Sailing Alone Around the World
One of my favorite books is “Sailing Alone Around the World ” by Canadian born Joshua Slocum. On July 2nd 1895 he left the port of Yarmouth Nova Scotia and left North American behind (Ref.1/p.23). In his book he writes: “On September 25, in the latitude of 5degN, longitude 26deg 30′ W, I spoke the ship North Star of London. The great ship was out forty-eight days from Norfolk, Virginia, and was bound for Rio, where we met again about two months later”.
Let’s look at Celestial Navigation to help us understand the Spray’s whereabouts on September 25th 1895. Captain Slocum was not a fan of chronometers, “At Yarmouth, too, I got my famous tin clock, the only timepiece I carried on the whole voyage” (Ref.1/p.22). First let’s assume that Capt Slocum had accurate time and he performed a Meridian Passage on the Sun, let’s try to figure out what his sextant reading was. First let’s use the current Nautical Almanac for 2021 (Ref.3). We need to interpolate to determine the GHA & DEC for when the Sun is directly over 26deg 30’W (Figure 2):
1300UTC: GHA=17deg 6.6′
1400UTC: GHA= 32deg 6.8′
delta = 9deg 23.4′
MP = GHA = 26deg 30′ & DEC = S01deg 4.5′ & UTC = 13hrs 37.55min
Figure 3 shows the geometry of the position. The Spray’s Zenith distance is equal to the latitude north of 5deg + the Sun’s dec south = 6deg 4.5min. Thus Ho = 83deg 55.5min.
Assuming a lower limb reading & height of eye = 4m
Ho = Ha -R + PA +/-SD
R = 1/tan(H + 7.32/(H+4.32)) = 0.1min
SD = 15.9min
Dip = 3.5min
Ha = 83deg 55.5min + 0.1min – 15.9min = 83deg 39.7min
Ha = Hs +/- SxtntEr +/-IndxEr – Dip
Hs = 83deg 39.7min + 3.5min = 83deg 43.2min
Back In Time
Capt Slocum claims he used a tin clock, but he was an experienced sailor and had extensive navigational experience. He had this to say about time: “To find local time is a simple matter. The difference between local and standard time is longitude expressed in time – four minutes, we all know, representing one degree. This briefly is the principle on which longitude is found independent of chronometers. The work of the lunarian, though seldom practised in these days of chronometers, is beautifully edifying, and there is nothing in the realm of navigation that lift’s one’s heart up more in adoration” (Ref.1/p149). In 1895, the navigational reference was the “American Ephemeris & Nautical Almanac”. Let’s go back in time and see what data was available. The closest that is available online is the 1900 edition (Ref.4).
#1. – “Sailing Alone Around the World”, Joshua Slocum
#2. – “Joshua Slocum”, Wikipedia
#3. – “Nautical Almanac”
#4. – “The American Ephemeris and Nautical Almanac for the year 1900”, Washington Bureau of Equipment 1899