiglas that I've taped together and I set them in a triangle on top of the basing pan it's really really important that you don't have any wind or any vibration shaking the water it's going to make your shots a lot easier to take once you've set up this little tent above the basting pan you also have to put pieces along the end of the tent so you don't get any wind inside this little tent area at all after you have this all set up you're then ready to start taking shots again you walk around behind the pan until you've found the spot where the reflection of the Sun in the water is coming directly up into your eye you'll find out where it is really quickly then at that point you take your sextant and you adjust the filters on the sextant see you can see the Sun and the image when we're using an artificial horizon it's important to remember that you may still have to have filters in for the horizon mirror here's the image coming out of the artificial horizon is going to be just as strong as the image you've looked when you're looking directly at the Sun you then take your shot and you bring it down into the pan of water so the image of the Sun is superimposed on top of the reflection from the pan you want them to be absolutely directly on top of each other after you've taken you a shot and you're doing your Corrections from height of Sexton to height observed because you weren't using either an upper or lower limb you were directly superimposing the images on top of each other you will not use the altitude corrections for a Sun you'll use the outdo corrections for a star and that will give you the proper altitude Corrections so that's how you take a Sexton shot you now have the angle of the Sun to the horizon and the exact time that angle was applicable it is now time to convert that information to a line of position on a chart please take a look at our celestial navigation course at E learn to sail comm this is a nine week twenty seven hour course that will explain in detail what the celestial triangle is how to solve it and how to plot that information on a universal plotting sheet thanks I hope you've enjoyed this demonstration you

Channel: James Cook

Views: 145,545

Rating: 4.8853602 out of 5

Keywords: Calibrating a Sextant, Taking a Sextant Shot, Sextants, Celestial Navigation, ASA 107, Using A Sextant, Sextant (Invention), Solving a Sextant Shot, Ocean Navigation, Sailing Accross An Ocean, Artificial Horizon, How To Use A Sextant, How To Calibrate A Sextant, eLearnToSail, Victoria Sailing School, Sextant Errors and Corrections

Id: i9gUs3cLgx0

Length: 14min 33sec (873 seconds)

Published: Fri Jan 25 2013