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
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).
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.
PLOTTING SHEET
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
