Author Topic: CelNav Four - Sight Reduction II  (Read 605 times)

Offline HenryC

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CelNav Four - Sight Reduction II
« on: September 13, 2010, 06:00:59 PM »
CelNav Four  Last Edit - 10/28/14

OK, we can't put this off any longer. Let's do this.  Take out your Pepperday table!

1) Look up the S value for the LHA, 308d 26'.  It is 10.000 and will be on page 18, below 308 on the top line, and to the right of the 26 on the left side. Write it in the S box next to the LHA.

2) Look up the C value for the dec 21d 07'.  It is 2.847 and will be on page 15.  The 21 is on the upper edge, and the 07 on the left side.  Write it down next to the dec, in the box with the black C+.

3) The companion S value to that C (right next to it on page 15) is 41.807.  Write it down in the S box just to the right of the box where you wrote the C.

4) Add up the S and C values from steps 1) and 2)  and put the result 12.847 in the S box directly beneath them.

5) Look up the closest C value you can find for the S in step 4) and write it in the box marked C-.   I will pick 15.635, right next to 12.845.  (P 19)

6) Repeat the C number in 5), 15.635, in the C box to its right (under "corr obs alt").

7) Repeat the S number in 4), 12.847, in the S box at the extreme right (under "GMT").

#8. Subtract the C value from 5), 15.635, from the value from 3), 41.807, and put the result, 26.172, underneath in the box marked S.

9) Follow the little arrow to the left and in the box marked K place the angle corresponding to an S value of 26.172 from #8.  According to rule (i) K must be less than 90 since LHA is NOT between 90 and 270.  Write 31d 51' in the box labeled K. (P 17)

10) Put the Latitude, 26, in the box labeled lat, circle the N.

11) Since the Lat  name is N and the dec name is S, by rule (ii) we add K and Lat and place the result 57d 51' in the box labeled K + lat.

12) Look up the C value of 57d 51' in step 11), 25.834, from page 17, And write that in the black C+ box at far right (follow the arrow).

13) Add up the numbers steps 6) and 12) and write the result, 41.469, below in the box labeled S.

14) The closest angle less than 90d corresponding to the S value in 13) is 21d 18', on page 15. Write it in the box labeled Comp alt.

15) The C value corresponding to the S value in 13) is 2.897. Enter it to the right in the box labeled C-. (P 15)

16) Subtract step 15) from step 7) and place the result, 9.950, in the box labeled S at the lower right.

17) The Meridian bearing corresponding to 9.950 is greater than 90 by rule iv, so it is 128d 21' on page 18. And by rule v,  Meridian bearing = Azimuth = 128.3d. This is confirmed by our estimate of the Azimuth when we first took the sight. BTW, this is why Pepperday has you write down a rough azimuth value at the top of the form, to help you avoid the confusing and error prone choices of rules v and vi.

18) The difference between observed and computed altitudes is 39' and Computed>Observed, so it is on the side of the AP AWAY from the GP. The way to remember this is rule iii. You can also use the memory aid "Coast Guard Academy = CGA = Computed Greater Away". Remember, since the angle you measured with your sextant is less than the predicted altitude from the AP, you must be further AWAY from the the GP than the AP.

The exact values from the N.O. website calculator are Az = 128.3d, Intercept = 39.7 Nautical miles.

Plotting the LOP

The final step is to go where you plotted the AP on your chart and draw the Azimuth line, pointing to the body's GP. Mark off the Intercept on the Azimuth line;  if the Intercept is positive, that is, the computed was greater than the observed, draw it on the Azimuth on the side AWAY from the body. If it is negative, i.e.,  the computed is less than the observed,  plot it on the side of the AP TOWARD the body. This makes sense, doesn't it?  The bigger the Altitude, the closer you are to the body's GP.  Now draw a line through the Intercept at right angles to the Azimuth and that will be the LOP.  It is a tiny piece of the huge circle on the earth's surface where the body's altitude is Ho.  You are somewhere on that line.


I suppose that by this point, many of you will have decided learning celestial by table is just not worth the effort for the type of sailing you plan to do.  There's no need to be ashamed, after all, CELNAV is becoming obsolete.  However, a backup for GPS is still a very good idea, and I will discuss a pocket calculator method in CelNav Six which requires the Nautical Almanac, timepiece and a sextant, but does not need sight reduction tables at all.  You will still be dependent on an electronic device, but it is a pretty robust and inexpensive one.  So stick around for the rest of the lessons, they will still come in handy even if you never  need to use Pepperday in earnest.

« Last Edit: October 29, 2014, 03:24:34 PM by HenryC »