Download prime meridian

The
Longitude
Question
On October 22nd, 1707 a British Naval Fleet sank of
within minutes after running onto the rocks of the
Scilly Isles.
1647 sailors died.
(Oh, and several treasure chests of great value also
went to the bottom of the English Channel.)
It was one of the greatest maritime disasters in the
history of the British Isles. People freaked.
It was later determined that the main cause of the
disaster was the navigators' inability to accurately
calculate their longitude, or east/west location.
Lands within
British
Empire at any
time
For the British and other imperialist powers like
Spain and France, accurate navigation at sea was a
key to controlling their growing empires.
The magnitude of the problem was illustrated by the 1497 voyage
of the Portuguese navigator Cabral who, on his way to the East
Indies, ran into the coast of Brazil, some 2,000 miles off course!
In response to the Scilly
Islands tragedy, the British
Parliament passed the
Longitude Act promising a
reward of £ 20,000 to
anyone who could solve
the so called “Longitude
Problem”, one of the most
vexing scientific problems
of the time.
Note that £20,000 in 1714 is around $4.7 million
dollars today!
What is my
latitude
(north/south
position)?
Easy!
What is my
longitude
(east/west
position)?
Hard!
But why was measuring longitude, one’s position east
to west, a problem at all? After all, sailors had been
measuring latitude (one’s north south position) with
ease since ancient times. Why the difference?
To answer that question, we need to
understand what latitude and
longitude are, what each measures,
and how one is scientifically derived
from the earth’s rotation around the
sun and the other is a function of
time and is therefore purely a human
creation.
The location of any place on the earth’s surface can be
described by parallels (lines parallel to the equator) and
meridians (lines running from pole to pole), two sets of
imaginary arcs drawn on the earth’s surface. Note that
parallels are circles while meridians are half circles.
The numbering system to indicate the location of any
parallel is called latitude. The numbering system to
indicate the location of meridians is called longitude.
Parallels are about 69 miles apart per degree.
(Arroyo Grande is at 35⁰ N and King City (about 70
miles north of AG) is at 36⁰ N latitude.
However, since meridians converge at the poles, the
distance between them is not constant. It becomes
smaller the closer you get to the poles.
Lines of latitude run
east to west but they
tell you your
north/south position
(how far you are
north or south of the
equator).
The equator is 0⁰ latitude. All other parallels have
latitude numbers between 0⁰ and 90⁰ either north or
south depending on whether they are north or south
of the equator.
Latitude lines above
the equator are
labeled N (North)
30⁰ N
Latitude lines below
the equator are
labeled S (South)
45⁰ S
Lines of longitude
run north to south
but they tell you your
east/west position
(how far you are east
or west of the prime
meridian.
The prime meridian, which runs through Greenwich,
England, is 0⁰ longitude. All other meridians have longitude
numbers between 0⁰ and 180⁰ east or west depending on
whether they are east or west of prime meridian.
All locations east of the Prime Meridian have
longitudes from 0⁰ to 180⁰ E
The Prime Meridian at
Greenwich, England is the
meridian a 0⁰ latitude.
All locations west of the
Prime Meridian have
longitudes from 0⁰ to
180⁰ W.
Arroyo Grande is at
about 120⁰ W latitude.
The prime meridian divides the earth into western and
eastern hemispheres. The equator divides the earth into
northern and southern hemispheres.
Five Key Parallels and their Latitudes:
North Pole
90°N
66°N
Arroyo Grande
35°N
23.5°N
0°
23.5°S
66°S
South Pole
The Tropics of Cancer and Capricorn are the latitudes at which
the sun is directly overhead at the summer solstices (June 21 in
the northern hemisphere and December 21 in the southern
hemisphere.) North and south of these lines the sun is never
directly overhead.
The zone in between the Tropic of Cancer and the Tropic
of Capricorn is known as – wait for it – the tropics.
The Arctic and Antarctic Circles are the latitudes above
which the sun is above the horizon for 24 continuous hours
at least once per year and below the horizon for 24
continuous hours at least once per year.
The areas between the tropics and the arctic
circles (marked in pink above) are known as the
Northern and Southern Temperate Zones.
Along with the five major parallels, you should know the two
main meridians: the prime meridian at 0⁰ longitude which runs
through Greenwich, England and, opposite from it at 180⁰ E
or W longitude (same place), the international date line.
Since the meridian at 180⁰
longitude runs through
several countries, it would
divide those countries not
simply into two different time
zones, but into two different
calendar days. To avoid such
unnecessary confusion, the
date line dips and bends
around countries to permit
them to share the same time.
At the 1884 international meridian conference in
Washington, DC, 24 standardized time zones were
agreed to, each spanning 15⁰ of longitude.
(24 x 15⁰=360⁰)
Measuring one’s latitude was fairly
easy because latitude is related to
the angle of the sun or a star above
the horizon, which is something
humans figured out how to do very
early on (around 4000 BCE).
From ancient times sailors determined their latitude
by using instruments such as the astrolab (left) and
later the sextant (right) to determine the angle of
the sun (or a star) above the horizon. The farther
one is from the equator, the lower the midday sun
(or star) appears in the sky.
Once you knew the angle of the sun or star, it was a
simple matter of consulting a table which would give
you your latitude based on the known angle of the
sun or star above the horizon for that day.
Measuring one’s longitude (east-west
position) was not so easy because
longitude is related to time which was
difficult to measure accurately,
especially at sea.
For centuries it was known that all one
needed to determine one’s longitude was a
clock on board that told you what time it was
back at the prime meridian at Greenwich,
England (or at any other place with a known
longitude). When the sun reached its highest
point above your ship, you knew it was 12
p.m. your time. Each hour of difference
between your time (12 p.m.) and the time
back at Greenwich was equal to 15⁰ of
longitude. So if your Greenwich clock said 2
p.m. you knew you were at 30⁰ W longitude.
The problem was that most clocks were extremely
inaccurate—gaining or losing a minute or more each
day—and even then, they were based on delicate
mechanisms (such as pendulums) that would be
thrown completely out of kilter on a rocking ship. A
few days out of port, you could of course still know
when it was 12 p.m. for you (the sun would be at its
highest point) but your reference clock (telling you
the time back at Greenwich) would be worthless.
In 1613 Galileo proposed a brilliant solution to the so called
“longitude problem”. He realized that the positions of Jupiter’s moons
could function as a clock that would be observable from any place on
earth. This failed however because the telescopes in use could not
measure the position of these moons accurately, especially at sea.
The solution to the
longitude problem was
eventually solved not by
any of the great scientists
of the day but rather by
an uneducated British
carpenter and mechanic
by the name of John
Harrison who was really
really good at making
clocks.
At age 20, for example, he
made a clock completely out of
wood that lubricated itself by
using the resins that naturally
seeped from the wood.
At age 42, after seven years of painstaking work in pursuit of
the Longitude Act Prize, he completed his H1, his first “sea”
clock meant to function at sea. However it was not accurate
enough to win the prize. So he went back to work.
At the age of 68, Harrison completed his masterpiece, the H4.
In a test voyage to Jamaica, sailors using the H4 calculated
their longitude to within two miles, well within the accuracy
required for payment of the £20,000 Longitude Prize.
Foul Play!
Getting the prize money from the board was a different
matter, however. They had intended that the prize go to a
proper gentleman, not a simple working class carpenter
like John Harrison. So they proclaimed the results of the
test voyage to have been pure luck and refused to give
Harrison the prize.
Finally in 1773, when Harrison was on this death bed,
King George III intervened on his behalf and granted
him £ 20,000 for his achievements in solving the
centuries old longitude problem.
But he never received the official award (which was
never awarded to anyone).
Harrison’s design for marine chronometers was so
good that even though other craftsmen found other
ways to manufacture them, his template remained in
use until microchips heralded the use of electronic
chronometers in the 1980’s.
Dava Sobel has written a
great book about the
longitude problem simply
called Longitude.
“Dava Sobel has written a gem of a
book. Longitude is one of the best
reads for the non-scientific mind to
come along in many a moon”.
Financial Times