The earth’s circumference (which has the form of a circle) at the
equator is (3.1416 × 7926), 24900 miles; which divided by 360, gives
69.17 miles for 1 degree of longitude at the equator. Leaving the
equator, degrees of longitude gradually diminish, since all meridians
converge at the poles. Thus, 1 degree of longitude, at 10 degrees of
latitude, is 68.1 miles; at 20 degrees 65 miles; at 30 degrees 59.9
miles; at 40 degrees 53 miles; at 50 degrees 44.5 miles; at 60 degrees
34.6 miles; at 70 degrees 23.7 miles; at 80 degrees 12 miles; at 90
degrees 0.
Imaginary lines running north and south, through these degrees, from
pole to pole, are called _meridians_. Those east and west, _parallels_.
One meridian which runs through Greenwich, near London, England, is
called the _prime meridian_, and all the other meridians are reckoned as
east or west of it.
_Longitude_ is distance east or west of the prime meridian. When we say
that the longitude of Paris is 2° 20′ East, we mean that the meridian
running through Paris is 2° 20′ east of the prime meridian that runs
through Greenwich, England. The longitude of Washington, D. C., is 77°
7′ West. That means that the meridian which passes through Washington is
77° 7′ west of the prime meridian.
The longitude of a place tells in degrees, minutes, and seconds, the
distance it is east or west of the prime meridian.
[Illustration]
RULE.--_To find the difference of time between two places, when the
difference of longitude is known, or vice versa, multiply the given
longitude, expressed in degrees, by 4. This gives the equivalent time in
minutes. Dividing the given time, expressed in minutes, by 4, gives the
equivalent longitude in degrees._
EXAMPLE: The difference of longitude between Boston and San Francisco is
nearly 51-1/4°, what is the difference of time?
51-1/4 × 4 = 205. 205 minutes equal 3 hours and 25 minutes. _Ans._ 3
hours and 25 minutes.
The difference of time between London and New York is about 4 hours and
55-1/2 minutes, what is the difference of longitude?
4 hours and 55-1/2 minutes equals 295-1/2 minutes. 295-1/2 ÷ 4 = 73-7/8.
_Ans._ 73-7/8°.