SUN SIGHT REDUCTION CALCULATIONS

Transcription

1 SUN SIGHT REDUCTION CALCULATIONS Date: Ship s time: GMT at sight: h m s DR Position: Lat º N/S Long º E/W Ass Position: Lat º N/S Long º E/W GHA: º Decl: º N/S d = +/- Incrmnt º Corr: +/- GHA: º Decl: º N/S Ass Long: º (add E, subtract W) LHA: º Sextant altitude º Index Error +/- º Dip (always minus) Apparent altitude º Altitude correction º True altitude (Ho) º Hc º d = +/- Z = Corr: Zn = Calc alt Hc º * (+/- 180) True alt Ho º * Zn = ºT Calc alt Hc º * Intercept = miles TOWARDS/AWAY** *Subtract smaller of Ho or Hc from the larger **If true alt is greater than calculated alt, intercept is towards, else away

2 Notes: 1) GMT is recorded in hrs mins and secs. 2) DR Lat and Long figures are whole degrees and decimal minutes 3) Assumed Lat = DR Lat rounded up or down to nearest whole degree ALMANAC 4) Find Almanac page for the current GMT date 5) Find (always on R/H page) correct day and hour of GMT time 6) Record GHA in degrees and decimal minutes 7) Record Declination from the same place, noting degrees N or S 8) DO NOT record only the minutes of declination! Look for degrees 9) Find d at the base of the column 10) d is plus if the decl is increasing down the page, minus if decreasing 11) Find Almanac grey page for the correct GMT minutes 12) Locate seconds down column under Sun/planets, read off increment 13) Increment is always added to GHA 14) Assumed Lon is calculated to produce an LHA in whole degrees 15) If Lon is E, calculate Ass Lon minutes to add up with GHA to 60 mins 16) If Lon is W the minutes will be the same as GHA minutes to derive zero 17) Assumed Lon is calculated to the whole degree nearest to DR Lon SEXTANT 18) If sight is difficult to take, record three to five sights, average the results 19) Average time of sights as well as altitudes. 20) If three sights appear logically correct, use middle one and it s time 21) Index error could be plus or minus 22) Estimate height of eye at time of sight and find dip on right of yellow card 23) Dip is always minus 24) Check altitude corr on left of yellow card in column for correct month 25) Ensure correction is plus for lower limb and minus for upper limb SIGHT REDUCTION TABLES 26) Find page for the Assumed Lat 27) If declination is in different hemisphere to Lat, use CONTRARY page 28) If declination and Lat are in the same hemisphere use SAME page 29) Find LHA in left or right column 30) Find column for correct Declination in whole degrees 31) Record Hc, d and Z. Note that d is stated as + or 32) Check top left or bottom left corner as appropriate to determine Zn calculation 33) Use Table 5 near back of tables to find Altitude correction 34) Find d along top and declination in º on sides, record the correction FINAL CALCULATION 35) Add/subtract correction and Hc to determine Calculated Altitude Hc 36) Subtract smaller of Hc and Ho from the larger to derive Intercept 37) If Ho is greater than Hc then Intercept is Towards 38) If Ho is less than Hc then Intercept is Away 39) Position line is drawn 90 º to Azimuth line and should pass close to DR position 40) If second sight is not possible the same day, a line from the DR position parallel to the course line to cross the position line will give a fair indication of actual position (not a fix).

3 PLOTTING THE POSITION LINE Setting up the grid 1. Using the Mercator grid on the lower right corner, mark the Assumed Latitude on the vertical scale (right hand side) 2. Draw a horizontal pencil line through the Mercator grid at that point. All longitude measurement for this plot will be made along this line. 3. Measure off 60º along the pencil line. This represents one degree of longitude. 4. Using this measurement, mark a longitude position on the horizontal centreline of the compass rose on both sides of the centre of the rose 5. Make similar marks on the top horizontal line to both sides of centre 6. Draw a vertical line down both sides of the compass rose through these points 7. Label the horizontal centre line as the Assumed latitude and the lines above and below 1º more and less. 8. Note: in S latitude, degrees will increase top to bottom, N latitudes will decrease 9. Label the centre vertical line as the assumed longitude in whole degrees and the lines left and right as 1 º more and less. 10. Note: in E longitude, degrees will increase left to right, in W longitudes they will decrease 11. This forms a chart grid for the position line plot. Plotting the position 12. Measure the Ass Long minutes off the Mercator grid and mark on the horizontal centreline line in the compass rose. 13. From the centre of the rose determine the Azimuth angle using the outer rim of the rose as reference. 14. Transfer this angle to the longitude mark made in 12 above and draw in the Azimuth line. 15. If the Intercept is AWAY, extend the Azimuth line in the opposite direction. 16. Measure off the Intercept on the vertical centre line of the rose. 17. Mark off the Intercept on the Azimuth line either TOWARDS or AWAY from the Long position as the case may be. 18. Draw the position line through this point at right angles to the Azimuth line 19. This the position line Next step 20. Draw a line of the course and distance from the EP at intercept to a new DR position at the time of the next sight 21. Transfer the first position line to the end of the course line 22. Plot the second sight the same as the first and draw in the position line to cross the first position line. 23. Where they cross is a fix.

4 PLOTTING SHEET

5 MERIDIAN PASSAGE SIGHT Ship s Date: Ship s time: GMT at sight: h m s DR Position: Lat º N/S Lon º E/W Use Arc to Time tables to Longitude to time: convert the longitude in Whole degrees: h 00 m 00s degrees to time in hours, + Mins & secs: 00h m s mins and seconds = h m s MERIDIAN PASSAGE Add longitude in time to GMT GMT: h m s of sight to derive MP ship. Long in time h m s Remember E minus W plus MP ship h m s GMT Then add Zone Time to get Zone time h m s Ship s time at Meridian Ship s Time h m s Passage Decl: º N/S Use ship s time at MP to d = +/- get declination data from Corr n +/- Almanac Decl: º N/S Sextant Sextant ALT º IE +/- Dip always - App alt º Alt corr n º - if upper limb True alt Ho º Establish ZX by subtracting Ho from 90 º 90 º 00.0 Establish relative position of the sun N or S Ho = º If ZX and decl are same hemisphere then +, else - ZX = º N/S Add or subtract declination Decl º N/S LATITUDE AT NOON º N/S