While affording such accurate methods of determining time, does not
directly furnish that by which we are accustomed to be guided, so that
astronomers are wont to speak of _solar time_, as _mean solar time_, and
_apparent solar time_, the former being that kept by our clocks and the
latter that from which the former is estimated. If the earth moved around
the sun at an unvarying rate, then each time the sun threw the shadow
of a vertical rod directly north (in the northern hemisphere) would
indicate noon at the point where the rod was located; and the interval
between two such successive shadows would be exactly one day, and we
could divide the time into twenty-four equal spaces and call each space
one hour. Now, this is what we imagine the earth to do; or rather, for
the sake of convenience, and because the results are the same in either
case, we conceive the sun to move around the earth, and make a _mean_ or
_mock sun_, move around the earth in a day of twenty-four hours of equal
unvarying length, and call this a _mean solar day_. But the _true_ or
_apparent solar day_ is considerably different from this, being sometimes
less and sometimes more than twenty-four hours in length, the _true sun_
reaching the meridian as much as 16¼ minutes before or after the _mean
sun_, both reaching it together only four times each year, viz.: On
April 1, June 15, September 1, and December 24. Of course it would be
impossible to construct a time-piece that would keep pace with the _true
sun_. Indeed, it is difficult enough to construct one that will keep with
the mean sun. But all difficulty is obviated by making clocks whose _rate
of error_ can be determined. This rate being known it is easy to estimate
the correction, and thus obtain exact time. For example, suppose a clock
to _gain_ 0.24 of a second per day, then in two days it will gain two
times 0.24 of a second, and in three days three times 0.24 of a second,
etc. These amounts subtracted from the noon-time of the clock would give
the correct noon. For any other hour, a part of the 0.24 depending on
the number of hours after noon must be subtracted. If the clock _loses_,
then in a similar manner the proportioned loss must be added. In actual
practice, we may say that even the best chronometers do not keep exact
time; and every one has to be “corrected” in the manner indicated.
To obtain mean time from apparent time we apply to apparent time a
correction called the equation of time. Thus, at noon, on October 1st,
in Washington, the equation of time is _minus_ 10 minutes 34.58s., which
means that if from 12 hours we subtract 10m. 34.58s. we shall have 11h.
49m. 25.42s., the mean time of apparent noon, or noon as indicated by the
north shadow of the vertical rod. Or, if at 10m. 34.58s. after the sun
crosses the meridian we set our watch at 12 o’clock, we shall have exact
mean time. On the 31st of October the real sun will be 16m. 17.56s. ahead
of the mean sun, while on the 31st of December it will be 3m. 38.11s.
behind the mean; that is, will reach the meridian at 12h. 3m. 38.11s. p.
m. mean time.
Beside the ordinary clocks, the chronometers used by navigators keep mean
solar time, and family almanacs usually in some form give the “equation
of time” under the headings, “clock slow” or “fast,” or “sun south.”
Astronomers use also another kind of time called “sidereal,” of which we
may have something to say in the future.
On the 1st, 16th and 31st of this month the sun rises at 5:57, 6:13
and 6:30 a. m., and sets on the same days at 5:42, 5:18 and 4:58 p. m.
respectively, showing a decrease of one hour and seventeen minutes in
thirty days, or at an average rate of 2m. 34s. per day. On the 18th and
19th the sun will be eclipsed, entering the moon’s shadow at 10:20 p. m.
on the 18th in longitude 132° 0.6′ east, and in latitude 63° 30.5′ north;
and leaving it at 2:15 a. m. on the 19th in longitude 134° 22.7′ west,
and in latitude 33° 25′ north. Greatest eclipse occurs at 12:18 a. m. on
the 19th. Magnitude, 0.638. As the entire eclipse is during our night,
it will of course be invisible to us, but can be seen by the inhabitants
of the northeastern part of Asia and the northwestern portion of North
America.