Computing the sextant altitude

Computing Altitudes i.e Calculating the setting to put on the sextant Computing altitudes is related to the altitude of a celestial body at the time meridian passage It is the process of calculating the altitude to ‘set’ on the sextant in order to observe a body at the time of meridian passage The process of computing the altitude of a body at meridian passage is therefore the reverse of the procedure in “Latitude by Meridian Altitude” problems • As you will remember from Latitude by Meridian Altitude problems, there is a relationship between Latitude, Declination, Zenith Distance (ZX) and True Altitude Applying Declination to ZX Let us now imagine the 2/O moves to 100 North and observes a body with Declination of 100 S Pn ZX Observers Meridian Equinoctial W Z Q Lat X Z E Dec Q TA X TA Ps This was how we solved Lat x Mer Alt problems • • • • • • • • • • • Sextant Altitude Index Error (I.E.) Observed Altitude Dip Apparent Altitude Total Correction True Altitude ZX + or – Dec Latitude 450 25.5’ S 00.2’ 450 25.7’ 8.7’ 450 17.0’ 000 15.3’ 450 32.3’ S ~900 00.0’ : 440 27.7’ N : 340 27.7’ S : 100 00.0’ N : : : : : : : P Z Lat Q Dec X ZX Computing Altitudes is the complete reverse • • • • • • • • • • • Sextant Altitude Index Error (I.E.) Observed Altitude Dip Apparent Altitude Total Correction True Altitude ZX + or – Dec Latitude 450 25.5’ S 00.2’ 450 25.7’ 8.7’ 450 17.0’ 000 15.3’ 450 32.3’ S ~900 00.0’ : 440 27.7’ N : 340 27.7’ S : 100 00.0’ N : : : : : : : P Z Lat Q Dec X ZX Procedure Begin by obtaining the LMT of Meridian Passage Apply longitude in time to obtain the GMT/UTC of Meridian Passage Obtain the Declination of the celestial body from the Nautical Almanac Then proceed as follows:- Computing Altitudes is the complete reverse S A I.E O.A Dip A.A T.C T.A ~ ZX Dec Lat 450 25.5’S 00.2’ 450 25.7’ 8.7’ 450 17.0’ 000 15.3’ 450 32.3’S 900 00.0’ 440 27.7’N 340 27.7’S 100 00.0’N • Apply Latitude to Declination to obtain the ZX • Difference the ZX from 900 00’ to obtain the TA • Apply the T.C using the TA as the argument in the Altitude Correction Tables, A2/A3 • As you are in fact doing a reverse procedure please remember the correction MUST be applied in reverse! • This applies to all corrections i.e T.C., Dip & I.E • Then apply ‘Dip’ to obtain the O.A followed by Index Error to obtain the S.A or conversely, the setting to put on the sextant Be Careful! I II III When combining Latitude and Declination Also when naming ZX A simple sketch will always help ZX Z Q X Lat Dec TA IV All corrections MUST be applied with the OPPOSITE sign Pn Be Careful! • Questions will also require the true bearing of the body at meridian passage (either 0000 or 1800) • This will be named opposite to the ZX and can be checked by using the Latitude, Declination and a small sketch ZX Z Q Lat X Dec TA Pn Proforma Layout Find the UTC and LMT of meridian passage of the Sun (LL) and also the setting to put on the sextant Date is July 29th 2000 in DR position 400 30’S, 0310 00’W Index Error 2.0’ off the arc and Height of Eye of 28m State the bearing of the body at meridian passage LMT Mer Pass Long UTC Mer Pass 29d 29d Lat Dec ZX ~ TA TC(+) AA Dip (-) OA IE (+) SA 12h 06m 02h 04m 14h 10m 400 30’S 180 35.1’N 590 05.1’S 900 00.0’ 300 54.9’N 14.4’ (-) 300 40.5’ 9.3 (+) 300 49.8’ 2.0’ (-) 300 47.8’N 00s 00s 00s Dec “d”(0.6) Dec 180 35.2’N 0.1 – 180 35.1’N ZX Z Q Lat X Dec TA [...]...Proforma Layout Find the UTC and LMT of meridian passage of the Sun (LL) and also the setting to put on the sextant Date is July 29th 2000 in DR position 400 30’S, 0310 00’W Index Error 2.0’ off the arc and Height of Eye of 28m State the bearing of the body at meridian passage LMT Mer Pass Long UTC Mer Pass 29d 29d Lat Dec ZX ~ TA TC(+)