Getting Started in Celestial Navigation (The Noon Sight for Latitude)

hi I'm Chris Nolan from sailing vessel navigator in the West Indies so far in this series we've learned how to use the marine sextant to measure the angle between the horizon and a celestial body in this episode we'll learn about the noon site for latitude including terms such as zenith distance and declination the noon site for latitude is one of the most historic and useful sites for you to perform in fact most of the early explorers relied exclusively on the new site so the general idea behind navigating by noon sites goes something like this if you were to depart Europe in your destination was the Caribbean what you would do is you would sail south to the latitude of your destination turn to the west and then every day take new insights adjusting as necessary until you got close enough to spot land the practical idea behind the noon site is that you measure the Sun when it's on your Meridian your meridian is your line of longitude the measurement of the Sun for a noon site should take place when the Sun is due north or due south of you but the easy way to tell is you just wait until it gets to its highest point in the sky and measure it then in fact if you go no further in celestial navigation than learning how to compute your latitude at noon you can find your way across the nation before we get into the specifics of the noon site for latitude there's a couple of theoretical concepts to cover the first is geographic position for every celestial object there's one spot on the earth directly beneath it if you were standing in that spot you would measure the celestial object at 90 degrees above your head or it would be directly at your zenith I'm here in the league of Dominica and I'm going to observe the Sun at local apparently today because today is a special day it's March 20th which is the vernal equinox that means the Sun is directly over the equator at right about now on the equinoxes the sun's geographic position is directly above the equator the equinox is a great day to demonstrate the concept of zenith distance now when we measure something in the sky we're using the horizon as a zero point and measuring up to the object however what we're really interested in is the distance from the spot directly over our head to the object that distance is called Zenith distance theoretically when you use the Sexton you are measuring the angle from the spot directly above your head or your Zenith to the spot directly beneath any celestial object living on a sphere this angular distance is a real distance in nautical miles in fact it's the amount of degrees between you times 60 this concept of zenith distance is one of the most fundamental aspects of celestial navigation theory it will become crucial to our understanding as we move through the series unfortunately we have no easy way of measuring it with the sextant so instead we measure the distance from the horizon to the object and subtract that from 90 to obtain Z so we've hoped to take the noon sight off the coast of Dominica and all I'm going to do is measure the distance from the Sun to the horizon and do some quick math and we'll have our latitude in fact on the equinox our latitude is equal to Z no distance so when I measure the Sun I come up with a figure of 74 degrees and 15 minutes of the compass you all I have to do now since today is the equinox is subtract that from 90 and I have my latitude so 90 minus 74 15 is 15 degrees in 45 minutes so let's check the GPS and see how close we can now that's fine if we only shot local apparent noon on the equinox but what if we shot one day after the equinox or one day before the equinox in that case we need to learn about declination so the spot directly beneath any celestial object has a definite latitude it's some number of degrees above below or right on the equator it's called declination declination is simply the latitude of the spot directly beneath any celestial object in fact there's another special case of the new insight for latitude we can talk about and that's when the Sun is directly overhead it's called Mahina noon and for me that's today today I measure the Sun at 90 degrees above the horizon which means I'm floating on the spot directly beneath the Sun and its declination is equal to my latitude the nautical Almanac is the book which allows us to look up the declination of the Sun for any minute of any day of any year so in the special case when the Sun is directly overhead you simply open the nautical almanac find the declination of the Sun for that time and that equals your latitude so far we've talked about a couple of special cases of the new insight for latitude one where the Sun was directly over the equator on the equinoxes another when we were directly beneath the Sun and we measured it exactly overhead however you can see there's a bunch of other cases that could happen and in order to get to the core of this we'll have to cut the earth in half let's use this melon to point out that the Equator stretches all the way from the center of the earth out to the sky similarly your position could be said to represent a line from the core of the earth outward but you needn't use the melon every time you need to think about this in fact I recommend you draw the relationship between the equator and your ship every time you have one of these problems aside from the two special cases we've talked about already you can draw the relationship between the equator your ship and the geographic position of the Sun in three ways in the first case the sun's geographic position could be in the opposite hemisphere of your ship in the second case your ship in the sun's geographic position could be in the same hemisphere but the Sun is closer to the equator finally your ship in the sun's Geographic position could be in the same hemisphere but the Sun could be further from the equator than you in the first case latitude in red will be equal to Z naught distance in green - declination in blue in the second case latitude in red is equal to Z naught distance in green plus declination in blue in the final case latitude in red is equal to declination in blue - zenith distance so to summarize aside from the two special cases we learned about there's only three real options for local apparent noon if you make the drawing every time you can't go wrong before we get into example problems we need to take a closer look at the nautical almanac you've already seen how you can pull out a value of declination for a given hour but typically observations take place between whole hour measurements in that case you must interpolate the value of declination for our case now it's typically sufficient to estimate declination to the nearest tenth of a minute so let's take a look at a couple examples all our example problems use the 1981 nautical Almanac for consistency in this case we're near the Yucatan Peninsula in Mexico in the northern hemisphere the time is 1800 UTC and our sextant reads 49 degrees 15 minutes the first thing to do is calculate our zenith distance which we know is 90 degrees minus our sex tonight using a trick of converting 90 degrees to 89 degrees in 60 minutes makes them at easy in this case we come up with 40 degrees and 45 minutes for zenith distance the second step is to pull your declination from the nautical Almanac in this case it's south 17 degrees and 16 minutes after finding our declination and zenith distance we create our typical drawing and we realize that we're in the opposite hemisphere from the sun's Geographic position in this case our latitude is equal to our zenith distance minus our declination all that's left now is to do the arithmetic to come up with our latitude in the second example problem we're further north near Nova Scotia Canada and our time is 1530 UTC at that time our sextant reads 53 degrees in 24 decimal 7 minutes the first step is to calculate zenith distance again using the trick of 90 degrees being equivalent to 89 degrees in 60 minutes this yields a zenith distance of 36 degrees and 35 decimal 3 minutes looking up our declination we must interpolate for the value between the hours of 15 and 1600 in this case our declination is north 8 degrees and 9 decimal 7 minutes we draw our picture finding out that the Sun is in the same hemisphere but further south where latitude is equal to Z net distance plus declination all that's left is to do the arithmetic to come up with the latitude in our third example problem we're in the Caribbean Sea in the time of measurement in 1618 UTC at that time our sextant measurement is 86 degrees and 53 decimal 3 minutes to find our Zenith distance we subtract the sextant height from 90 using our trick of 90 being equal to 89 degrees and 60 minutes and we come up with the zenith distance of three degrees in six decimal seven minutes to find our declination we must interpolate in this case it's sufficient to estimate that we're 20 minutes into the hour or one third of the way through we come up with a figure of 17 degrees in 8 decimal 7 minutes after drawing our figure we'll determine that latitude will be equal to declination minus zenith distance all that's left is to do the arithmetic and come up with our latitude I have to ask yourself about the goal of celestial navigation if it's to find a Caribbean islands an accuracy of about 15 nautical miles is generally sufficient now so far we've neglected a few Corrections that we could make to become a little bit more accurate we'll learn about those in lesson 3 but for now it's sufficient to practice your accuracy down to about 15 to 20 nautical so in this episode we've looked at the noon sight for latitude we've covered the concepts of Zenith distance and declination refer to the notes below practice what you've learned in this lesson and when you're ready we'll move on you

Channel: Practical Navigator

Views: 192,613

Rating: 4.9370222 out of 5

Keywords: navigation, coast guard, coast, guard, sextant, navigationtraining, chris nolan, noon site, local apparent noon, latitude at noon, latitude by sun, equinox, lahaina noon, declination, zenith, zenith distance, latitude, celestial navigation, celestial, learning celestial navigation, celnav, education, learning, navigator, merchant mariner, LAN, noonsite, subsolar point, solar noon, geographic position, nautical almanac, noon sight

Id: BWGOlpj4YwE

Length: 11min 0sec (660 seconds)

Published: Sat Apr 06 2013