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The Earth in Space
LATITUDE, LONGITUDE AND TIME
LATITUDE
A Series of imaginary lines, known as an
earth grid, is A Series of imaginary lines,
known as an earth grid, is drawn on
maps and globes in order that places can
easily be located. The imaginary lines
called lines of latitude and longitude
intersect one another at right angles as
they run from north to south (longitude)
and east to west (latitude).
Latitude is the distance in degrees on the
surface of the earth north or south of the
equator. Since the earth is a sphere
which rotates on an axis, two fixed
points of reference are available. These
points, called the North Pole and South
Pole (geographic poles) are located
where the axis intersects the surface of
the earth.
If we draw a circle around the earth
half way between the geographic poles
and perpendicular to the line joining
them, we have established a third point
of reference. This line, called the
equator, is an east-west line. It represents
the largest of a series of circles called
parallels of latitude. The distance
between any two parallels is always the
same.
As the parallels of latitude approach
the poles, they become smaller and
smaller circles, although all cover 360°.
The north-south location of any point on
the earth’s surface is called the latitude
of that point.
Latitude is measured in degrees. He
earth is a sphere containing 360°. The
distance from the equator to either of the
poles is one-quarter of 360°; therefore,
each quarter segment of the earth is 90°.
The equator has a latitude of 0°; thus,
both the north and south poles have
latitudes of 90°.
Compass directions are assigned to
the parallels of latitude which lie above
and below the equator. All parallels
drawn between the equator and the
North Pole are north latitudes. In the
same way, all parallels of latitude drawn
between the equator and the South Pole
are called south latitudes. For example, a
point in the Northern Hemisphere
located halfway between the equator and
the North Pole would have a latitude of
45° N.
The latitude of New York City is
approximately 41° N., which places it
41° north of the equator. The boundary
of the United States is between 25° N
and 50° N.
Latitude Can Be Measured In Miles.
Since the circle of latitudes are parallel,
the distance between them never
changes and can be measured. Each
degree of latitude, whether near the
equator or near the poles, is 70 miles
wide.
In order to be more exact in
determining location, each degree of
latitude is divided into 60 equal units
called minutes. Each minute is further
subdivided into 60 seconds.
As we have learned, there are 70miles to one degree of latitude and, since
each degree of latitude is 60 minutes
wide, each minute is equal to 1 1/6 miles
or 6,000 ft. In turn, each second of
latitude is equal to 100 ft.
This breakdown of distance along the
Earth’s surface enables us to calculate
distance in latitude and to express them
in miles and feet. The following
illustration will show you how you can
calculate the distance between two
points of latitude that lie on the same
line of longitude.
Example: What is the distance
expressed in miles between a point on
the equator and a point located 30° 12’
03” N., with both points having the same
longitude?
Solution: Keeping in mind that each
degree of latitude is 70 miles wide, we
can calculate the number of miles
represented by 30°.
70 x 30 = 2,100 miles
1. Each minute of latitude is 1 1/6
miles wide. Therefore, 12 minutes
of latitude are 14 miles wide.
1 1/6 x 12 = 14 miles
2. If each second of latitude is 100
feet wide, 3 seconds of latitude is
300 feet wide.
3. By adding the number of miles in
30° of latitude to the number of
miles in 12 minutes of latitude, we
find the mileage in 30° 12’ of
latitude.
2,100 – Number of miles in 30° of latitude
14 – Number of miles in 12 minutes of
latitude
______
2,114 Total number of miles in 30° 12’ of
latitude
4. If we now add the number of feet
in 3 seconds of latitude, we obtain
the total distance between these
two points to be 2,114 miles, 300
feet.
2,114 + 300 ft. = 2,114 mi., 300 ft.
Determining Latitude. Being able to
determine the latitude of a position is of
great importance to a navigator. In most
instances, the navigator uses celestial
navigation; that is, he determines his
position in relation to the positions of the
heavenly bodies the positions of these
objects appear to change as the navigator
moves north or south of his present
position.
Polaris, the North Star, is often used by
navigators to determine their latitude.
Polaris is about 90° above the horizon at
the North Pole (90° N). At the equator
(0°), Polaris would have an altitude of
0°. Therefore, if an observer standing at
the North Pole would change his
position to 89° N, one degree away from
the pole, Polaris would sink one degree
lower in the sky.
The angle that a heavenly body, such
as the sun or Polaris, is above the
horizon is called its altitude. An observer
can determine the latitude by measuring
the altitude of Polaris. The latitude of an
observer is approximately equal to the
number of degrees Polaris is above the
horizon. The instrument used to
determine the altitude of a star is called a
sextant.
Many other stars are used as reference
points for determining latitude.
However, corrections must be made
since other stars do not lie directly over
the North Pole as does Polaris.
The Nautical Almanac contains charts
(Astronomical Tables) which provide
corrected data for determining latitude.
These charts take into account the date
(since the change in season affects a
star’s position in the heavens) in the time
of day that the observation is made.
The navigator can also determine the
latitude of his position by finding the
altitude of the sun. This is known as
“shooting the sun”. Again, he must
consult the astronomical tables to obtain
the correct values for his latitude
readings.
LONGITUDE
Meridians are semi-circles drawn
around the earth from the North to the
South Pole. Like parallels, they are
measured in degrees. These imaginary
lines lie at right angles to the parallel
lines of latitude. The position of a place
on the earth in relation to a particular
meridian is called longitude.
While the equator provides us with a
natural starting point and is designated
as 0o latitude, there is no natural
reference line for longitude. A meridian
called the prime meridian passes
through Greenwich, England.
Since meridians are drawn east ans
west of the prime meridian, they can be
drawn only halfway around the earth in
each direction. Thus, the meridian
which lies directly opposite the prime
meridian is 180o. This is called the
International Dateline. Chicago which
lies about halfway between the prime
meridian and the International Dateline
and is located west of the prime
meridian, has a longitude of about 90oW.
All meridians converge at the poles.
The width of each meridian, therefore,
changes as the latitude of a point on the
meridian changes, the width becoming
narrower as the latitude increase. For
this reason, no standards or constants of
distance can be used in calculating
longitude.
Finding Longitude. All points on a
given meridian have the same time.
Thus, if it is 12 noon at a point on the
75o meridian, it is 12 noon at ANY point
on that meridian. A day on earth—24
hours—is defined as the time it takes the
earth to make one complete rotation on
its axis. The earth must rotate at a rate
of 1o every 4 minutes or 15o per hour to
rotate through 360o in 24 hours.
Therefore, there is a difference of one
hour for every 15o of longitude.
You know that the sun rises in the
east and sets in the west. For this reason
a place in the east will receive the sun
before a place in the west. Therefore, a
traveler moving westward must turn his
watch back one hour for every 15o of
longitude traveled.
How can this information be used to
determine longitude? The prime
meridian is used as a reference point in
determining longitude. Navigators use
an instrument called a chronometer to
tell them Greenwich time. The
chronometer is a special clock with
numbers ranging from 1 to 24. Numbers
1 through 12 represent the time from
1am to 12 noon. Numbers 13 through
24 represent the hours from 1:00pm to
12:00 midnight. Therefore, it the
chronometer reads 15 o’clock
Greenwich time, it is 3pm in Greenwich,
England.
The Following illustrations
should help you understand how a
knowledge of Greenwich time helps a
navigator determine his longitude.
Example 1. Suppose the time in
Greenwich according to your
chronometer is 8pm and your local time
according to your wristwatch is 10pm.
What is your longitude?
(a) The time difference between
your position and that of Greenwich,
England, is two hours.
10pm – Local time.
-- 8pm -- Greenwich time
2 hour difference in time
(b) We know that for every hour
of time there are 15º of longitude. A
difference of two hours would indicate
the passage of 30º of longitude.
Therefore, you must be located on a
meridian of 30 º away from Greenwich,
England (0º longitude).
(c) Would you be located at
30ºW or 30ºE? Since your local time is
later than Greenwich time, you must be
located to the east of Greenwich. Your
longitude, therefore, must be 30º E.
Example 2. If you were located ion
New York City (75oW) and your local
time is 5am, what time would the
chronometer read?
(a) Since you are located on the
75oW meridian and
Greenwich, England is at 0o,
there is a five-hour time
difference.
(b) Because you are west of
Greenwich, Greenwich time
must be later than your local
time. Therefore, you must
add five hours to your local
time to find Greenwich time.
(c) Thus, the chronometer would
read 10am.
Example 3. What time would it be in
Greenwich, England, if the hands on the
chronometer indicated 18 o’clock.
(a) Since the numbers 1 through
12 indicate ante meridian
(am) time, the higher
numbers 13 through 24 must
indicate post meridian (pm)
time.
(b) All you need do to find
Greenwich time, if the
chronometer reads greater
than 12 noon, is to subtract
12 from that number.
(c) If your chronometer reads 18
and you subtract 12, then
Greenwich time must be
6pm.
Other Methods for Determining
Position. Heavy clouds may make it
impossible to find latitude of a position;
therefore, other methods must be used.
One method is called dead reckoning.
IN this method, the navigator carefully
records the speed of his craft and the
direction of travel. Knowing the point of
departure, the speed he is traveling, wind
or current movements, and the time
moved in a particular direction, the
navigator can locate his position on a
map.
Although plotting of positions on a
map by dead reckoning is not easy and
the results are not as accurate as those
obtained by other navigational methods,
dead reckoning is often used in bad
weather.
A very effective way of determining
position is the long range navigation
(Loran) method of electronic navigation.
Although it is an accurate way to
determine location in all kinds of
weather, there are still many areas of the
world beyond the range at which radio
signals can be received.
In its operation, radio signals are sent
out from pairs of radio stations. Aircraft
and ships equipped with special radio
receiving equipment can pick up the
radio signals and locate their positions
with much accuracy.
TIME
The measurement of time is based
largely upon the rate at which the earth
rotates on its axis and revolves around
the sun. A year, for example, is
measured by the time it takes the earth to
make one complete revolution around
the sun.
This poses a serious problem to
astronomers and space scientists since
such a measurement cannot be
accurately made. The period of the
earth’s revolution about the sun will
vary, depending upon the starting point
and the point at which the year ends.
How then can we tell how long a year
is?
The Year. In general, what we mean by
a year is a tropical year; that is, the
interval of time between two vernal
equinoxes. (The vernal equinox occurs
on or about March 21 and marks the
beginning of spring in the Northern
Hemisphere.) The tropical year has a
length of 365 days, 5 hours, 48 minutes,
and 46 seconds.
Since our calendar year is exactly 365
days, we are left with an extra ¼ day
(about 6 hours) every tropical year. To
make up for this excess or lost time,
Julius Caesar devised a calendar as the
Julian Calendar. Caesar ordered that one
extra day (February 29) be added to the
calendar every fourth year (which was to
be known as the “leap year”). However,
a new problem was created when one
day was added every fourth year. The
year became 11 minutes, 14 seconds too
long.
In 1582, a new more accurate
was introduced by Pope Gregory XIII.
This calendar, known as the Gregorian
Calendar, kept Caesar’s principle of
every fourth year being a leap year so
long as the numeral of the year was
divisible by 4. Thus, 1584 was a leap
year, but 1585 was not. The next leap
year occurred in 1588.
The major difference between the
Julian Calendar and the Gregorian
Calendar concerned the century years.
While all century years would be leap
years (divisible by 4) according to the
Julian Calendar, only those century years
divisible by 400 would be leap years
according to the Gregorian Calendar.
Thus, the year 1600 was a leap year, but
the years 1700, 1800, 1900 were not.
The next century year which will
be a leap year will be the year 2000. An
error of less than one day in 300 years
still exists in the Gregorian Calendar
which is the one we still use today.
A Day. The length of a day is based on
the rotation on its axis. It is determined
by the passing of the sun of another star
across a meridian twice in succession. If
the heavenly body crossing the meridian
is the sun, we call the day a solar day; if
it is another star then we refer to the day
as a sidereal day.
Apparent Solar Day. The time it takes
the sun to cross the observers meridian
twice in succession is called an apparent
solar day. Apparent solar noon occurs
when the sun is directly over the
observers meridian. However, since the
earth is moving in it’s orbit around the
sun at the same time it is rotating, the
earth must rotate about 1˚ more on it’s
axis to bring the sun over the meridian
again. Thus, the sun would arrive about
4 minutes (1̊ = 4 minutes) late the next
day.
During the course of the year, the
observed solar day would vary in length.
To compensate for the variation in time,
an average, or mean length of soalr time
has been calculated. The result is a 24-
hour day which is called a mean solar
day. Mean solar time is the time at
which we set our clocks.
across the earth with the prime meridian
as their starting point.
INTERNATIONAL DATELINE
Sidereal Day. It is important to realize
that although the earth rotates on it’s
axis we do not feel the sensation of
movement. Instead, it seems as if all the
stars in the celestial sphere revolve
around us. Therefore, a sidereal day is
defined as the period of time for a
particular star to cross a meridian twice
in succession as the celestial makes one
full turn.
If we compare the length of a
sidereal day to that of a mean solar day
we find the sidereal day to be 4 minutes
short. Thus, the length of a sidereal day
is 23-hours, 56-minutes.
Standard Time. Before 1883, each city
or town set its clocks to correspond to
apparent solar time. Thus, 12 o’clock
noon would occur in each city when the
sun was in its highest point in the sky.
As people began traveling across the
United States, they found they had to
keep resetting their watches every time
they crossed another meridian. Railroad
timetables caused confusion due to this
lack of standardized time-keeping.
Finally, in 1883, four standard
time zones were set up across the
country. Each time zone was 15o wide
with the time within each zone
uniformly that of the meridian located
near its center. Therefore, standard is the
mean solar time of the meridian of that
particular zone.
The four standard time zones
with their center points are (1) Eastern
Standard time (750 W.), Central
Standard time (900 W.), Mountain
Standard time (1050 W.) and Pacific
Standard time (1200 W.). Twenty-four
standard time zones have been set up
Opposite the prime meridian on the other
side of the earth lies the 1800 meridian.
Running close to this meridian is the
International Dateline. It is here that the
new day begins. If you were to travel
westward across the International
Dateline, you would continue to turn
your clock back one hour. In addition,
you would have to move your calendar
ahead one full day when you crossed the
International Dateline.
If you crossed the dateline while
traveling eastward, you would have to
turn your calendar back one full day.
Thus, if you left New York on
Wednesday at 3 pm, and could travel
completely around the world going
westward in six hours, you would arrive
back in New York the same day. If not
for the dateline, you might arrive in New
York the day before you left.