Geography 140
Introduction to Physical Geography
Lecture: The Geographic Grid (Longitude)
IV. Longitude
A. Longitude is distance east or west of a base line or prime meridian
B. The longitude of any given place is its distance, measured in degrees
of arc, from this base line.
C. Picking a base line from which to begin numbering longitude was not
the easy matter that it was with latitude.
1. Latitude uses the midpoint between the two poles, the equator, as a
base line, and this is a pretty obvious line given by the rotation
of the earth.
2. There is no such naturally obvious base line for longitude, and so
each country figured that the meridian of its capital should merit
the honor. This led to years of international bickering and
cartographic confusion. Some cities that have been used for prime
meridians include Munich, Warsaw, Brussels, Rio de Janeiro,
Copenhagen, Amsterdam, Lisbon, Paris, Madrid, Rome, Stockholm, and,
of course, Washington, DC.
3. These excesses of patriotism, of course, made communications among
ships at sea difficult and even on board a single ship with its
often multicultural crews kidnapped from various ports!
4. In 1871, the International Geographical Congress met to resolve the
issue and recommended that the meridian passing through the old
(1675) Royal Observatory in Greenwich, England (a borough of
London) should be the common zero. The proposal didn't get too
far, given all the national pride problems that kept erupting.
5. The IGC met again in 1875 and, again, things weren't proceeding too
well. The French did suggest that they might be willing to
relinquish their demand for Paris as the Prime Meridian if everyone
else agreed to sign onto the metric system, which had been
developed during the French Revolution. This did pave the way for
a break in the logjam, though.
6. In 1884, the British agreed to adopt the metric system in exchange
for the honor of having the Prime Meridian passing through a London
suburb. So, the Greenwich Meridian was finally passed at the
International Meridian Conference held in Washington, DC, and
attended by delegates for 41 nations (Santo Domingo was the sole no
vote, and France and Brazil abstained).
a. Greenwich probably won because the USA had already decided to
use it rather than Washington, DC.
b. Also, at the time, 72% of the entire world's trade was carried
on ships that used the Greenwich Meridian: This was the age of
empire, and Britain had colonies all over the world ("the sun
never sets on the British Empire") and was, therefore, in a
position to strongarm the rest of the world into giving its own
Royal Observatory the honor.
c. Despite the imperialism involved in this choice, it worked out
fairly well internationally:
i. The antipodal meridian to the Greenwich Meridian makes a
mathematically convenient International Date Line (more on
that in another lecture) of 180°
ii. The antipodal meridian to Greenwich's is out in the middle
of the Pacific Ocean, where the date issue can
inconvenience the fewest people in the sparsely settled
mid-Pacific. Not that the British gave a hoot about that,
but it worked out rather well in the end.
d. Greenwich, incidentally, is pronounced "GREN-itch," not "Green-
witch" (go figure)
D. Anyhow, latitude is reckoned in both directions from the Greenwich
Meridian, so this base line is numbered 0° and the antipodal line
is numbered 180°.
1. The reason that 180° is the top number possible for longitude
is that, by starting at the base line (an arbitrary choice of a
meridian, which is one half of a great circle stretching from the
North Pole to the South Pole), we measure one half of a circle to
get to the antipodal meridian (the other half of the same great
circle that the prime meridian is on).
a. A circle is 360° of arc.
b. 360° divided by 2 is 180°
![[ 1/4 circle ]](https://home.csulb.edu/~rodrigue/geog140/180degrees.jpg)
2. Except for the prime meridian and the antipodal meridian (most of
which is the International Date Line), the suffix "E" or "W" must
appear after the number given for the longitude: It definitely
helps to know which hemisphere we're talking about, since the
numbering is the same in each hemisphere.
E. Subdividing longitude:
1. There are no linear equivalents for degrees, minutes, and seconds
of longitude that are applicable all over the world, the way there
are for latitude. This is because the meridians of longitude are
spread out the farthest at the equator (where a degree of longitude
would be about 110 km on the ground) but they converge as you
approach the poles: By 60° N or S, one degree of longitude is
down to about 55 km on the ground and, at the poles, it's zero.
2. So, other than that caveat, we use the same units of arc for
pinpointing longitude as we do for latitude: degrees, minutes, and
seconds.
a. long. 118°W
b. long. 118°09'W
c. long. 118°09'06"W (Long Beach Airport again)
d. Some nice geotrivia for you: If you would like to know the
latitude and longitude of any American city, you can click here.
F. How longitude is represented on a globe or map:
1. Cartographers use meridians to depict that part of the geographic
grid that refers to longitude.
2. All meridians are half sections of great circles.
3. Meridians are spaced the farthest apart at the equator and converge
closer and closer together until they actually touch at each pole.
At the equator, then, a degree of longitude is roughly 110 km wide
but this drops to about 55 km by 60° N or S, and down to 0°
at each pole.
4. Other characteristics of meridians:
a. All meridians are true north-south lines used to represent east-
west longitude with respect to the Prime Meridian.
c. Meridians always cross lines of latitude at right angles (except
the poles, which are points of latitude).
d. An infinite number of meridians can, theoretically, be drawn on
the globe, which means all locations on Earth lie on a meridian.
G. How longitude can be determined if you have no idea where you are (you
came to college to find yourself, right?).
1. Actually, you'll be relieved to learn that the concept of finding
longitude is easier than that of finding latitude. Basically,
longitude is a function of time:
L = f(t)
2. By this I mean that, since the sun appears to move 15° an hour,
if you know where the sun is as seen by two places at the same
time, you can figure out the difference in time and convert it into
a difference in longitude. Since you can't directly observe the
sun in two places at once, what you need is some indication of what
the exact time is somewhere else that you can refer to and do the
math.
3. The most important requirement in its reckoning, then, is an
accurate timepiece. Historically, this technical requirement
turned out to be a real sticking point in coming up with this
relatively simple process of determining longitude. Mind you,
accurate timepieces had been around for centuries, if not millenia
(a sundial is a pretty accurate timepiece). The problem was a
timepiece that was accurate under the conditions of travel, so that
you could know what time it was somewhere else.
a. So, how did people navigate long ago? Well, up until the end of
the fifteenth century (before 1492), Old World sailors pretty
much stayed close to land, sailing or rowing along the coasts so
they could find their way readily. This was a bit wasteful in
creating longer "scenic routes" than would be necessary if they
could figure out longitude on the fly.
b. Polynesian sailors rather generously defined "land" and learned
to sail across the open Pacific (and even the Indian Ocean) by
reference to established currents that were affected by the
presence of islands.
c. With the huge expansion in trade and then conquest set off by
Columbus' voyage to the New World in 1492 and Diaz' voyage
around the southern tip of Africa (the Cape of Good Hope) in
1486 and da Gama's voyage to India in 1497, this longitude
problem began to be a major pain as it made for a certain
reluctance to set off across the open seas. Imperialism-minded
countries began to offer big money to anyone who could figure an
accurate way around the problem, beginning with Spain in the
late 1500s and followed by France, the Netherlands, and England
in the 1600s.
d. A pretty cool idea was based on Galileo's discovery of the moons
of Jupiter back in 1610. By observing them, he and others
quickly realized that their eclipses (when they slid behind the
gas-giant planet) were predictable. If a navigator on the high
seas could note the local time of such an eclipse and compare it
with the local time at which it was predicted to happen at some
other reference location back home, the difference in times and,
therefore, longitude could easily be found. So, Galileo tried
for all that government money available to find a solution,
first from Spain and then from the Netherlands. He struggled to
create tables of eclipse times for base lines in Europe, but his
observations never got precise enough. He also labored mightily
to create telescopes that could be used to observe and time the
eclipses on board a ship but that, too, was more easily said
than done. Then, the Catholic Church got after him for arguing
that the earth was not the center of the cosmos and he had to
focus on that (the consequences very realistically could have
been a truly awful execution). He never collected the prizes.
Some other people made attempts to tighten his observations,
most notably and best of all Jean Dominique Cassini in 1668.
The British came up with some excellent telescopes that could be
used on board a ship. In fact, King Charles II founded the
Royal Observatory in 1675 specifically to work on the longitude
problem! That's how big a problem this was!
e. All these struggles fizzed in the 1760s, however, when a rather
obscure British clockmaker, of all things, one John Harrison,
came up with a very simple system based on a really accurate
timepiece, basically, a watch.
f. This guy was a working class fellow, a carpenter and joiner who
made wooden clocks in his day job. He had hardly any formal
education. Even so, he was an imaginative and creative guy and
came up with a number of really quite original inventions:
i. A wooden clock that didn't need any lubricating oil (which
is what usually messed up clocks back then).
ii. A counterbalanced spring-driven portable clock that wasn't
affected by the motion of the clock itself (pendulum clocks
are affected by motion and the constant directional pull of
gravity):
iii. Caged roller bearings, still used in contemporary machines
of all types!
iv. Bimetallic strips, made of two metals that respond
differently to temperature changes and compensate for them.
v. Finally, a watch that was accurate enough to be used for
oceanic navigation in the late 1750s.
g. So, this working-class schmo kept applying for the British prize
for solving the longitude problem (to the Board of Longitude),
and they kept saying that his watch was a fluke and insisting on
more and more trials of more and more copies that had to be made
by him. It really looks as though they were trying to do him
out of the £10,000 prize (which was a, pardon the pun,
astronomical sum back then), maybe because he was just a regular
hoi polloi fellah (the British class system is very rigid
to the present day).
h. Finally, he appealed to George III, King of England, who said
he'd been "cruelly wronged" and "By God, Harrison, I will see
you righted" (this was the king on whose watch the American
colonies got away during the American Revolution -- you might
enjoy renting the movie, "The Madness of King George," which was
about him and is really a riveting story). Even the King's
trials of the watch in 1772 wouldn't move the Board of
Longitude, so Harrison appealed to Parliament, which finally did
right the wrong and awarded him £8,750 in June 1773. He
was vindicated, enriched, and recognized as the author of the
solution to the longitude problem. He didn't have long to savor
his victory, though: He died four years later, on his 83rd
birthday.
4. The upshot of all this (and, no, I don't expect you to memorize
these historical details and dates and names) is that, if you have
a watch set to some base meridian's time, you don't need Cassini's
tables, fancy astronomical telescopes, and base-isolated chairs to
use them on a rocking and rolling ship deck! You just need a good
watch.
5. To understand Harrison's achievement, you have to recall that the
basis of longitude is the 24 hour day. By observing the sun, we
have a way of converting time to longitude.
a. As the earth rotates, the sun appears to make a complete circle
(360°) in 24 hours.
b. Each hour of time, the sun then appears to move 15° (360°/24=15°)
of longitude
c. Each minute of time, the sun appears to move ¼° of longitude
d. Alternatively, it takes the sun 4 minutes to move 1° (60
minutes of time divided by 15° of longitude).
6. Let's go back to Harrison's accurate timepiece, or "chronometer"
(it measures ["meter"] time ["chronos"]). It differs from your
Rolex or Timex in a variety of ways:
a. It is extremely accurate.
b. It lists all 24 hours on its face, instead of having the arm
pass through 12 hours twice a day. This puts an end to a.m. and
p.m. questions: 09:00 is unambiguously 9 a.m. and 13:45 is
unambiguously 1:45 p.m. You might have encountered this system
in the military or if you attended other colleges, such as CSUN
or Chico State.
c. Most importantly, it is always set to UT (Universal Time) or UTC
(Universal Time Coördinated) or Zulu Time (long story, that
last bit, and I'll get to it when we talk about time zones in
the next lecture), also and formerly known as Greenwich Mean
Time (GMT) or mean solar time at Greenwich.
7. So, how we're ready to learn the process by which an amateur
navigator can determine longitude (with your chronometer packed
away with your analemma, sextant, and 140 notes).
a. First step: Make sure that it is noon, just as you did for
latitude.
b. If so, consult your chronometer and learn what time it is in
Greenwich, England.
c. Calculate the difference in time between your local sun time and
Greenwich time.
d. Convert the difference in time into a difference in longitude.
Don't remember what the conversions are? Here they are:
i. First, figure out how many full hours of difference there
are between you and Greenwich and multiply by 15°.
ii. Second, for any minutes left over, the sun would have moved
¼°. The easiest thing to do is take those
minutes left over and divide them by 4 (because the sun
takes 4 minutes to move one degree).
iii. Add the two numbers together (i + ii) to get the total
longitudinal difference between you and Greenwich.
iv. Very importantly, convert the decimal notation of your
answer into degrees, minutes, and seconds of arc OR into
fractions. So, a longitudinal difference of 153.75°
should be converted to 153¾° or to 153°45'.
Either way is correct, but it is not correct to leave the
calculation in the decimal system (those darn Chaldeans).
e. Now, ask yourself the burning question: Is Greenwich time
earlier than you or later than you?
i. If Greenwich time is a smaller number than your local time
(12:00 for noon), then you are EAST of Greenwich. You got
under the noon sun before the folks in Greenwich, which,
since the earth moves from west to east, means you are
farther along the rotational cycle, or east of Greenwich.
Add the "E" suffix to your longitude, and you are done.
ii. If Greenwich time is a larger number than your local time
(e.g., noon here or 12:00), then you are WEST of Greenwich.
They beat you under the noon sun, so you are lagging in the
direction from which the earth is turning: west. Add the
"W" suffix to your longitude, and all is well.
f. That's all there is to longitude, then. Even though
historically this was the more difficult concept to develop, it
is procedurally a lot simpler to do than figuring out latitude.
And that's a wrap on longitude (and on the whole subject of the
geographic grid, for that matter). This is a lot of material, and it
can be a bit confusing. So you don't forget all this, I have prepared
a lab exercise for you, to reïnforce the concepts by applying
them, hands-on. You can get to the lab by clicking here
or by going to the course home page and clicking on "Lab 1."
The printed-out labs are due filled out in my mailbox in LA4-106 (with
your names emblazoned on them!). Remember, I am not grading for the
right answers so much as for watching you make a sincere effort to
work through the material.
One thing about the lab, though: Right now, you have enough material
to make it through Lab 1 a and b, but not enough to tackle Lab 1c, so
don't panic when you see it. The lecture material on time will get
you through that.
Document and © maintained by Dr.
Rodrigue
First placed on web: 09/09/00
Last revised: 09/13/01