Getting Started in Celestial Navigation (The Marine Sextant)

hi I'm Chris Nolan aboard the sailing vessel navigator in the West Indies in this episode we'll talk a bit about the parts to the marine sextant and how to use it to measure an angle between the horizon and a celestial object the first thing to do is familiarize ourselves with the sextant itself the marine Sexton is a device used for measuring angles between two objects in the sky it's nothing more than a glorified protractor but in order to understand the concepts of celestial navigation in the Sexson itself let's take a closer look in the parts to the device the telescope enables the user to get an amplified view of objects the shades come in various magnitudes and are used to dim bright objects such as the Sun and the moon the way the sexton actually works is by moving two mirrors in relation to each other the index mirror moves while the horizon mirror does not there are two ways to move the index mirror the first is with the squeeze trigger on the index arm for gross adjustments the second is with the micrometer drum which is used for fine adjustments let's take a closer look at how to read the sextant this section is set to zero degrees in zero decimal zero minutes note that zero is not all the way back on the sextant a very small adjustment gets the sextant to zero degrees in 1.0 minutes one minute of Arc on the sextant equates to one mile of accuracy and measurement on a chart this is zero degrees and 30 point-0 minutes note that the degree reading is noticeably not zero anymore let's zoom in even a little bit further every section is a bit different but as the micrometer drum turns you can see the degrees change slowly and you can get even more precise down to the tenth of a minute in fact in reality this is far greater than the human eye can distinguish in order to read the tenths on this sextant you note which decimal on the vernier scale on the right most closely matches the hash mark on the left watch as we progress through the decimals this is 0.2 0.4 0.6 zero point eight and one point zero here's a couple examples of how to read the Sexton this is 60 whole degrees and zero decimal zero minutes this is 60 whole degrees plus thirty one decimal four minutes don't get too caught up in reading the tenths remember that the human eye is not as precise as the sextant typically an accuracy of one minute of Arc is sufficient you're getting started in celestial navigation the best place to start is with the biggest object in the sky the Sun now when we measure the Sun in the sky we're actually interested in shooting to the center of the Sun but that's really difficult to do with the sextant so what we do instead is we shoot to the lower limb of the Sun let's use this orange to represent the Sun theoretically we want to shoot to the center of the Sun that's imprecise to do because it's tough to measure so instead we shoot to the lower limb of the Sun the picture we want to see through the Sexton is the Sun balanced on the horizon like a golf ball on a fairway it's also possible to shoot to the upper limb of the Sun with a different correction however this typically is not done instead we always shoot to the lower limb of the Sun so I'm a tanker and Martinique in the French West Indies and being an acre is a great time to practice taking sights with the Sexton the first thing that I do is I set the sextant to zero I read the instrument carefully to make sure that I'm actually at zero once I have the Sexson set at zero I set shades I know that on a day like today I need one shade on the horizon glass and two on the index glass to make sure that I don't hurt myself but you'll have to experiment with your own sextant be very careful not to hurt yourself by looking directly at the Sun without proper protection once I have my shade situation figured out I look through the telescope directly at the Sun I look quickly to make sure that everything is good and I'm not going to hurt myself once I'm satisfied that everything is good with sexing the first thing I do is I adjust the micrometer drum by about thirty minutes of Arc while I'm looking at the Sun this allows me to separate the Sun into two distinct images in this case we've exaggerated a little bit it's about four or five degrees difference so you can see that there are two distinct images of the Sun once I've separated the Sun into two images I take the lower image and I follow it down to the horizon using two distinct images in the sexy the first is that the index arm moves forward the second is that the Sexton itself comes down to the horizon together it looks like this so when I take the sight I do those two distinct motions and bring the Sun down until it's just about at the horizon once it's close to the horizon I use the micrometer jump to fine-tune the adjustment keeping the Sun in sight the entire time the index arm moves forward as the sextant comes down we get the Sun as close to the horizon as possible once I've got the Sun pretty much there and I've adjusted it with the micrometer drill the last step is to swing the Sexton in an arc while adjusting the micrometer drum so that the Sun pitches an arc through the image and touches the horizon at the very bottom of the arc this ensures maximum accuracy as the sextant rocks back and forth the Sun traces an arc through the image we want the lower limb of the Sun to touch the horizon at the bottom of the arc once I've got it I note the exact time to the second read the sextant two or three times to make sure I didn't make a mistake and do some calculations there's a couple other ways to take the sights that you may have been taught or seen people doing one way is to pre calculate how high the Sun should be or make a logical guess set the sextant to that angle and then simply sweep the horizon until you're close once you get there you adjust with the micrometer drum swing the sextant back and forth until you have it exactly right and then record your time and measurement another way to do it is to invert the sextant by turning it upside down what that enables me to do is instead of bringing the Sun down to the horizon I bring the horizon up to the Sun that way it tends to be a little bit more complicated for beginners but it's out there if you need it either way the important thing is to note the exact angle and the exact time to the second that you measured that the actual act of measuring a celestial body is probably the most important aspect of celestial navigation it's where the most common errors occur so the good thing is you consider thank ER or sit at home on a winter's day and practice taking measurements to your heart's content you can use the horizon edit at a beach or you can purchase or create an artificial horizon and use it at home so that you can practice enough so that when you're at sea your measurements are second nature in this episode we've talked a bit about the parts of the marine sex and how to use it to measure an angle to the Sun practice the techniques you learned refer to the notes at the bottom of this lesson and when you're ready we'll move on you

Channel: Practical Navigator

Views: 635,759

Rating: 4.9084291 out of 5

Keywords: celestial navigaton, celestial, navigation, navigation training, navigationtraining, uscg, sextant, celestaire, astra IIIb, astra 3b, astra, coast guard, merchant mariner, chris nolan, navigator, coast guard training, education, marine navigation, marine sextant, sailing, s/v navigator, learning celestial navigation, navigating by the stars, navigating by the sun, how to use a sextant, how to celestial navigation, how to navigate by the stars

Id: DrAkrgZRb9Y

Length: 7min 51sec (471 seconds)

Published: Fri Apr 05 2013