|
Latitude and Longitude
Author and Curator:
Dr. David P.
Stern
| Any location on Earth is described by two
numbers--its latitude and its longitude. If a pilot or a
ship's captain wants to specify position on a map, these are the
"coordinates" they would use.
Actually, these are two angles, measured in
degrees, "minutes of arc" and "seconds of arc." These are denoted by the
symbols ( , ', " ) e.g. 35 43' 9" means an angle of 35 degrees,
43 minutes and 9 seconds (do not confuse this with the notation (', ")
for feet and inches!). A degree contains 60 minutes of arc and a minute
contains 60 seconds of arc--and you may omit the words "of arc" where
the context makes it absolutely clear that these are not units of
time.
Calculations often represent angles by small letters
of the Greek alphabet, and that way latitude will be represented by λ
(lambda, Greek L), and longitude by φ (phi, Greek F). Here is how they
are defined.
Latitude
|
| |
![[IMAGE: Defining latitude]](Slatitud.gif) |
|
The
latitude angle lambda |
|
Imagine the Earth was a transparent sphere
(actually the shape is slightly oval; because of the Earth's rotation,
its equator bulges out a little). Through the transparent Earth
(drawing) we can see its equatorial plane, and its middle the point is
O, the center of the Earth.
To specify the latitude of some point P on the
surface, draw the radius OP to that point. Then the elevation angle
of that point above the equator is its latitude λ--northern
latitude if north of the equator, southern (or negative) latitude if
south of it.
|
[How can one define the angle between a line and a
plane, you may well ask? After all, angles are usually measured
between two lines!
Good question. We must use the angle which completes it to 90
degrees, the one between the given line and one perpendicular
to the plane. Here that would be the angle (90-λ) between OP and
the Earth's axis, known as the co-latitude of P.]
|
| |
![[IMAGE: Lines of latitude]](Slatline.gif) |
|
Lines of latitude |
|
On a globe of the Earth, lines of latitude are
circles of different size. The longest is the equator, whose
latitude is zero, while at the poles--at latitudes 90 north and 90
south (or -90) the circles shrink to a point.
Longitude
On the globe, lines of constant longitude ("meridians")
extend from pole to pole, like the segment boundaries on a peeled
orange.
|
| Every meridian must cross the equator. Since the
equator is a circle, we can divide it--like any circle--into 360
degrees, and the longitude φ of a point is then the marked value
of that division where its meridian meets the equator.
|
| |
![[IMAGE: lines of longitude]](Slonglne.gif) |
Longitude
lines or "meridians" |
|
| What that value is depends of course on where we
begin to count--on where zero longitude is. For historical
reasons, the meridian passing the old Royal Astronomical Observatory in
Greenwich, England, is the one chosen as zero longitude. Located at the
eastern edge of London, the British capital, the observatory is now a
public museum and a brass band stretching across its yard marks the
"prime meridian." Tourists often get photographed as they straddle
it--one foot in the eastern hemisphere of the Earth, the other in the
western hemisphere.
A lines of longitude is also called a meridian,
derived from the Latin, from meri, a variation of "medius"
which denotes "middle", and diem, meaning "day." The word
once meant "noon", and times of the day before noon were known as
"ante meridian", while times after it were "post meridian." Today's
abbreviations a.m. and p.m. come from these terms, and
the Sun at noon was said to be "passing meridian". All points on the
same line of longitude experienced noon (and any other hour) at the
same time and were therefore said to be on the same "meridian line",
which became "meridian" for short.
|
|
|
e-publication
