the angles between the horizon and the Sun during the day and the Pole Star
1
at
night, they were able to find their north-south direction – their latitude.
2
The location of the stars in the sky appeared to be the same each season, and so they could
be used as reliable reference points to calculate someone’s position at sea. Over
the centuries, celestial maps became fundamental tools for navigation.
On the other hand, the measure of longitude – east-west movement – was so
complicated that it remained an enigma with no solution. The ancient Greeks
understood that if they could measure time in two different places at the same
moment, they could calculate the distance between the two and therefore the position.
3
Yet despite this insight, they never managed to build the right tool to put the
theory into practice. The invention of the marine clock, the tool that would one
day solve the “longitude problem”, was still many centuries away.
Over time, finding the solution to the problem became a geopolitical imperative, as no nation could dominate the world without maritime supremacy, and the
lack of a solution continued to cause tragedy after tragedy at sea. In 1707, Admiral
Sir Cloudesley Shovell fatally misjudged the position of the British fleet while
returning from the Battle of Gibraltar. Due to his error, all of the ships crashed
against the Isles of Scilly to the southwest of England, with over 2,000 casualties.
In light of this accident, the London Parliament decided that the longitude problem had to be resolved for good and set up a special committee to award a £20,000
prize – several million dollars today – to anyone who made a precision marine
clock. Sir Isaac Newton said before the committee that “such a clock has not yet
been designed or built”.
4
In her best-selling book Longitude: The True Story of a Lone Genius Who
Solved the Greatest Scientific Problem of His Time,
5
Dava Sobel tells how a poor
English carpenter, John Harrison, built in 1761 the first such clock. He had already
made a wooden pendulum at the age of 20 and later improved his design to the
1
Budlong, John. 1978. Sky and Sextant: Practical Celestial Navigation.
2
Idem
3
The hours indicated simultaneously by two clocks at two different sites make it possible to
calculate the respective geographical distance. Since the Earth rotates 360 degrees in 24 hours,
one hour equals 15 degrees (one 24
th
of a turn), so the one-hour difference between a ship’s
position and its starting point indicates that it has travelled a distance such as to have covered
fifteen degrees of longitude to the east or to the west depending on the direction of departure.
At the equator, where the circumference of the Earth is at its largest, fifteen degrees correspond
to one thousand nautical miles. As we move away from latitude 0, the value of each degree
measured in miles decreases, approaching zero at the poles, and is calculated with complex
trigonometric formulas already elaborated in ancient times.
4
Idem, p. 121.
5
Sobel, Dava. 1995. Longitude: The True Story of a Lone Genius Who Solved the Greatest
Scientific Problem of his Time. London: Walker Publisher.
52 GPS: The Stellar Clock
1
at
night, they were able to find their north-south direction – their latitude.
2
The location of the stars in the sky appeared to be the same each season, and so they could
be used as reliable reference points to calculate someone’s position at sea. Over
the centuries, celestial maps became fundamental tools for navigation.
On the other hand, the measure of longitude – east-west movement – was so
complicated that it remained an enigma with no solution. The ancient Greeks
understood that if they could measure time in two different places at the same
moment, they could calculate the distance between the two and therefore the position.
3
Yet despite this insight, they never managed to build the right tool to put the
theory into practice. The invention of the marine clock, the tool that would one
day solve the “longitude problem”, was still many centuries away.
Over time, finding the solution to the problem became a geopolitical imperative, as no nation could dominate the world without maritime supremacy, and the
lack of a solution continued to cause tragedy after tragedy at sea. In 1707, Admiral
Sir Cloudesley Shovell fatally misjudged the position of the British fleet while
returning from the Battle of Gibraltar. Due to his error, all of the ships crashed
against the Isles of Scilly to the southwest of England, with over 2,000 casualties.
In light of this accident, the London Parliament decided that the longitude problem had to be resolved for good and set up a special committee to award a £20,000
prize – several million dollars today – to anyone who made a precision marine
clock. Sir Isaac Newton said before the committee that “such a clock has not yet
been designed or built”.
4
In her best-selling book Longitude: The True Story of a Lone Genius Who
Solved the Greatest Scientific Problem of His Time,
5
Dava Sobel tells how a poor
English carpenter, John Harrison, built in 1761 the first such clock. He had already
made a wooden pendulum at the age of 20 and later improved his design to the
1
Budlong, John. 1978. Sky and Sextant: Practical Celestial Navigation.
2
Idem
3
The hours indicated simultaneously by two clocks at two different sites make it possible to
calculate the respective geographical distance. Since the Earth rotates 360 degrees in 24 hours,
one hour equals 15 degrees (one 24
th
of a turn), so the one-hour difference between a ship’s
position and its starting point indicates that it has travelled a distance such as to have covered
fifteen degrees of longitude to the east or to the west depending on the direction of departure.
At the equator, where the circumference of the Earth is at its largest, fifteen degrees correspond
to one thousand nautical miles. As we move away from latitude 0, the value of each degree
measured in miles decreases, approaching zero at the poles, and is calculated with complex
trigonometric formulas already elaborated in ancient times.
4
Idem, p. 121.
5
Sobel, Dava. 1995. Longitude: The True Story of a Lone Genius Who Solved the Greatest
Scientific Problem of his Time. London: Walker Publisher.
52 GPS: The Stellar Clock