Celestial Observations Amplitude (Visible Horizon ) - Moon - Center on VH

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1 LEGEND: USCG 2119-NP-5 Blue = information provided Black = calculated values Celestial Observations Amplitude (Visible Horizon ) - Moon - Center on VH Red = table values, etc. Green = ANSWER STEPS At 1622 ZT, on 15 June 1981, in DR position LAT ' N, LONG ' W, you observe an amplitude of the Moon. The center of the Moon is on the visible horizon and bears psc. The variation is 5 E. What is the deviation? SOLUTION: ZT 16h 22m 00s 6/15 ZD + 9 (135 W = 9) GMT 01h 22m 00s 6/16 SEE PAGE 5 for excerpts from the Nautical Almanac June 16, h 22m 00s GMT Dec S ' d (6.3) 2.4' Dec S ' Using the provided ZT determine your GMT. Watch for a change in GMT date. Lookup the Moon's Declination for the GMT in the Almanac. The d correction is applied based upon how Declination is increasing or decreasing with time on the date used. Apply corr d from the minutes table A bearing (psc or pgc) of a body can be observed when it is on either the Celestial or the Visible Horizon. The Moon is on the celestial horizon when its upper limb is on the visible horizon. When observing a body on the Visible Horizon, an additional correction from Bowditch Table 28 (New Table 23) must be applied. This correction accounts for the change in bearing as the body moves between the Visible and Celestial Horizons. It reduces the bearing on the Visible Horizon to the Celestial Horizon, from which the table is computed. No additional corrections needed! Moon Visible Horizon Celestial Horizon Correction from Bowditch Table 28 (New Table 23) needs to be applied in this case! Moon Visible Horizon 2119-np-5.pdf Page 1 of 5 Copyright LAPWARE, LLC

2 The best time to observe an amplitude of a celestial body is on the Celestial Horizon. There are fewer corrections that have to be applied. xx*xxthe Moon is on the Celestial Horizon when its upper limb is on the Visible Horizon. xxxxtable 28 Excerpt Declination xxlatxxxxx16 xxxxx ' x10 xxxxx0.1xxxxx ' x15 xxxxx0.2xxxxx0.2 Use Bowditch Table 28 (New Table 23) Correction of Amplitude as Observed on the Visible Horizon to determine the correction. Interpolate as needed. Enter this table with the determined values of: Lat ' N and Dec S ' There is no need to interpolate. Corr: 0.1 For the moon apply half the correction toward the elevated pole. Therefore, due to N Latitude and Rising, SUBTRACT Observed bearing: psc Table 28 Correction: Corrected bearing: psc 2119-np-5.pdf Page 2 of 5 Copyright LAPWARE, LLC

3 Dec S ' LAT ' N xxxxtable 27 Excerpt Declination xxlatxxxx17.0 xxx ' xx10 xxx 17.3xxx ' xx15 xxx 17.6xxx18.1 The angle is prefixed E if the body is rising and W if it is setting. The angle is suffixed N or S to agree with the name of the body s declination. Interpolate 4 ways for the exact Amplitude. Amplitude = E 17.7 S (Rising and Declination) BY FORMULA: Sin Amp = Sec Lat x Sin Dec Use the formula in the Explanation of tables for Bowditch Table 27 (New Table 22) Amplitudes. Sin Amp = Sin Dec / Cos Lat = Sin Cos Convert the Dec and LAT into degrees and tenths of degrees. Insert into the formula and solve for Sin Amp = = = E 17.7 S (Rising and Declination) This angle is prefixed E if the body is rising and W if it is setting. This is the only angle in celestial navigation referenced FROM East or West, i.e. from the prime vertical. The angle is suffixed N or S to agree with the name of the body s declination. A body, whose declination is zero, rises and sets exactly on the prime vertical. In this example, the angle is labeled East and South np-5.pdf Page 3 of 5 Copyright LAPWARE, LLC

4 090.0 The bearing is in the SE quadrant. Add Amplitude E 17.7 S (a) to/from 090 to determine the Amplitude. True Bearing This is the TRUE bearing of the Sun. Bearing TRUE T Variation V 5 E + W Magnetic M Deviation D -1.5 E Compass C psc Mnemonic - "TVMDC" East is least, West is best when uncorrecting. Uncorrecting is going away from T to C or T to G. Compass is best, error is WestxxxxGyro is best, error is West Compass is least, error is EastxxxxGyro is least, error is East Select the closest answer. A) 1.5 E ANSWER B) 1.5 W C) 0.5 E D) 0.5 W 2119-np-5.pdf Page 4 of 5 Copyright LAPWARE, LLC

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