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QUESTION-TYPE BASED TESTS
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TEST 3 – Timekeeper: Invention of Marine Chronometer
A.
Up to the middle of the 18th century, the navigators were still unable to exactly identify the
position at sea, so they might face a great number of risks such as the shipwreck or running out of supplies
before arriving at the destination. Knowing one’s position on the earth requires two simple but essential
coordinates, one of which is the longitude.
B.
The longitude is a term that can be used to measure the distance that one has covered from one’s
home to another place around the world without the limitations of naturally occurring baseline like the
equator. To determine longitude, navigators had no choice but to measure the angle with the naval sextant
between Moon centre and a specific star lunar distance along with the height of both heavenly bodies.
Together with the nautical almanac, Greenwich Mean Time (GMT) was determined, which could be adopted
to calculate longitude because one hour in GMT means 15-degree longitude. Unfortunately, this approach
laid great reliance on the weather conditions, which brought great inconvenience to the crew members.
Therefore, another method was proposed, that is, the time difference between the home time and the local
time served for the measurement. Theoretically, knowing the longitude position was quite simple, even for
the people in the middle of the sea with no land in sight. The key element for calculating the distance
travelled was to know, at the very moment, the accurate home time. But the greatest problem is: how can a
sailor know the home time at sea?
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