PRELIMINARY RULES IN NAUTICAL ASTRONOMY.

CIVIL AND ASTRONOMICAL DAY.

261. The Civil Day, or common method of reckoning time, begins at midnight, and ends the following midnight, the interval being divided into two periods of 12 hours each; the first twelve hours, from midnight to noon, are denoted by A.M. (ante meridian); the latter, from noon to midnight are styled P.M. (post meridian); thus we say 10 A. M. when an event occured at 10 o'clock in the morning, and 10 P.M. when it occured at 10 o'clock in the evening.

262. The Astronomical Day begins at noon and ends at the following noon, and is later than the civil day by twelve hours. The hours are reckoned throughout, or continuously from oh to 24". The distinction of A.M. and P.M. is not recognised in astronomical time. Thus, 11 o'clock in the forenoon of the second of January in the civil reckoning of time corresponds to January 1 day 23 hours in the astronomical reckoning; and 1 o'clock in the afternoon of the former to January 2 days 1 hour of the latter reckoning.

263. Since the civil day commences at the midnight preceding the noon which commences the astronomical day, it is evident that the civil mode of reckoning is always twelve hours in advance of the civil reckoning, and hence we have the following Rule for converting civil into astronomical time.

Given civil time at ship, to reduce it to astronomical time.

RULE LXXVII.

1o. If the civil time at ship be P.M., it will also be astronomical time, P.M. being omitted.

2°. If the civil time be A.M., add twelve to the hours and subtract one from the days of the month; also omit A.M. The result in each case is the Astronomical Date.

EXAMPLES.

Ex. 1. May 10th, at 5h 30m P.M., civil time is 5h 30m astronomical time of the same date; because the 10th astronomical day begins at noon of the 10th civil day, and 5h 30m have elapsed since that noon. But 5h 30m A.M. civil time on May 10th is 17h 30m astronomical time on the 9th of May, for the 9th day of the month, according to the astronomical reckoning, commences at noon of the 9th civil time, and ends at noon of the 10th civil day (the hours being reckoned up to 24), and 5h 30m A.M. of the 10th is 17" 30m from noon of the 9th. Ex. 2. October 7th, at 3h 20m P.M., civil time, is October 7th, at 3h 20m astronomical time. (See 1° of Rule LXXVII.)

Ex. 3.

October 7th, at 3h 20m A.M., civil date, is October 6 15h 20m astronomical date; since 7d less id is 6d, and 12 added to 3h 20m is 15h 20m. (See 2° of Rule).

Ex. 4. January 31st, at 7h 20m P.M., civil time, is January 31st, at 7h 20 astronomical time. (Rule LXXVII, 1°.)