Solution to: "The Days of Yacht Deliveries"Jul 25, 2007
The taking of a planet and star sight is not as difficult as it may first appear. Although there is more skill involved in taking the sights, the calculations are not much more difficult than a sun sight. Venus has one more correction that has to be found on the front pages of the Nautical Almanac, but the math is not difficult. The important thing, of course is to follow the procedure.
If I had Vol. 1 HO 249 the star sights would have been child's play. Vol. 1 is issued every five years and it is entered with the LHA of Aries. The volume provides the seven best available stars with their Hc and Zn. However, for the sake of consistency I used Vol. II HO 249 which is a bit more cumbersome.
Taking sights of stars and plants is predicated on the amount of ambient light. We need to have it dark enough to see the stars, yet light enough to make out the horizon. The specific time is known as civil and nautical twilights. In the morning we use the time of nautical twilight, though usually the navigator will have the watch and be able to tell when the time is right. The twilight times are listed in the daily pages of the Nautical Almanac.
The sights take place at a DR of 28° 53' N, 63° 25' W. The Height of Eye is 10 feet and there is no Index Error. The Hs of Venus is 22°40.2'. The time of the sight is 9:05:35 GMT. Let's find the Ho
3rd corr- 2.3
+ Planet corr .1
The last additional correction is for Venus found on the front pages of the NA. It is always positive.
Next let's find the GHA of Venus.
GHA of Venus at 9Hrs 348°31.2'
Inc and corr 1°23.8'
-v corr .1'
GHA Venus 349°54.9'
-Ass long 63°54.9
LHA Venus 286°
Dec N 18°52.9' d +0.7
Dec N 18°53'
We now enter into HO 249 at Ass latitude 29°
Hc 22°17' d +25 Z 81°
Table 5 + 22'
Intercept is 4.1 nm Away
Next we go to shoot the star Formalhaut. Remember when taking a shot of star and planets there are no limbs as there are when shooting the sun or moon. The star or planet is split by the horizon. It takes some practice!
One note here: Because the declinations of HO 249 only go to 29° we got lucky with this shot as the declination is S 29° 35.1'. If the declination is greater than 29° one must use HO 229 or Vol.1 of HO 249, if you have one that is up-to-date.
The Hs of Formalhaut is 31° 31' the time of the sight is 9:07;10 GMT. Let's find the Ho.
3rd corr - 1.6'
GHA Aires 48°21.9'
Inc and corr 1°47.8'
+ SHA Star 15°29.7'
GHA Star 65°39.4'
-Ass Long 63°39.4'
LHA Star 2°
Declination S 29°35.1'
We enter HO 249 and find the following:
Hc 31°58' d -60 Z 172°
Table 5 - 35
Intercept 3.3 nm Toward
When the LHA is less than 180°, Z is subtracted from 360° in order to find Zn which is the true bearing of the celestial object. In this case the Zn is 182°.
The last part of this puzzle is to plot the fix and find out where we are in relation to the DR. When the plot is done (and I hope you take the time to do it) the fix falls at 28° 57' N by 63° 58' W. This is where we would begin our new plot.