Longitude and Time
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 [865] 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.
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.
TABLE OF THE DIVISIONS OF TIME
| 60 | seconds (sec.) | = | 1 minute (min.) |
| 60 | minutes | = | 1 hour (hr.) |
| 24 | hours | = | 1 day (da.) |
| 7 | days | = | 1 week (wk.) |
| 30 | days | = | 1 commercial month (mo.) |
| 52 | weeks | = | 1 year (yr.) |
| 12 | months | = | 1 year |
| 360 | days | = | 1 commercial year |
| 365 | days | = | 1 common year |
| 366 | days | = | 1 leap year |
| 100 | years | = | 1 century |
| Century | Years | Months | Days | Hours | Minutes | |||||
| 1 | = | 100 | = | 1,200 | = | 36,500 | = | 876,000 | = | 52,560,000 |
Centennial years exactly divisible by 400, and other years exactly divisible by 4, are Leap Years.
Why We Have Leap Year
The average time it takes the earth to revolve once around the sun (one year) is 365 days, 5 hours, 48 minutes, 47.8 seconds, or about 3651⁄4 days.
The change in the length of the mean sidereal day, i.e., of the time of the earth’s rotation upon its axis, amounts to 0.01252 seconds in 2400 mean solar years.
Instead of reckoning this part of a day each year, it is disregarded, and an addition is made when this amounts to one day, which is very nearly every fourth year. This addition of one day is made to the month of February. Since the part of a day that is disregarded when 365 days are considered as a year, is a little less than one quarter of a day, the addition of one day every fourth year is a little too much, and, to correct this excess, addition is made to only every fourth centennial year.