HARMONIC ANALYSIS! AND PREDICTION OF TIDES 5 
calations of months and days being arbitrarily made by the priesthood 
and magistrates to bring the calendar into accord with the seasons 
and for other purposes. 
16. The Julian calendar received its name from Julius Cesar, who 
introduced it in the year 45 B. C. This calendar provided that the 
common year should consist of 365 days and every fourth year of 366 
days, each year to begin on January 1. As proposed by Julius Cesar, 
the 12 months beginning with January were to be alternately 31 days 
and 30 days in length with the exception that February should have 
only 29 days in the common years. When Augustus succeeded 
Julius Cesar a few years later, he slightly modified this arrangement 
‘by transferring one day from February to the month of Sextilis, or 
August as it was then renamed, and also transferred the 31st day of 
September and November to October and December to avoid having 
three 31-day months in succession. 
17. The Gregorian calendar received its name from Pope Gregory, 
who introduced it in the year 1582. It was immediately adopted by 
the Catholic countries but was not accepted by England until 1752. 
This calendar differs from the Julian calendar in having the century 
years not exactly divisible by 400 to consist of only 365 days, while 
in the Julian calendar every century year as well as every other year 
divisible by 4 is taken as a leap year with 366 days. For dates before 
Christ the year number must be diminished by 1 before testing its 
divisibility by 4 or 400 since the year 1 B. C. corresponds to the year 
0 A. D. The Gregorian calendar will gain on the Julian calendar 
three days in each 400 years. When originally adopted, in order to 
adjust the Gregorian calendar so that the vernal equinox should 
fall upon March 21, as it had at the time of the Council of Nice in 
325 A. D., 10 days were dropped and it was ordered that the day 
following October 4, 1582 of the Julian calendar should be designated 
as October 15, 1582 of the Gregorian calendar. This difference of 
10 days between the dates of the two calendars continued until 1700, 
which was a leap year according to the Julian calendar and a com- 
mon year by the Gregorian calendar. The difference between the 
two then became 11 days and in 1800 was increased to 12 days. 
Since 1900 the difference has been 13 days and will remain the same 
until the year 2100. 
18. Dates of the Christian era prior to October 4, 1582, will, in 
general, conform to the Julian calendar. Since that time both cal- 
endars have been used. The Gregorian calendar was adopted in 
England by an act of Parliament passed in 1751, which provided 
that the day following September 2, 1752, should be called September 
14, 1752, and also that the year 1752 and subsequent years should 
commence on the Ist day of January. Previous to this the legal 
year in England commenced on March 25. Except for this arbitrary 
beginning of the year, the old English calendar was the same as the 
Julian calendar. When Alaska was purchased from Russia by the 
United States, its calendar was altered by 11 days, one of these days 
being necessary because of the difference between the Asiatic and 
American dates when compared across the one hundred and eightieth 
meridian. Dates in the tables at the back of this volume refer to 
the Gregorian calendar. 
19. The three great circles formed by the intersections of the planes 
of the earth’s equator, the evliptic, and the moon’s orbit with the 
