Solution to: “Petrel heads south”

This navigation problem deals with the solution of a sun/moon sight. This sight is done during daylight hours, when both the sun and moon are visible and are at an angular distance from each other that creates two good intersecting Lines of Position, i.e., more than 45 degrees.

I am a big fan of this particular sight, because you can get a lot of information without much extra effort. It beats the heck out of a sun line alone, where all you get is a Line of Position. If the moon is visible during the daylight hours, go ahead and shoot it. This combined with the sun sight will give you a good fix — depending, of course, on how accurate your shot is.

Your skill with a sextant will make you a better celestial navigator. The tables we enter are pretty straightforward. If you know where to look to get the numbers and you are reasonably careful, you should never have any problem reducing the sights. The art of celestial navigation is in the hand-eye coordination — the ability to get a shot of a celestial object while the vessel is heeled over and you are grasping the mizzen with one arm while holding the sextant in the other, in the ability to drop the sun’s limb right on the lip of the horizon. So get out there and practice!

We will first reduce the sun sight. We are using the 2005 Nautical Almanac. The day is Nov. 23. The D.R. at the time of the shot is 37 degrees 40 minutes N by 74 degrees 25 minutes W. The shot is of the Lower Limb. Height of Eye is 10 feet, and the Sextant Error is 5 minutes Off the arc. The sun shot is taken at 14:37:10 GMT, and the Hs is 23 degrees 58 minutes. Since we are using H.O. 249 to solve the celestial triangle, I have included the corrections from Vol. 11.

Since we can’t subtract the Assumed Longitude from the GHA, we add 360 degrees to the GHA to get the following:

We can now enter the sight reduction tables under Assumed Latitude 38 degrees N, LHA 328 degrees and Dec 20 degrees S. For the minutes of declination, I round off the 26.6 minutes to 27 minutes.

H.O. 249 yields the following:

Table 5 for the correction for minutes of declination is -27 minutes

Intercept is 1 nm Toward. Remember when plotting that the intercept is 1 nm toward the sun in a direction of 147 degrees from the assumed position of 38 degrees N by 74 degrees 40.4 minutes W.

Now we can do the moon sight. This is also a lower limb sight, and we have more numbers to wade through. The moon sights are not that difficult, although it is easy to make math errors. There are specific directions in the moon tables at the rear of the Nautical Almanac.

On the moon page, you will also see the following corrections:

The answer for the GHA of the moon is:

These last numbers are retrieved from the moon tables at the rear of the Nautical Almanac.

We now enter H.O. 249 under Assumed Latitude 38 degrees N with a declination of 15 degrees N and an LHA of 56 degrees. The tables read:

Table 5 for 33 minutes of declination is +19 minutes. Thus:

Note also that if the LHA is less than 180 degrees, Z has to be subtracted from 360 degrees to get the correct Zn or azimuth. Thus, we have to subtract 99 degrees from 360 degrees to get a correct azimuth of 261 degrees.

When we plot the answer, we find that van Nes got a fix at 37 degrees 45 minutes N by 75 degrees 08 minutes W.