Daily Kennebec journal. [microfilm reel] (Augusta, Me.) 1870-1975, 1913-01-20
THE EARTH'S JOURNEY.
Turns once More Than Days in Year
Number.


A day is generally supposed to be
the time that the earth takes to mako
one complete revolution on its axis
but actually the earth makes every
year one more revolution than there
are days in the year.
When we speak of day we mean a
day of exactly "a hours as measured
on our clocks, and this day fixed by
law To regulate the affairs of every
civic, or citizen. But how has it
come about that our day is exactly "a
hours long? The time taken by the
earth to turn round once on its axis
is not "a hours. but 2S hours Sc min
| utes seconds, nearly four minutes
shorter than the civil day.
There are three kinds of days-the
sidereal, or star, day; the solar, or
sun, day; and the mean solar day,
which is the same as the civil day of
our business and social life.
Astronomers always reckon time by
sidereal days, says the Chileren's
Magazine, because they are the most
accurate for measuring. and there are
1861 of them in a year. The sidereal,
or star, day gets its name from the
Latin word for star. sidus, and its
length is measured by means of the
stars. Some particular star is taken
and the exact moment at which it
reaches the meridian--that is, its
highest point in the heavens--is not
ed Then the slime that elapses he
fore it reaches the same point in the
sky on the next night is counted, and
that time is a sidereal day. No mat
ter what star is observed. or what
time in the year the calculation is
made, a sidereal day is always the
same length-namely. % hours Sc
minutes 4 seconds, and this is the
time that the earth takes to make one
revolution on it's axis. The appar-
ent movement of a star in the heaV-
ems as it goes up to its highest point
bis, of course, caused by the rotation
of the earth.
The day is measured by the sun,
too. This is the solar day, which,
| however is not of the same length as
the sidereal day, and is not the same
length at all times of the year, 1t
| is always longer than a star day, but
at some periods of the year the sun
day is nearly a minute longer than at
others. owing to the earth having
moved along in its orbit, it has to
turn round on it's axis a little more to
bring the sun exactly to the meridian.
The result is that q solar day is not
simply the time the earth takes to
turn completely on its axis but this
time plus the time it takes to make
the little extra turn to bring the sun
| to the meridian.
Now, the passage of the earth
1 or
Gle but in an ellipse, and therefore in
some parts of it's journeys the earth
travels faster than at other times.
The result is that the time occupied
by the extra bit of turning to bring
the sun to the meridian varies ac-
cording to the speed at which the
earth happens to be travelling round
the sun, and the solar day is there
fore different lengths at different
times in the year. 1t is longest when
the earth is travelling fastest, and
shortest when the earth moves slow
est in its orbit. II would be very
awkward If in business and in school
we had days varying in length at
different times of the year, and so a
uniform day for all the year has to be
| worked out by taking the average of
all the solar days. This is the civil
days of our clocks. 1t is called the
mean solar day, and is exactly "a
| hours long