![]() Work from the East to the West because in the evening you lose the eastern horizon first, and in the morning you get the eastern horizon first. Repeat for some other stars, and try to get shots of stars which give a good angle of "cut". Plot the position-line thus obtained from the comparison of calculated versus observed. Your Assumed Position is the origin of the azimuth-line on which you plot your line of position (LOP). In other words, if your calculated altitude is greater than your corrected observed altitude, then you are further away from the star along the azimuth. Remember the useful acronym "Coast Guard Academy" - Computed Greater Away. The difference is your error in position, which you will plot on a plotting-sheet. There will be an error of some number of minutes of angle (i.e. Now, perform your Marc St Hilaire sight-reduction using the values of time, Sidereal Hour Angle and Declination for the star for the time of your shot obtained from the Nautical Almanac,and compare it against the observed altitude (corrected for Dip or height-of-eye, sextant Index-Error and refraction) for your assumed position. Using the adjusting-drum, bring the star to the horizon, rock the sextant to make sure the star just grazes the horizon, and note the time of the shot to the nearest second. Somewhere in the field you should see a small bright spot.that's the star! You might not be able to see it with your naked eye, but the sextant star-telescope will help. ![]() ![]() To get a star shot (at dawn or dusk), calculate the local time of Nautical Twilight (for dawn or dusk), and at that time (or preferably a few minutes earlier,) pre-set your sextant to the altitude of the star you wish to shoot, then go to your compass (gyro or magnetic, in which case you will have to correct for magnetic variation), line your sextant up on the bearing (i.e. You don't need to know them by sight, the sextant will find them for you using the following procedure. In that row you will find the tabulated Altitudes and Azimuths (True) for seven selected navigational stars at various azimuths. Then, find the page for your your Latitude (in whole degrees) in "Selected Stars" and look down the column on the extreme left marked LHA Aries until you get to your calculated LHA Aries. Now you have the Local Hour Angle of Aries. If it's greater than 360, subtract 360 degrees. If it's less than 360 degrees then add 360. LHA Aries = GHA Aries (for the Hour GMT, from Nautical Almanac) + Increment (Minutes, again from Nautical almanac) + Longitude East (OR - Longitude if you are West or Prime Meridian). However, Sirius usually rises early enough and it is nice to know the easily identifiable stars in the Southern Cross and Orion's Belt.įor stars, use "Selected Stars" (AP 3270/NP 303) Epoch 2010.0, and calculate Local Hour Angle of Aries at nautical twilight (time in GMT for nautical twilight taken from the Nautical Almanac) for your Assumed Position thus:. Stars are not as useful as you would hope, because you need a clear horizon and by the time most stars are visible, the horizon is not. Once you can get a reasonable sight with the sextant, and have accurate time, all you need is Bennett's Almanac and an ability to add and subtract.įorget the theory.just follow the directions in the Almanac.Īs for the bodies, start with sun, moon and planets. Bennett from one of our Aussie Universities published his own version of an almanac good for (I think) 10 years. In fact easier to hold than heavy metal jobbies when all is pitching and tossing, and if you do drop it in the drink it won't break the bank.Īs for tables, a guy named Prof. ![]() It will take all of 30 minutes to master it.įor what it's worth, the Davis Mark 2 (the one with the vernier scale) is quite adequate for use on a small boat. Ozdork if you are ever in Tassie I would be pleased to show you how to use that Davis sextant. ![]()
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