520 FIRST REPORT UPON MAGNETIC WORK

IV.——DETERMINATION OF THE AZIMUTH OF POLARIS AND TRUE MERIDIAN

AT ANY HOUR, THE STAR BEING VISIBLE AND THE CORRECT LOCAL

MEAN TIME BEING KNOWN.

Many years ago a table was published giving azimuths of Polaris
at stated times during one year, but as it was arranged for a kind of
time with which surveyors are generally unacquainted, and was ex-
plained in unfamiliar astronomical terms, and required the use of
tables and data not always accessible, it met with little favor and
never came into general use.

In this article it is proposed to simplify the work, omit all tech-
nicalities requiring a knowledge of astronomy, and present the method
with two new and compact tables adapted to common clock time, with
such plain directions for use that any person of ordinary intelligence
can understand and apply them.¹

As the surveyor should have a perfectly clear idea of what is meant
by astronomical time (used to simplify computations) and the hour
angle of Polaris, these terms will now be explained.

The Civil Day, according to customs of society, commences at midnight
and comprises twenty-four hours from one midnight to the next following.
The hours are counted from O to 12 from midnight to noon, after which they
are again reckoned from O to 12 from noon to midnight. Thus the day is
divided into two portions of 12 hours each, the first of which is marked
a. m., the last p. m.

The Astronomical Day commences at noon on the civil day of the same
date. It also comprises twenty-four hours; but they are reckoned from O
to 24, and from the noon of one clay to that of the next following.

The civil day begins twelve hours before the astronomical day; therefore the
first period of the civil day answers to the last part of the preceding astro-
nomical day, and the last part of the civil day corresponds to the first part
of the astronomical day. Thus, January 9, 2 o'clock p. m., civil time, is
also January 9, 2h astronomical time; and January 9, 2 o'clock a. m., civil
time, is January 8, 14h, astronomical time.

The rule, then, for the transformation of civil time into astrono-
mical time is this: // the civil time is marked p, m., take away the
designation p. m., and the astronomical time is had without further
change; if the civil time is marked a. m., take one from the day and
add twelve to the hours, remove the initials a. m., and the result is the
astronomical time wanted.

¹ See foot-note, p. 511.