NAVIGATIONAL DEVICES
From the earliest of times, sailors have found ways to navigate their ships on the seas and oceans of the world. The earliest sailors navigated by simply following the coastline. Aside from being a rather slow method of navigating, this method was also rather dangerous and limited. It was dangerous in that waters close to the shoreline could be shallow enough to strand a ship or the waters could be full of rocky protrusions capable of sending ships to their graves. When seafarers began sailing out of sight of land more than 4,000 years ago, they used the stars to determine their direction. They calculated the distance traveled from their speed and sailing time, and they drew rough charts and maps to find their way and to exchange information about navigational routes with others. It had also been known as early as 300 B.C. that a sundial casts a longer shadow as it is moved farther north of the equator, and this information was used by sailors from that time to get an idea of how far north of the equator a ship was. All of these methods provided only very rudimentary means of navigating.
It was not until more than 3,000 years after sailors set out on the seas that the compass was developed. The premise of a compass is that the magnetized needle of a compass, when it is balanced on a central pivot or left to float on liquid, will always turn to point in the direction of magnetic north. Navigators on Chinese ships were the first ones who were known to use compasses to determine the direction their ships were heading, as early as 1100.
Numerous inventions were created to determine a ship’s latitude. With the invention of the astrolabe in the fourteenth century, sailors were able to measure the Sun’s height with better accuracy than with a sundial, and they were able to use the information provided by the astrolabe to determine how far north of the equator they were. An astrolabe was a metal circle with a sighting rule that rotated in the circle; the rule could be aligned with the Sun, and measurements on the ring indicated the Sun’s height. Other devices followed that were better able to determine the Sun’s height and thus provide an idea of the ship’s latitude. The backstaff, invented in 1595, and the sextant, invented in 1757, were devices that each improved a navigator’s ability to determine latitude. The backstaff was a device that required the navigator to face away from the Sun and make a calculation of the shadow in relation to the horizon. The sextant was a measuring device that required the navigator to look into an eyepiece and calculate the Sun’s position relative to the horizon and then check printed tables to convert this information into latitude.
The missing piece in the navigational puzzle was the ability to calculate longitude, or how far east or west a ship had traveled. (1) The need for a device to calculate longitude was so important to navigators that the English Parliament offered a reward of 20,000 pounds (an extraordinarily large sum at the time, perhaps $10 million in today’s currency) to anyone who could invent a method for calculating longitude. (2) In 1759, English clockmaker John Harrison built a chronometer that was accurate enough for navigation. (3) The premise of the device was that the Sun rises two seconds later each day for each kilometer traveled in a westerly direction, so the accurately measured change in time was an accurate way to calculate longitude. (4) Harrison was easily able to convince Parliament that the reward was warranted.