208
4 Navigations from Ground to Space
and heading accuracy of the ships. By using the celestial navigation techniques, the
Zheng’s fleets smoothly arrived in the Gulf of Aden, the Red Sea and the east coast
of Africa, while European ocean voyage industry was then still in infancy.
In 1434, for the first time, the Portuguese fleets crossed the Cape Bojadorcalled
“death limit” on the west coast of Africa, which started the adventure of European
ocean voyage and colonial plunder. In 1492, an Italian navigator and explorer, C.
Columbus, led his fleet across through the Atlantic Ocean to the American continent. In 1497, a Portuguese, V. da Gama, bypassed the Cape of Good Hope at the
southern tip of Africa and opened up a route to India. In 1519, a Portuguese explorer
F. Magellan led a fleet of about 270 crews, which took 3 years to achieve the first
ocean voyage round the world. With the expansion of European capitalism and the
increasing demand of ocean shipping, the celestial navigation technology had developed rapidly. By the eighteenth century, the European fleets had commonly been
equipped with the measuring-angle instruments like sextants and the chronometers
like astronomical clocks to further improve the ship positioning accuracy based on
the celestial navigation technology.
The sextant is an optical instrument used to measure the angular separation
between two distant visible objects, with a shape of fan. It is composed of a small
telescope, a semi-transparent fixed plane mirror (also known as the horizon mirror)
and a movably reflecting mirror (also known as the index mirror), with a measuring
arc of one-sixth of the circumference. Generally, the sextant is used to measure the
angle between the Sun or other celestial bodies and the horizon at a certain time, so
as to determine the longitude and latitude of ships or aircrafts. As early as 1699, the
sextant’s principle was first proposed by Isaac Newton (1643−1727), but his notes
were not published until 1742. Around 1730, a British mathematician John Hadley
and an American inventor Thomas Godfrey independently invented the octant, a
double-reflecting quadrant instrument with the measuring arc of one-eighth of the
circumference. By 1757, a British naval officer John Campbell altered the octant’s
double-reflecting quadrant into the circular arc of one-sixth of its circumference
to attain a measuring range of 120°, which was renamed sextant. Such simple and
convenient stellar measuring instrument has widely been applied to the ocean voyage
since the middle eighteenth century.
In 1735, John Harrison, a British clock mechanist, invented a mechanical marine
chronometer with temperature compensation, which was the first practical astronomical clock on board of ships and a revolution of the celestial navigation technology, enabling navigators to accurately compute their longitudes at sea. Before the
invention of astronomical clock, the latitude could be estimated but the estimates
of longitude were usually very inaccurate. The marine chronometers had been used
for more than 200 years by continual improvement, and later were replaced by electronic astronomical quartz clocks. In 1837, the American Captain Thomas H. Sumner
proposed the concept of celestial position lines, the circle of equal altitude, as he was
approaching the Smalls Lighthouse in thick weather. The longitude and latitude of
the ship’s position could be estimated by using the circle of equal altitude. In 1875, on
the basis of the circle of equal altitude, the French Naval Commander Marcq de SaintHilaire proposed the astronomical drawing way and conversion method to determine
4 Navigations from Ground to Space
and heading accuracy of the ships. By using the celestial navigation techniques, the
Zheng’s fleets smoothly arrived in the Gulf of Aden, the Red Sea and the east coast
of Africa, while European ocean voyage industry was then still in infancy.
In 1434, for the first time, the Portuguese fleets crossed the Cape Bojadorcalled
“death limit” on the west coast of Africa, which started the adventure of European
ocean voyage and colonial plunder. In 1492, an Italian navigator and explorer, C.
Columbus, led his fleet across through the Atlantic Ocean to the American continent. In 1497, a Portuguese, V. da Gama, bypassed the Cape of Good Hope at the
southern tip of Africa and opened up a route to India. In 1519, a Portuguese explorer
F. Magellan led a fleet of about 270 crews, which took 3 years to achieve the first
ocean voyage round the world. With the expansion of European capitalism and the
increasing demand of ocean shipping, the celestial navigation technology had developed rapidly. By the eighteenth century, the European fleets had commonly been
equipped with the measuring-angle instruments like sextants and the chronometers
like astronomical clocks to further improve the ship positioning accuracy based on
the celestial navigation technology.
The sextant is an optical instrument used to measure the angular separation
between two distant visible objects, with a shape of fan. It is composed of a small
telescope, a semi-transparent fixed plane mirror (also known as the horizon mirror)
and a movably reflecting mirror (also known as the index mirror), with a measuring
arc of one-sixth of the circumference. Generally, the sextant is used to measure the
angle between the Sun or other celestial bodies and the horizon at a certain time, so
as to determine the longitude and latitude of ships or aircrafts. As early as 1699, the
sextant’s principle was first proposed by Isaac Newton (1643−1727), but his notes
were not published until 1742. Around 1730, a British mathematician John Hadley
and an American inventor Thomas Godfrey independently invented the octant, a
double-reflecting quadrant instrument with the measuring arc of one-eighth of the
circumference. By 1757, a British naval officer John Campbell altered the octant’s
double-reflecting quadrant into the circular arc of one-sixth of its circumference
to attain a measuring range of 120°, which was renamed sextant. Such simple and
convenient stellar measuring instrument has widely been applied to the ocean voyage
since the middle eighteenth century.
In 1735, John Harrison, a British clock mechanist, invented a mechanical marine
chronometer with temperature compensation, which was the first practical astronomical clock on board of ships and a revolution of the celestial navigation technology, enabling navigators to accurately compute their longitudes at sea. Before the
invention of astronomical clock, the latitude could be estimated but the estimates
of longitude were usually very inaccurate. The marine chronometers had been used
for more than 200 years by continual improvement, and later were replaced by electronic astronomical quartz clocks. In 1837, the American Captain Thomas H. Sumner
proposed the concept of celestial position lines, the circle of equal altitude, as he was
approaching the Smalls Lighthouse in thick weather. The longitude and latitude of
the ship’s position could be estimated by using the circle of equal altitude. In 1875, on
the basis of the circle of equal altitude, the French Naval Commander Marcq de SaintHilaire proposed the astronomical drawing way and conversion method to determine