126 
THE GUIDE TO NATURE 
is brightest March 17 in the evening 
and May 28 in the morning. 
Since the angle of elongation is in- 
creasing it is evident that Venus will 
be farther east of the sun each night 
and set later until February 9, and then 
earlier each evening until it sets with 
the sun on April 22. Starting with 
February 9 the line joining the earth 
with Venus will be seen to be moving 
farther from a horizontal direction until 
March 31 and then again more nearly 
horizontal on April 22. When the line 
is farther from the horizontal as it is 
on March 31 the planet is said to be 
stationary. It then ceases its eastward 
motion among the stars and begins its 
westward motion. This is illustrated 
in Figure 3, in which we draw lines 
from the earth in the direction and of 
the length shown in Figure 2 for each 
position of the earth. On May 1 1 the 
westward motion ceases and the east- 
ward motion begins again. 
What is a Year? 
At the beginning of a new year it 
may be well to ask what a year is? 
Nearly if not everyone thinks he knows 
what a year is, yet their definitions 
would probably differ widely. There 
are several kinds of years. We ordi- 
narily think of the year as the interval 
designated on the calendar as a year — 
namely, the civil year. Civil years are 
either common years of 365 days or 
leap years of 366 days. It begins at 
midnight of December 31 and ends at 
midnight of the following December 
31. The year did not always begin at 
this time. Using each fourth year as a 
leap year, the average length of a year 
is 36 5)4 days. This is called the Julian 
year because it is the year of the Julian 
calendar. If we use the Gregorian 
calendar as we do, omitting as leap 
years the century years which are not 
evenly divisible by 400, we use a Greg- 
orian year. These years are merely 
attempts to express years as a given 
number of days. There is no necessary 
relation between the length of a year 
and the length of a day, hence no per- 
fect adjustment can be made. But 
what is a year? Some one says that 
it is the time required by the earth to 
make its revolution around the sun. 
Observation of the sun’s position with 
respect to the stars shows that the earth 
is moving around the sun. If we could 
not see the stars we would not know 
that the earth moved around the sun. 
Since we cannot see the stars in the 
same direction as the sun we must get 
around this difficulty. It may be, too, 
that the stars have moved while the 
earth has made its revolution. When 
these difficulties are overcome and we 
find the interval from the time that the 
sun appears in a fixed direction until it 
appears in the same direction again, we 
have determined what is known as a 
sidereal year. 
The most obvious effect of the 
earth’s revolution about the sun is the 
change of seasons. It is the sun’s po- 
sition with respect to the earth’s 
equator which causes this change. The 
year of the seasons is the interval from 
the time that the sun crosses the 
equator until it crosses again in the 
same direction. This interval is the 
tropical year. It is not quite the same 
as the sidereal year, because the points 
at which the sun crosses the equator 
(the equinoxes) change their position. 
Our calendars try to adjust this tropical 
year to the day. 
How long are these years? Are they 
uniform? Is one year as defined equal 
to another? It is not. The years differ 
from each other in length by either of 
these definitions. The earth’s motion 
about the sun is so full of irregularities 
that much computation is required to 
determine the length of any particular 
rear. Even when the irregularities 
are omitted the length of the year 
