When he was 18 years old, Scottish-born Chay Blyth enlisted in the British Army paratroopers. Paratrooper units are clite formations, and one has to be tough and determined to excel. This Blyth did, becoming a sergeant as a 21-year-old. Blyth took every oppertunity to advance. He first came onto the world stage when he and Captain John Ridgeway rowed across the North Atlantic in a 20-foot open boat making the passage in 92 days. For this feat Blyth was awarded the British Empire medal.
In 1968, with no sailing experience, Blyth entered into the first non-stop Golden Glove Around the World Race in a 30-foot production boat. This was the race won by Robin Knox-Johnston. Blyth didn’t didn’t fair as well, leaving the race after rounding the Cape of Good Hope.
Far from being deterred Blyth seized the next opportunity, and in 1971 sailed around the world single-handed non-stop westerly against the prevailing winds – the first to achieve this.
His appetite whetted, in 1973 he entered into the first Whitbread Around the world Race. For crew he took along an assembly of paratroopers (most didn’t know how to sail) on the yacht Great Britain II and took line honors. He was knighted in 1997.
Even though that race took place only three decades ago, the changes in technology since then have been rapid and sophisticated. Navigation, for instance, was still done by celestial, and yachtsmen depended on their skills with a sextant to find their way around the world. One went to sea with an accurate chronometer that had been rated, a set of sight reduction tables, a nautical almanac, plotting sheets and a good sextant.
Blyth was a good navigator, constantly taking star sights, when he could, as well as the standard noon sights of the sun. In this example of Blyth’s navigational ability I have him doing a Polarius shot at evening twilight. For information concerning the Whitbread Race see Blyth’s book about the race, Theirs Is the Glory.
We are aboard Great Britain II on September 18. We are sailing toward Cape Town and our DR position is 12° 15;N by 48° 25’W. The height of the eye is 10 feet and there is no Index Error. All times are in GMT and we are consulting the 2007 Nautical Almanac. The Hs of Polaris at the time of the sight is 11° 56.6′. The time of the sight, when it is dark enough to see Polaris, is at 21:32:55.
Note: There is a table at the rear of HO249 for finding latitude from Polaris but I use the more accurate method in the rear of the Nautical Almanac.
A. Find out what time Civil Twilight is for the DR longitude
B. What is LHA Aires?
C. Calculate Ho and thus latitude from Hs?
Answers
A. Civil Twilight is at 21:33:40
B. LHA aries is 272°
C. Latitude is 12° 15.2
EXTENDED ANSWER:
By David Berson
The Nautical Almanac is full of so much information that it takes practice to recall how to use the all the tables correctly. The Polaris sight forces us to review the methodology of calculating twilight times and of finding latitude by using Polaris.
First off we go to the daily pages of September 18 and find that the time for Civil twilight is at Greenwich. The time is 18:20 L2MT. There is a slight catch though. The times given are for the central meridian of the time zone, so unless we are at 0°, 15°, 30°, etc. we have to calculate the time of twilight for our position. Our DR states that we are at 48° 25’ west.
We go to the rear of the Nautical Almanac to the Conversion of Arc to Time Table and find that it takes the sun 3 hours 13 minutes and 40 seconds to arrive at our position. We add this number to the time given in the tables and come up with our time for civil twilight at our DR.
18:20
+3:13:40
21:33:40 GMT time of Civil Twilight.
We use civil twilight by the way because that calculates when the center of the sun is 6° below the horizon. Just getting dark. This time doesn’t have to be spot on. Most navigators are up at the time of evening twilight and it just a matter really of waiting until you can see some stars using the sextant. The morning twilight time is far more important because the navigator wants to stay in the rack as long as possible.
As you can see by this shot the actual time of the shot was just a tad before the time of calculated civil twilight. This was due to the probable face that the vessel was a little more to the east than the DR calculated.. The time of the actual sight is at 21:32:55. We first have to calculate the LHA of Aires. We do this by going to the daily pages and looking under the Aires column. We find the following
GHA Aires 21 hours 312° 24.0
+ 32 min,55 sec 8° 15.1
GHA Aires 320° 39.1
-ass long 48° 39.1
LHA Aires 272°
Remember to find LHA we use assumed longitude based on DR longitude. We need a whole number of degrees.
Next we reduce the HS to latitude. The first few steps are familiar. We go as far as the 3rd correction before we enter the Polaris tables.
Hs 11° 56.6
-dip 3.1
11° 53.5
3rd corr- 4.5
Ha 11° 49.0
This is standard procedure just remember to use the correction table at the front the Nautical Almanac for stars and planets. Next we go to the Polaris tables at page 276.
These tables are self-evident. The formula is given to find latitude. It is -1° +ao +a1+a2. Thus we have the following:
Ha 11° 49.0
– 1°
+a0 1° 24.9
+a1 .4
+a2 .9
Latitude 12° 15.2
If you want you can see the azimuth at the bottom table which for this case will be 0°.6’
