navigate
navigate
導航；確定...的方向
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
protrusion
突出物；凸起
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
sundial
日晷
casts a longer shadow as it is moved farther north of theequator
, and this information was used by sailors from that time to get an idea of how far north
of the equatorequator
赤道
a ship was. All of these methods provided only very rudimentary
rudimentary
基本的；粗淺的
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
premise
假設；前提
of a compass is that the magnetized
magnetize
使磁化
needle of a compass, when it is balanced on a central pivot
pivot
支點
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
latitude
緯度.
With the invention of the astrolabe
astrolabe
星盤（天文儀器）.
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
rotate
旋轉
in the circle; the rule could be aligned
align
使成一條直線
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
sextant
六星儀（天文儀器）
, 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
longitude
經度
, or how far east or west a ship had traveled. 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. In 1759, English clockmaker John Harrison built a chronometer that was
accurate enough for navigation. 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. Harrison was easily
able to convince Parliament that the reward was warranted.
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