Geoscience Reference
In-Depth Information
made observations of the stars and the planets as they crossed the meridian. From the
outset the techniques relied on the meticulous construction of accurate instruments
made by scientists and engineers according to continuously developing engineering
principles. Flamsteed's successor Edmund Halley (1656-1742) concentrated on observing
the motion of the Moon using the theory of gravitation discovered by his friend Isaac
Newton in order to develop his Moon motion theory. The third Astronomer Royal
James Bradley (1693-1762) installed even larger meridian instruments, and the fourth,
fifth and sixth Astronomers Royal (Nathaniel Bliss (1700-1764), Neville Maskelyne
(1732-1811) and John Pond (1767-1836) respectively) continued improvements.
It took until the end of the eighteenth century for the methods and measurements
to bear fruit. The required data was first published in the
Nautical Almanac
in 1767,
under Maskelyne's name. The invention of the required instrument, the octant, was
by John Hadley (1682-1744), and was called so because its scale of angular measure
was based on one-eighth of a circle. When for practical reasons this was changed
to one-sixth of a circle the instrument's name changed to a sextant.
Eventually the method of lunar distances was rendered obsolete by the invention of
the chronometer by John Harrison in 1761, and the Greenwich Observatory developed
its instruments for determining star positions using transit instruments. Under George
Airy (1801-1892), seventh Astronomer Royal, the calculations of the positions of the
moon, the planets and the stars were carried out by factory-like methods through
human computers or “intelligent drudges” - young men, mathematically well educated
but not too much so. Their calculations on forms printed for the purpose were checked
at key stages by senior permanent staff. Airy brought the system to a very high state
of great reliability, insisting that everything was done in the best possible way (namely
his
way). He treated his staff rather ruthlessly, dismissing the computers when they
reached the age of 21 at which time they would be entitled to higher rates of pay.
In 1850 Airy built a new transit circle and this instrument and its observations
defined the Greenwich Meridian as it has come to be today, producing observations of
unprecedented accuracy. Their accuracy and that of the calculations, coupled with
the emergence of Britain as the dominant marine power and the extensive Empire
over which it had influence, meant that the British system of latitude and longitude
came to predominate over the French, as I shall relate in this Chapter.
HAVING FOUND LATITUDE AND LONGITUDE a sailor would need to relate
this to his position on the sea meaning he would need to have accurate maps of the
sea, islands and coast lines of the continents. The British maps were executed by
the Army using surveying techniques independently of any astronomical measure-
ments and starting on a baseline laid out on Hounslow Heath (now the position of
Heathrow Airport
20
). Its northwest end is now marked by an upturned cannon
20
It is not a coincidence that the fundamental bases of both the trigonometrical survey of France
and Britain both lie now partly within major airports serving their capital cities (Orly and
Heathrow respectively, serving Paris and London). The needs of state-sponsored surveyors for an
extensive, accessible and low population-density flat plain were the same as the needs of airlines
who needed to land aircraft near their major city destinations.
