The modern reader of the 21st century will find it Hard to understand the problems a sea captain of the 17th and 18th- century faced in navigating his wooden ship through relatively unknown waters. It is fascinating to look at what they accomplished with the tools they had at their disposal. They regularly sailed across the oceans from one port to another, much as we travel from one city to another today. What were these tools the mariners used to find their way?

Since the 15th century mariners have adapted the tools used by astronomers. One of the first was the Astrolabe. This tool was probably invented by the Greeks in ancient times and means, "start taker". (Fisher, p. 22) It is a circular device, with the degrees marked off around its circumference and a pointer with sighting vanes on it that rotates from the center of the circle. In use, the Astrolabe is hung vertically by a ring held in the hand. The stars are sighted by rotating the pointer until the star is seen through both sights. The degree marked by the pointer on the arc could then be converted into the degree of latitude of where the observer was located. This tool could also be used to find the heights of mountains, buildings, etc.

By laying the Astrolabe flat, instead of hanging it verically, an observer could also find the azimuth, the bearing from true north, of a celestial object. This was important in establish- ing lines of position to a navigator. The major change mariners made to adapt the Astrolabe to their use was to use two degree arcs on either side of the hanging ring. Each had its 0-degree mark on the horizon or horizontal base and the 90 degree mark (zenith) at the hanging ring. This allowed the altitude to be read directly from the Astrolabe. It also made it easier to read for both right- and left-handed observers.

It was very difficult, however, to get accurate sightings with the Astrolabe from the rolling deck of ship at sea. A mariners Astrolabe was therefore often made small in diameter and out of a heavy material to try and counteract the motion on board. They were also made with open work to help counter the deflection of the wind. Unfortunately,though, the accuracy of the Astrolabe is in direct proportion to its size (Fisher, p.25). "Joao de Barros, the historian, reported that when Vasco da Gama reached the bay of St. Helena on his first voyage round the Cape of Good Hope in 1497, he went ashore and set up a large wooden astrolabe to get his bearings. He had been unable to get a trustworthy meridian altitude of the sun from the deck of his ship with his portable instrument." (Brown, p.181). It often took three men to take a sight, one holding the instrument and bracing himself is much as possible, one to sight the star through the pointer, and one to read the altitude. (Brown, p.182) Despite these difficulties the Astrolabe continued to be used by mariners well into the 18th-century.

The cross-staff was another tool adapted from astronomers by mariners for navigation use. It was made of a long staff, usually 36 inches long, and a shorter sliding cross piece, that moves up and down the longer staff. In use the long staff was held up to the eye and the cross piece moved along the staff until it fills the area between the star and the horizon. A scale was engraved on the side of the long staff to show the angular measurement. This was a much simpler device than the Astrolabe and could be used by one person. It was still in- accurate, though, because the observer had to look at two objects at the same time (ocular parallax), and it, at times, forced the observer to look directly at the sun when taking a noon sight.

Another instrument introduced at about the same time as the Astrolabe and cross-staff was the quadrant. The quadrant was nothing more than a quarter circle with its arc marked from zero degrees to 90 degrees. It had a plumb bob that hung from its center radius (apex). There was also a set of sights along one side (radial line) of the instrument. The observer sighted the star or other object through the sights and where the plumb bob cord cut the degree arc, that was the altitude which like the other instruments mentioned could then be converted to latitude. If the North Star was used for the sightings, the altitude was then your latitude, but if the sun was used, allowance had to be made to seasonal changes with the sun's movement. Tables found in a nautical almanac would help the mariner make these adjustments. The quadrant shared the same problems with the Astrolabe, in that it was difficult to take a sight on the moving deck of a ship and if the observer was taking a sun sighting, it meant looking directly at the sun. Its accuracy again was also directly proportional to its size.

Most quadrants made had a 10 to 12 inch radius. The following quotes are interesting in that they addressed the size problem. Basal Ringrose, was the surgeon and acting navigator, aboard captain Bartholomew Sharp's ship in 1680. They were on a three- year voyage around South America, to attack and pillage the Spanish settlements and shipping from 1680 to 1683. He wrote in his journal, while in the Pacific Ocean, on August 21st, 1680: "... that day I finished two quadrants, each of which were two feet and a half radius"(Esquemeling, p. 355). Another reference to the size of quadrants used with respect to accuracy is the following quote from Thomas Clarke, a surveyor to the Massachusetts Bay government in November of 1672: "... observed with a large quadrant, with the approbation of Mr. Whiswall, who is well skilled in mathematics; and is, to my best skill and judgment, our east line from the above said Island. (Clap- board island in Casco Bay). If the honored court were pleased to go twenty minutes more northerly in the Merrimack River, it would take in all the inhabitants and places east along, and they seemed much to desire it" (Chandler, p. 189). To explain further, the settlers in the county of Cornwall, Maine, had petitioned the Massachusetts Bay government to be included under the Mass Bay Colony jurisdiction, so the Massachusetts Bay government resurveyed their grant with a larger, i.e. more accurate quadrant and found they could stretch their bounds a little to include these settlements, even though these settle- ments were considered to be part of New France by a treaty between England and France. Fudging the figures "twenty minutes," in this case, to suit one's purposes is nothing new.

Around the beginning of the 1600s an English seaman named John Davis, invented an instrument called the back staff. With this instrument it was possible to take sights of the sun much easier and more accurately. The back staff had three vanes, a horizon vane, a shadow vane, and the sight vane. Two arcs were attached to a staff and the arcs are were marked off in degrees. The radius of the two arcs have the same center point which was fixed at the horizon vane. The sight arc was the larger arc and was marked from 0 degrees to 30 degrees. It was located at the opposite end of the staff from the horizon vane. The shadow arc was smaller and measured from 0 degrees to 60 degrees. It was located above the horizon vane. These two arcs added together yield the maximum altitude of 90 degrees.

The observer stood with his back to the sun and moved the shadow vane and the sight vane until the sun's shadow was brought in direct line with the horizon. This also eliminated looking at two objects at the same time (ocular parallax). With this instrument a mariner no longer had to be satisfied with a degree of accuracy, instead he could find his position at sea within a minute of the degree, which would translate into one sea mile (Fisher, p.76). The backstaff became one of the primary tools of the mariners in determining latitude until well into the 18th-century.

The last instrument for determining latitude to be discussed here is the octant, which was developed in 1732. John Hadley, an Englishman is credited with the invention. The octant is referred to as a double reflecting instrument. It uses two mirrors to bring a star's or sun's reflection down level with the horizon. Like the back staff this eliminated the ocular parallax, but with the octant you didn't have your back to the sun and it could also be used to take a sight on the stars (the back staff with a shadow vane could not be used for this purpose).

The octant works on the principal that when the observer views an object in the mirror, the angle that the object makes with the mirror is a same as the angle the mirror makes to the observer's eye. The angle of reflection equals the angle of incidence. The ease in using this instrument as well as its accuracy, made it very popular with mariners and it quickly replaced the back staff with all but the most conservative captains.

All the instruments discussed so far, were used by a mariner to calculate latitude, the distance north or south of the equator. Determining longitude, the distance traveled east or west of a point, in Colonial times was another problem altogether. This required the mariner to know the time it was at the port he departed from and the time it was at his present position. For this he needed an accurate time piece set at the time of the port of his departure. Adding or subtracting the hours and minutes differing from his present time, he could then calculated the distance in miles from the port (longitude).

John Harrison, an Englishman, developed such a time piece in 1735. Yet it was still being refined, developed, and tested well into the late 1700s, so for the Colonial navigator there was little chance of using one. What most ship captains did instead was to use what was called "deduced reckoning," shortened to "ded reckoning," that being corrupted to "dead reckoning." This involved keeping track of the time traveled (time was clocked on-board ship with a half-hour glass, in four hour watchs), direction of travel, and the speed of travel (calculated with a log line). All these were recorded in the log book each day. By plotting this information on a chart or map, it would give the ship's captain a pretty good idea of where his ship was. Using this method along with taking sites to determine latitude, a mariner could set his course until he reached the latitude of his destination and then travel East or West on that course until he made his land fall.

This is just a brief look at the tools use by mariners of the Colonial times to sail safely from one destination to another.