4 U. S. COAST AND GEODETIC SURVEY 
Prior to the year 1925, the astronomical day was in general use by 
astronomers for the recording of astronomical data, but beginning 
with the Ephemeris and Nautical Almanac published in 1925 the 
civil day has been adopted for the calculations. Each day of what- 
ever kind may be divided into 24 equal parts known as hours which 
are qualified by the name of the kind of day of which they are a part, 
as sidereal hour, solar hour, lunar hour, or constituent hour. 
12. The moon revolves around the earth in an elliptical orbit. Al- 
though the average eccentricity of this orbit remains approximately 
constant for long periods of time, there are a number of perturbations 
in the moon’s motion due, primarily, to the attractive force of the sun. 
Besides the revolution of the line of apsides and the regression of the 
nodes which take place more or less slowly, the principal inequalities 
in the moon’s motion which affect the tides are the evection and 
variation. The evection depends upon the alternate increase and 
decrease of the eccentricity of the moon’s orbit, which is always a 
maximum when the sun is passing the moon’s line of apsides, and a 
minimum when the sun is at right angles to it. The variation ine- 
quality is due mainly to the tangential component of the disturbing - 
force. The period of the revolution of the moon around the earth 
is called a month. The month is designated as sidereal, tropical, 
anomalistic, nodical, or synodical, according to whether the revolution 
is relative to a fixed star, the vernal equinox, the perigee, the ascend- 
ing node, or the sun. The calendar month is a rough approximation 
to the synodical month. 
13. It is customary to refer to the revolution of the earth around 
the sun, although it may be more accurately stated that they both 
revolve around their common center of gravity; but if we imagine 
the earth as fixed, the sun will describe an apparent path around the 
earth which is the same in size and form as the orbit of the earth 
around the sun, and the effect upon the tides would be the same. 
This orbit is an ellipse with an eccentricity that changes so slowly 
that it may be considered as practically constant. The period of 
the revolution of the earth around the sun is a year, but there are 
several kinds of years. The sidereal year is a revolution with respect 
to a fixed star, the tropical year is a revolution with respect to the 
vernal equinox, the eclipse year is a revolution with respect to the 
moon’s ascending node, and the anomalistic year is a revolution with 
respect to the solar perigee. 
14. A calendar year consists of an integral number of mean solar 
days and may be a common year of 365 days or a leap year of 366 days, 
these years being selected according to the calendars described below 
so that the average length will agree as nearly as practicable with 
the length of the tropical year which fixes the periodic changes in the 
seasons. The average length of the calendar year by the Julian 
calendar is exactly 365.25 days and by the Gregorian calendar 365.2425 
days and these may be designated respectively as a Julian year and a 
Gregorian year. 
15. The two principal kinds of calendars in use by most of the 
civilized world since the beginning of the Christian era are the Julian 
and the Gregorian calendars, the latter being the modern calendar in 
which the dates are sometimes referred to as ‘‘new style” to dis- 
tinguish them from the dates of the older calendars. Prior to the 
year 45 B. C. there was more or less confusion in the calendars, inter- 
