The pole star

aka : “POLARIS” The AIM today is to introduce you to the theory of obtaining Latitude by Pole Star observation Why we obtain it How we calculate the observer’s latitude from a Pole Star observation Explain the theory of Latitude by Pole Star Explain why navigators use this theory to obtain a position line and hence a latitude Demonstrate how to obtain and apply the necessary corrections from the Nautical Almanac Demonstrate how to obtain the azimuth of Polaris at the time of observation Calculate a Latitude by Pole Star using the supplied pro-forma layout POLARIS Polaris (aka Ursae Minor, aka The Pole Star) lies very close to the North Celestial Pole If it was exactly on the North Celestial Pole what you think it’s Declination would be? If it’s position coincided exactly with the North Celestial Pole (i.e Dec’l = 900N) then the problem of finding the Observer’s Latitude could be easily solved as the altitude of Polaris would be equal to the Observer’s Latitude Consider the following diagram Diagrammatically:Diagrammatically:An observer at a Latitude of about 450 North P In this diagram is the following true? QP = 900? QZ = Latitude? ZP = CoLat or 900 – Lat? Polaris Z Lat Q Diagrammatically P Polaris If we now introduce the Observers rational horizon ( HR), is the following also true? • • • • • • CoLat Z H ZH = 900? ZR = 900? PH = Altitude of Polaris? PZ = 900 – Lat, or the CoLat ? QR = 900- Lat, or the CoLat ? PH = Altitude which also = Lat ?? Lat Q R CoLat Ok! So you don’t believe me? P Polaris If we say that the observer’s lat is 45N0 then:-then: • • • • • • ZH = 900? ZR = 900? QZ = 450 PZ = 450 QR = 450 PH = 450 CoLat Z H Lat Q R CoLat POLARIS However the declination of Polaris is approximately 890 and some corrections need to be made to the Altitude to find our exact latitude This is done using the Polaris Tables in the Nautical Almanac As we are taking a Sextant Altitude we must also correct for index error, dip and total correction before entering the Polaris Tables The Tables are entered with the LHA of Aries for the time of the observation How we find the LHA of Aries? The instructions at the bottom of the page must be followed POLARIS There are four corrections which we have to apply to the True Altitude of Polaris in order to find our latitude and these are:-are: I Deduct 10 00.0’ from the TA (why should this be ?) II Add correction A0 to the TA III Add correction A1 to the TA IV Add correction A2 to the TA These corrections are found in the Polaris Tables as follows:follows:- So, for example:Latitude = 260 05.2’ Month of May The LHA of Aries = 0350 32.0’ A0 = 00 14.7’ A1 = 0.6’ A2 = 0.4’ Azimuth = 000.00 One for you to do! Your Latitude is :-:- 360 29.5’S The month is : December The LHA of Aries is :-:- 3570 15.0’ Find the corrections A0, A1 & A2 to apply to the true altitude Also the azimuth A0 = A1 = A2 = Azimuth = 00 25.7’ (25.65) 0.6’ (0.55) 1.0’ 0.70 (0.665) POLARIS The last section in the Polaris Tables is the Azimuth and the Position Line runs at ?? 90O to the Azimuth The Latitude found is the Observer’s Latitude which, when combined with the DR Longitude gives the position through which to draw the Position Line (P/L) However, upon inspection of the Azimuth Table it will be seen that the Azimuth (True Bearing) is always between 3580 and 0020 which means that the P/L will never be far from the Observer’s latitude POLARIS PROBLEM SOLVING I II III IV V VI Find an approximate GMT (Z) Find the correct GMT using the chronometer time Find the LHA of Aries Correct the SA for index error, dip & refraction Enter Polaris Tables and follow the instructions given Find Azimuth and P/L Polaris Pro Forma [...]... 29.5’S The month is : December The LHA of Aries is :-:- 3570 15.0’ Find the corrections A0, A1 & A2 to apply to the true altitude Also the azimuth A0 = A1 = A2 = Azimuth = 00 25.7’ (25.65) 0.6’ (0.55) 1.0’ 0.70 (0.665) POLARIS The last section in the Polaris Tables is the Azimuth and the Position Line runs at ?? 90O to the Azimuth The Latitude found is the Observer’s Latitude which, when combined with the. .. Longitude gives the position through which to draw the Position Line (P/L) However, upon inspection of the Azimuth Table it will be seen that the Azimuth (True Bearing) is always between 3580 and 0020 which means that the P/L will never be far from the Observer’s latitude POLARIS PROBLEM SOLVING I II III IV V VI Find an approximate GMT (Z) Find the correct GMT using the chronometer time Find the LHA of... Observer’s latitude POLARIS PROBLEM SOLVING I II III IV V VI Find an approximate GMT (Z) Find the correct GMT using the chronometer time Find the LHA of Aries Correct the SA for index error, dip & refraction Enter Polaris Tables and follow the instructions given Find Azimuth and P/L Polaris Pro Forma