The longitude of a place tells in degrees, minutes, and seconds, the distance it is east or west of the prime meridian.
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 511⁄4°, what is the difference of time?
511⁄4 × 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 551⁄2 minutes, what is the difference of longitude?
4 hours and 551⁄2 minutes equals 2951⁄2 minutes. 2951⁄2 ÷ 4 = 737⁄8. Ans. 737⁄8°.
Measures of Time
The unit of time measurement is the same among all nations. Practically it is 1-86400 of the mean solar day, but really it is a perfectly arbitrary unit, as the length of the mean solar day is not constant for any two periods of time. There is no constant natural unit of time.
Time measures are used in telling the time of day, in problems in longitude and time, in figuring interest on notes and bills, and in numerous other ways.