REPRINTED WITH PERMISSION FROM
SPATIAL SYNTHESIS,
http://www.imagenet.org/
Latitude
and
Longitude
Figures and text
based on
images and text from a GeoSystems webpage which no longer exists.
Permission was
granted for
an earlier use of selected images when the previous site was contacted.
Consider
the Earth to be modeled as a
sphere:
the Earthsphere. The Earth is not actually a sphere, but a sphere
is a good approximation to its shape and the sphere is easy to work
with
using classical mathematics of Euclid and others.
- Given a sphere
and a plane.
There are
only a few logical possibilities about the relationship between the
plane
and the sphere.
- The sphere and
the plane do not
intersect.
- The plane
touches the sphere at
exactly one
point: the plane is tangent to the sphere.
- The plane
intersects the sphere.
- and does not
pass through the
center of the
sphere: in that case, the circle of intersection is called a
small
circle.
- and does
pass through the center
of the sphere:
in that case, the circle of intersection is as large as possible and is
called a great circle.
- Great circles
are the lines along
which distance
is measured on a sphere: they are the geodesics on the sphere.
- In the plane,
the shortest distance
between
two points is measured along a line segment and is unique.
- On the sphere,
the shortest distance
between
two points is measured along an arc of a great circle.
- If the two
points are not at
opposite ends
of a diameter of the sphere, then the shortest distance is unique.
- If the two
points are at opposite
ends of
a diameter of the sphere, then the shortest distance is not
unique:
one may traverse either half of a great circle. Diametrally
opposed
points are called antipodal points: anti+pedes, opposite+feet, as
in drilling through the center of the Earth to come out on the other
side.
- To reference
measurement on the
Earthsphere
in a systematic manner, introduce a coordinate system.
- One set of
reference lines is
produced using
a great circle in a unique position (bisecting the distance between the
poles): the Equator. A set of evenly spaced planes,
parallel
to the equatorial plane, produces a set of evenly spaced small circles,
commonly called parallels. They are called that because it is the
planes that are parallel to each other.
- Another set of
reference lines is
produced
using a half of a great circle, joining one pole to another, that has a
unique position: the half of a great circle that passes through
the
Royal Observatory in Greenwich, England (three points determine a
circle).
Here it is historical consideration that produces the uniqueness in
selection.
Choose a set of evenly spaced halves of great circles obtained by
rotating
the diametral plane along the polar axis of the Earth. These
lines
are called meridians: meri+dies=half day, the situation of the
Earth
at the equinoxes (see page on seasons).
The unique line is called the Prime Meridian; other halves of great
circles
are called meridians.
This particular reference system
for the
Earth
is not unique; an infinite number is possible. There is abstract
similarity between this particular geometric arrangement and the
geometric
pattern of Cartesian coordinates in the plane.
- To use this
arrangement, one might
describe
the location of a point, P, on the Earthsphere as being at the
3rd
parallel north of the Equator and at the 4th meridian to the west of
the
Prime Meridian. While this might serve to locate P
according
to one reference system, someone else might employ a reference system
with
a finer mesh (halving the distances between success lines) and for that
person, a correct description of the location of P would be at
the
6th parallel north of the Equator and at the 8th meridian to the west
of
the Prime Meridian. Indeed, an infinite number of locally correct
designations might be given for a single point: an unsatisfactory
situation in terms of being able to replicate results. The
problem
lies in the use of a relative, rather than an absolute, locational
system.
- To convert
this system to an
absolute system,
that is replicable, employ some commonly agreed upon measurement
strategy
to standardize measurement. One such method is the assumption
that
there are 360 degrees of angular measure in a circle.
- Thus, P
might be described
as lying
42 degrees north of the equator, and 71 degrees west of the Prime
Meridian.
The degrees north are measured along a meridian; the degrees west are
measured
along the Equator or along a parallel (the one at 42 north is another
natural
choice). The north/south angular measure is called Latitude; the
east/west angular measure is called Longitude.
- The use of
standard circular
measure creates
a designation that is unique for P; at least unique to all
whose
mathematics rests on having 360 degrees in the circle.
Parts of degrees may be noted as
minutes
and seconds, or as decimal degrees. A degree (°) of latitude or
longitude
can be subdivided into 60 parts called minutes ('). Each minute can be
further subdivided into 60 seconds ("). Thus, 42 degrees 30
minutes
is the same as 42.50 degrees because 30/60 = 50/100. Current
computerized
mapping software often employs decimal degrees as a default; older
printed
maps may employ degrees, minutes, and seconds. Thus, the human
mapper
needs to take care to analyze the situation and make appropriate
conversions
prior to making measurements of position. Such conversion is
simple
to execute using a calculator. For example, 42 degrees 21 minutes
30 seconds converts to 42 + 21/60 +30/3600 degrees = 42.358333 degrees;
powers of ten replace powers of 60.
- The figure below
shows the reference
system
described above placed on a sphere. What might be called a
Cartesian
grid in the plane is called a graticule on the sphere.
- All parallels
have the same
latitude; they
are the same distance above or below, north of or south of, the Equator.
- All meridians
have the same
longitude; they
are the same distance east or west of the Prime Meridian.
- Spacing
between successive parallels
or meridians
might be at any level of detail; however, when circular measure
describes
the position of these lines, that description is unique up to agreement
to use 360 degrees in a circle. One spacing for the set of
meridians
that is convenient on maps of the world, is to choose spacing of 15
degrees
between successive meridians. The reason for this is that since
the
meridians converge at the ends of the polar axis, that each meridian
then
represents the passage of one hour of time. Given that we agree
to
partition a day into 24 hours, 24 times 15 is 360, meridians may also
mark
time.
- Bounds of
measurement (see the figure
below).
- Latitude runs
from 0° at the
equator to
90°N or 90°S at the poles.
- Longitude runs
from 0° at the
prime meridian
to 180° east or west, halfway around the globe. The International
Date
Line follows the 180° meridian, making a few jogs to avoid cutting
through land areas.
- Length of one
degree on the
Earthsphere.
- One degree of
latitude, measured
along a meridian
or half of a great circle, equals approximately 69 miles (111 km). One
minute is just over a mile, and one second is around 100 feet (a pretty
precise location on a globe with a circumference of 25,000
miles).
Calculation: 25,000/360 = 69.444.
- Because
meridians converge at the
poles, the
length of a degree of longitude varies, from 69 miles at the equator to
0 at the poles (longitude becomes a point at the poles).
Calculation:
at latitude theta, find the radius, r, of the parallel, small circle,
at
that latitude. The radius, R, of the Earthsphere is R = 25,000 /
(2*pi)=3978.8769 miles. Thus, cos theta = r/R (using a theorem of
Euclid that alternate interior angles of parallel lines cut by a
transversal
are equal). Therefore, r=R cos theta. Then, the
circumference
of the small circle is 2r*pi and the length of one degree at theta
degrees
of latitude is 2r*pi / 360. For another application of this
particular
theorem of Euclid, see the linked
page concerning Eratosthenes measurement of the Earthsphere.
- For example,
at 42 degrees of
latitude, r
= 2956.882. Thus, the circumference of the parallel at 42
degrees
north is approximately 18578.6205 miles. Thus, the length of one
degree of longitude, measured along the small circle at 42 degrees of
latitude,
is: 51.607 miles.
- This
particular calculation scheme
is a rich
source of elementary problems using geometry and trigonometry.
Consider
the following question: at what latitude is the length of one
degree
on longitude exactly half the value of one degree of longitude at the
equator?
- Readers
wishing a visual review of
trigonometry
may find this link
to be of use.
- The position of
the sun in the
sky.
On June 21, the direct ray of the sun is overheard, or perpendicular to
a plane tangent to the Earthsphere, at 23.5 degrees north latitude (link
to page about seasons). The angle of the sun in the sky at noon
on
that day is 90 degrees. What is the angle of the sun in the sky,
at noon on June 21, at 42 degrees north latitude? Again, simple
geometry
and trigonometry solve the problem for this value and for any
other.
Use the fact that 42-23.5=18.5 degrees; that there are 180 degrees in a
triangle (look for a right triangle with the right angle at 42 degrees
north latitude); and that corresponding angles of parallel lines cut by
a transversal are equal. The answer works out to be 71.5
degrees.
Thus, on June 21 at local noon, in the northern hemisphere at 42
degrees
north latitude the sun will appear in the south at 71.5 degrees above
the
horizon; in the southern hemisphere at 42 degrees south it will appear
in the northern sky at 71.5 degrees above the horizon. Between
the
tropics, some interesting situations prevail (link to Parallels
between Parallels, pages 74-86). Use of this technique is
important
in calculating shadow and related matters in electronic mapping:
it was employed in making several virtual reality models of in this
book.
Further Directions:
- The north and
south poles are the
earth's
geographic poles, located at each end of its axis of rotation. All
meridians
meet at these poles. The compass needle points to either of the earth's
two magnetic poles. The north magnetic pole is located in the Queen
Elizabeth
Islands group, in the Canadian Northwest Territories. The south
magnetic
pole lies near the edge of the continent of Antarctica, off the
Adélie
Coast. The magnetic poles are constantly moving. What are
the
implications of this fact for the stability of our graticule?
- All of our
geometric analysis is based
on
Euclidean geometry, assuming Euclid's Parallel Postulate: given a
line and a point not on the line--through that point there passes
exactly
one line that does not intersect the given line. Non-Euclidean
geometries
violate this Postulate. What does the geometry of the Earthsphere
become in the non-Euclidean world?
Institute
of Mathematical Geography. Copyright, 2005, held by authors.
Spatial
Synthesis:
Centrality and Hierarchy, Volume I, Book 1.
Sandra
Lach Arlinghaus and
William Charles Arlinghaus