Finding Your Way at Sea

The transoceanic voyages of the Age of Discovery, from the 16th century onward, led to new demands in the field of navigation.

In meeting these new challenges, the art of navigation underwent a series of profound changes.

Francisco Albo, born on the Greek island Chios, but living in Rhodes, was part of the 1519 expedition. He began the voyage as boatswain of the carrack Trinidad, but had become pilot of the Victoria by the time the expedition returned in 1522.

He used his logbook, a brief, technical journal, to record navigational data gathered on the voyage on a daily basis. This voyage was to be the first to go all the way around the world. It is thanks to Albo that it is known which techniques they used to find their way at sea, as will be shown below.

The fleet's Expenses List, now held in Seville's Archivo General de Indias, is an accurate record of the navigational instruments carried on board the expedition's five carracks (sailing ships):

Wooden quadrants - 21
Metal astrolabes - 6
Stick astrolabe - 1
Ship's compasses - 35
Large boxes - 4 for four compasses
Hourglasses - 18

Ship's Compass

An indispensable navigational instrument; essential for orientation and dead reckoning. They were used for navigation in the Mediterranean Sea, and later on ships sailing across the Atlantic.

Originally, they consisted of a basic design of a needle on a float in a bowl of water. Later designs included a circle with graduated markings (compass rose), a wooden housing, and a system to ensure that it remained horizontal. This system was designed by Gerolamo Cardano, after whom it was named.

Compass Rose

A graduated circle inside a compass that shows the orientation of the cardinal points.

The cardinal points were originally named after the winds of the Mediterranean Sea. This changed when people began to sail across the Atlantic, with the circle of the horizon divided into 32 equidistant directions or quarters measuring 11.250 degrees. This degree of accuracy was sufficient for the needs of the time.

As we know, a compass does not point to the geographic north pole, but to the earth's magnetic north pole. At sea, it was essential for sailors to be able to correct this magnetic anomaly, otherwise known as the deviation between magnetic north and the compass needle.

The usual way of doing this was to compare the alignment of the compass needle with the geographic north pole, either by identifying the Pole Star, or by observing the shadow cast by the sun on the compass rose at noon.

Albo occasionally mentioned that the needle "declined to the north east."

Hourglass (Marine Sandglass)

This governed life at sea and watchkeeping shifts. It was adjusted at midday, at the sun-meridian transit time. The days were governed by these clocks.

The most common type lasted 30 minutes, and cabin boys and servant boys were responsible for turning them when the sand had stopped flowing. This often resulted in mistakes being made, either through a lapse in concentration or due to tiredness.

Nautical Quadrant

An instrument used for measuring the altitude of a star above the horizon.
It consists of a quarter-circle panel, usually made of wood, with graduated markings from 00 to 900. Two metal sight pinholes lie along one edge. A small plumb bob hangs from the apex.

Held vertically, the sight pinholes would be held up in the direction of the star. The reading would be obtained by looking at the position of the plumb bob against the graduations on the arc.

Astronomical Astrolabe

The word astrolabe means star taker, and its origins lie in classical and Islamic cosmology.
It is an ingenious calculator based on stereographic projection of the celestial sphere, which calculates celestial coordinates.

They were usually made of brass and were elaborately decorated. The main parts are the mater, the rim, the tympan, the back, and the rete.

Designed to be used in astronomy (and even astrology), they were not generally used in navigation.

Mariner's Astrolabe

This instrument is much simpler than an astronomical astrolabe. It was used for calculating the altitude of stars above the horizon.

It is essentially a graduated bronze or brass circle (some were also made of wood) with the center cut out. It has a rotating alidade with pinhole sights.

To view the sun, the instrument was held aloft and the alidade rotated until the sun's ray passing through the upper pinhole aligned with the lower one.

Dead Reckoning Position

Until it became possible to use astronomical observations, a type of navigation based on estimations of the distance traveled and the course was used. The position of the ship, calculated using this method, is known as the dead reckoning position.

Needless to say, deviations of the compass needle, variations in the course of a sailing ship, and errors when measuring speed led to variable results.

Interestingly, the Spanish name for the dead reckoning position is called fantasy position, which gives an indication of how accurate (or otherwise) this method was considered to be at the time.

Estimated Position

From the 15th century, a new method of navigation based on observations of the stars began to be used. This allowed sailors to calculate latitude using the altitude of the Pole Star or the sun's meridian.

The dead reckoning position, with the latitude corrected, was known as the estimated position.

Albo's logbook shows that this was the main method used on the first circumnavigation of the world.

Regiments of the Sun

The passing of the sun over the meridian of a particular place at noon (meridian) is the ideal time to calculate latitude.

Once the altitude of the sun has been noted, the calculation is no more than a simple exercise in addition and subtraction. In summary, it can be said that latitude is the result of subtracting the declination from the distance of the zenith, taking into account the sign of both.

In order to carry out this calculation, a series of mnemonics and tables were published in texts and manuals known as Regiments of the Sun.

Francisco Albo's logbook includes 158 examples of calculations of latitude using the meridian altitude of the sun. His very first entry notes the method used for calculating latitude.

Year 1519:

"Tuesday, 29th day of November, I began to take the altitude of the sun whilst following the said voyage ….

Wednesday, 30th of said month, I took the sun in 76°, and its declination was 22° 59′, and its polar altitude was 8° 59′, and the course was S.S.W."

Weighing the Sun

Measuring the altitude of the sun using an astrolabe was known as Weighing the sun.. As described above, to calculate latitude at noon, we need to know the altitude of the sun above the horizon and the corresponding declination of the sun.

A number of different instruments were used to measure its altitude, and the declination was obtained using astronomical tables.

Astronomical Tables

As we have seen, to calculate latitude using the meridian of the sun, we need to know the solar declination.

Put simply, this is the height in degrees of the sun over the celestial equator. Its value varies constantly between +-230 27´, depending on its position along the ecliptic.
This coordinate has been recorded in astronomical treatises since time immemorial.

Adapted versions of these tables were published for use in navigation.

The best known of these was Almanach perpetuum coelestium motuum (Perpetual Almanac). It was written by Abraham Zacuto, a Salamanca-born Jew who sought refuge in Portugal following the expulsion of the Jews from Spain in 1492.

Written in Hebrew, it was later translated into Latin by his disciple, José Vizinho. It may have been the basis of most of the astronomical tables written at that time.

There is a direct link between time and longitude.

The Earth turns at an average speed of 360° per day, with an average solar day lasting 24 hours (a sidereal day = 23 h 56 m 4 s).

Therefore, the difference between the time at the point of departure and that on board ship, expressed in degrees, gives the difference in longitude (East / West).

Finding out the time while at sea was relatively simple. However, ultimately the problem was knowing what the time was at the point of departure. This issue was not resolved until the mid-18th century, with the invention of the marine chronometer.

The main objective of the 1519 expedition was to find an alternative route to the Maluku Islands.

This meant a route other than the one used by the Portuguese. They sought a route that led to the east through the west, and, in particular, to show that they were traveling through the territory assigned to the Crown of Castile in the Treaty of Tordesillas.

It was therefore essential that they knew the geographical longitude of the Maluku Islands. The person given this task was Rui Faleiro, a cosmographer and colleague of Magellan.

In the end, Rui Faleiro was dismissed from the expedition, although he was required to present his Regimento da altura leste oeste (Rules for Finding Longitude) .

This volume included several methods for calculating longitude, which is the height from East to West, "from the latitude of the Moon, from the conjunction of the Moon with set positions of stars, and from the Sun and the variation of the compass needle."

The Sevillian Andrés de San Martín took the place of Rui Faleiro, in the post of Royal Pilot on board the San Antonio.

Andrés de San Martín carried out various astronomical observations on dry land to determine the geographical longitude of specific, significant points on the journey. He took the meridian in Seville as his reference point.

The results of his measurements were mixed, and he himself dismissed some of them.

Andrés de San Martín died in the ambush at Cebu in May 1521. His journal fell into Portuguese hands when the Trinidad was captured, and unfortunately did not survive.

Some of his work merits a special mention; in particular the events of April 17, 1520 in Port St. Julian (Argentina), where he was able to take advantage of an astronomical phenomenon: a solar eclipse.

The distance that he calculated to the meridian in Seville was 610. Since the actual measurement is 610 43´, this was an extraordinarily accurate result given the capabilities of the period.

It is not clear which method he used, since the eclipse could not be seen from Europe and was therefore not recorded, either in Zacuto's almanac or in Regiomontanus' Ephemerides.

Was it Wednesday or Thursday?

On July 9, 1522, the Victoria reached Ribeira Grande (Ciudade Velha) on the Island of Santiago, Cape Verde. Surprised, Albo wrote in his logbook:

“and this day was Wednesday, and they reckoned this day as Thursday, and so I believe that we had made a mistake of a day.”

This was the first record of the change in the date that occurs on traveling all the way around the globe. Since the expedition had traveled west, they needed to adjust the date forward by one day.

Francisco Albo wrote his last entry in his logbook on catching sight of the long-awaited Cape St Vincent, on their return journey.

“On the 4th of the said month, in the morning, we saw land, and it was Cape St. Vincent, and it was to the north-east of us. And so we changed our course to the E.S.E., to get away from that Cape."

They reached Sanlúcar de Barrameda scarcely two days later, having been away for 14 days short of three years. According to their own accounts, they had traveled 14,000 leagues and had“traveled around the entire circumference of the world, setting off to the west and returning from the east."