MARYLAND GEOLOGICAL SURVEY 521

The substance of the above rule may be otherwise stated as follows:
The astronomical time is the hours and minutes elapsed since the
NOON LAST PAST, the astronomical DATE being that of the civil day to
which the noon belongs. Thus, April 23, 4.15 p. m., civil time, is
April 23, 4h. 15m., astronomical time, and April 23, 4.15 a. in., civil
time, is April 22, 16h. 15m., astronomical time.

Hour Angle of Polaris.—In Fig. 9 the full vertical line repre-
sents a portion of the meridian passing through the zenith Z (the
point directly overhead) and intersecting the northern horizon at the
north point N, from which, for surveying purposes, the azimuths of
Polaris are reckoned east or west. The meridian is pointed out by
the plumb-line when it is in the same plane with the eye of the
observer and Polaris on the meridian, and a visual representation is
also seen in the vertical wire of the transit, when it bisects the star
on the meridian.

When Polaris crosses the meridian it is said to culminate; above
the pole (at S} the passage is called the upper culmination, in con-
tradistinction to the lower culmination (at S'}.

In the diagram—which the surveyor may better understand by
holding it up perpendicular to the line of sight when he looks toward
the pole—Polaris is supposed to be on the meridian, where it will be
about noon on April 10 of each year. The star appears to revolve
around the pole, in the direction of the arrows, once in every 23h.
56.1m. (23 hours, 56 minutes, 4.09 seconds) of mean solar time; it
consequently comes to and crosses the meridian, or culminates, nearly
four minutes earlier each successive day. The apparent motion of
the star being uniform, one quarter of the circle will (omitting frac-
tions) be described in 5h. 59m., one-half in llh. 58m., and three-
quarters in 17h. 57m. For the positions Si, s₂, S₃, etc., the angles
SPs!, SPs₂ SPs₃, etc., are called Hour Angles of Polaris for the
instant the star is at Si, s₂, or s₃, etc., and they are measured by the
arcs Ss_i} Ss_2} Ss₃₎ etc., expressed (in these instructions) in mean solar
(common clock) time, and are always counted from the upper meridian
(at S) to the west, around the circle from Oh. Om. to 23h. 56m.l, and
may have any value between the limits named. The hour angles,