May/June 2010 Issue 186: At sea again

Jun 18, 2010
The Schooner Virginia under sail.

The Schooner Virginia under sail.

A gentle voice calls my name. I am in a dream and opening my eyes I see that I am laying on rumpled sheets, still dressed. There is gurgling noise, like water overflowing a tub. I take a few seconds to remember that I am aboard a sailing ship, somewhere between Bermuda and St. Thomas. The gurgling is water rushing past the hull. “Time for stars,” the gentle voice says beyond the curtain separating my bunk from the main saloon. “Okay,” I mumble and am left wondering why I left the softness of home to be roused from bed at somewhere around 0500. Then I remember: Stars! Got to go.

It has been a long time since I have been aboard a well-handled schooner out at sea, and I am still a bit discombobulated. I recall the procedures; the watches, the dish washing techniques, the sail evolutions; but my body having achieved three score years, seems a little less enthusiastic about my decision to make a passage. My spirits are good though and even if I creak and groan like the rigging of this ship, I am looking forward to my first morning star sights in a decade. I am, however, counting on there being some coffee from the night before — just for an extra boost.

The most important thing to remember about morning star sights, done at the time of nautical twilight, when the sun is 12° below the horizon, is to not show up on deck too much ahead of time. Sleep is a precious commodity at sea and one hates to squander it by getting on deck too soon and waiting for the horizon to appear. It is the difference between a professional and a wannabe. The crew, none of whom I know, watch slyly to see just how good I am. I do know how to show up just in time, looking casual and relaxed, sextant in hand. Ready to do the morning stars. If nothing else, showing up on time — especially when taking star sights at dawn — creates the appearance that maybe ‘this guy knows what he’s doing.’

To ensure this illusion, I did my work the previous night. Knowing the speed and the course of the schooner Virginia, I calculated the dead reckoning (DR) for the time of nautical twilight, give or take a few minutes since spot-on precision isn’t necessary for this exercise. I got my time and then I went into the Nautical Almanac and calculated the local hour angle of Aries based on the time of nautical twilight. Then, I took out my Rude Star Finder, shook off the dust, put on the disk corresponding to my latitude and set it to the LHA. I was good to go. I just hoped that I had done it correctly!

Sure enough, when I got on deck the sky was beginning to lighten and the horizon was showing as a faint line. I took my position, jammed in the main shrouds, cradled my beautiful old Plath, checked my watch, and began that beautiful, magical process of bringing a speck of light down to the horizon. It was great! Other than the fact that I now need glasses in order to read the numbers off the sextant arc and that my German sextant felt like it weighed 20 pounds, my stars were just where I had hoped. The altitudes were close to what the star finder predicted.

I shot Arcturus, Alphard, and Gienah, trisecting the sky so that I could have good cuts should my sights prove to be good. I checked the index error by using a star instead of the horizon, and double checked my watch time. Satisfied with my sights, it was time to go below, crunch the dreaded numbers, and hope the cook had put out the morning coffee. All in all, a good beginning!

My DR position was 32° N by 64° 27’ W on Nov. 26. For this problem we will use the 2010 Nautical Almanac and H.O. 249 Vol. 1 for Arcturus and Alphard and Vol. II for Gienah. The height of eye is 16 feet. There is no watch error and no sextant error.

 

A. Calculate the time for nautical twilight at the DR.

Next, reduce the three star sights.

B. Arcturus at 10:01:15 GMT. The Hs was 32° 35.8’.

C. Alphard was shot at 10:02:20 with an Hs of 48° 28.3’.

D. Gienah  at 10:03:30 and the Hs was 31° 48.8’

Find the Ho for each sight.

E. Reduce and plot the three star fix and find the estimated position (EP).

 

Answers:

 

A. Nautical twilight at 9:57:48 GMT

B. Arcturus Ho: 32° 30.4

C. Alphard Ho: 48° 23.5

D. Gienah Ho: 31° 42.3

E. EP: 31° 50’ N by 64° 24’ W

Edit Module

Old to new | New to old
Nov 12, 2012 03:24 pm
 Posted by  Ocean Navigator

Solution part I

Three star fix aboard schooner Virginia. DR: 32°N by 64°27’ W on Nov. 26. Use 2010 Nautical Almanac. HE: 16 feet. No watch or sextant error. Shooting morning stars so first calculate time of Nautical Twilight at our DR:

I’m going to interpolate for lat. and long. At 30° time of Nautical Twilight is 5hrs 38 min. At 35° the time is 5hrs 45 min. A 7 minute difference in the 5 degrees of latitude so take 2/5x7 and get about 2 minutes which add to time of 5hrs 38 min and get time of 5hrs 40 min at latitude 32°.
Next find how long it takes sun to travel 64°27’ (long.). It will take 4hrs 16 minutes.

Nautical Twilight at 32° (LMT) 5 hrs 40 min
+ DR Long. 4hrs 16 min
Nautical Twilight 9hrs 57 min 48 sec

Reduce the sights. Use HO 249 Vol 1.

Arcturus: Time of shot 10:01:15 Hs 32°35.8’
GHA Aires 10 hours 215°13,8’
O1:15 18.8’ Declination N 19°07.7’
GHA Aires 215°32.6’
+SHA Star 145°57.7’
GHA Star 360°30.3’
-Ass long 64°30.3’
LHA Aires 297°

Hs 32° 35.8’
-dip 3.9’
-3rd corr 1.5’
Ho 32°30.4’

Into HO 249 under assumed latitude 32°

Hc 32°27’ d +26’ Z 087°
Table 5 corr + 3’
Hc 32°30’
Ho 32°30.4’
Intercept .4 nm Toward

Nov 12, 2012 03:25 pm
 Posted by  Ocean Navigator

Solution part II


Alphard @ 10:02:20. Hs 48°28.3’
GHA 10 hrs 215°13.8’
2min 20 sec. + 35.1’
GHA Aires 215°48.9’
+ SHA Star 217°58.4’ Declination S 8°42.3’
GHA Star 433° 47.3’
-360°
GHA Star 73°47.3’
- Ass Long 64°47.3’
LHA Aires 9°

Hs 48°28.3
-dip 3.9’
-3rd .9’
Ho 48°23.5’


HO 249

Hc 49°05’ d – 59’ Z to Zn 194°
Table 5 -41’
Hc 48°24’
HO 48°23.5’
Intercept .5 nm Away

Gienah @ 10:03:30 Hs 31°48.8’

GHA Aires 10 hrs 215°13.8’
03:30+ 52.6’
GHA Aires 216°06.4’ Declination S 17°36.1’
+ SHA Star 175°54.7’
GHA Star 392°01.1’
-Ass. Long 64°01.1’
LHA Aires 328°

Hs 31° 48.8’
-dip 3.9’
-3rd 1.6’
Ho 31°42.3’

HO 249

Hc 32°12’ d -51’ Z 143°
Table 5- 31’
Hc 31°41’
Ho 31°42.3’
Intercept 1.3 nm Toward

EP 31°50’N by 64°24’W

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