Longitude and the Sea Clock
Written by Studio AM.
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For centuries, sailors could estimate how far north or south they were by watching the Sun and stars. Finding longitude, their east-west position, was harder. A navigator needed to compare local noon with the time at a known reference place. Every hour of difference represented part of Earth’s daily turn.
The idea required a clock that kept accurate time during a long voyage. Pendulum clocks worked on land, but a rolling ship and changing temperature disturbed them. In eighteenth-century Britain, carpenter and clockmaker John Harrison spent decades developing sea clocks. He reduced friction, allowed parts to adjust for temperature, and eventually built a watchlike timekeeper that performed well at sea.
Harrison was not the only person studying longitude, and astronomy remained another important method. Yet reliable marine timekeepers gave navigators a practical new tool. By carrying reference time across the ocean, a clock could help turn two observations into a position on a chart.
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Question 1 of 4
What is the main idea of the passage?
The answer is B: Reliable sea clocks helped navigators find longitude by preserving a reference time during voyages.
The passage connects the longitude problem with Harrison’s durable timekeepers and the comparison of local noon to reference time.
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Question 2 of 4
Why could a difference between local and reference time reveal longitude?
The answer is C: Different east-west positions reach local noon at different points in Earth’s turn.
Because Earth turns, local noon occurs at different times from place to place; comparing those times indicates east-west separation.
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Question 3 of 4
What does “friction” mean in the clockmaker’s work?
The answer is D: Resistance produced when moving parts rub against one another
Harrison reduced friction while improving a mechanism, so the word refers to resistance between the clock’s moving surfaces.
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Question 4 of 4
What two conditions disturbed pendulum clocks at sea?
The answer is A: A rolling ship and changing temperature
The second paragraph directly identifies a ship’s rolling motion and temperature changes as problems for pendulum clocks.
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