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S. W. N. by W. 3 5 4 4 6 4
N. W. b. N. 7 2
70 W. by S. 7 6 1 6 8 6 4
7 0 10 8 0 11 7 6 N. W. by W. S. W. b.W. 12 7
6 2 6 4 3 6 2 4 5 6 5 5 6 4
S. by E. 7 4 8 9 10
West. S.S. W. 11 6 4 12 6 6
A current set the ship E. by S.AS. (by
compass) 15 miles in the 24 hours.
1 2 3
P.M. I take my departure from a point
of land, in Lat. 34° 26'S. Long. 1720 38’E. bearing by compass E. by S.As. distant 17 miles.
6 4 26 N. W.b. W. S. E. 6 6 0 5 8 S.b. W. W. S. E. E. 6
5 6 7 8 9 10 11 12
4 7 6 4 5
Variation 1 pts E.
TIME being a measured portion of infinite duration, any event which recurs at equal intervals might be taken as the unit of admeasurement, and to this end, nothing seems more appropriate than that the required standard should be sought among the innumerable celestial orbs, which appear to be continually revolving around our Earth.
Numberless observations, made in different ages of the world, have proved that the time intervening between two consecutive passages of the same star over a given meridian, is uniform and unchanging. Here then, we have a standard more exact than any that could be devised by art. Owing, however, to the elipticity of the earth's orbit, and the plane of the equator not being coincident with that of the ecliptic, this measure of time, (called by astronomers the sidereal day,) which is wholly uninfluenced by such phenomena, does not agree with the interval measured by the apparent revolution of the Sun around our earth; and since, if it had been adopted as the unit of admeasurement, the day (by which term is understood a certain fixed and uniform period of time,) would have commenced at no regular instant, as regards the rising, setting, or meridian transit of that luminary, which is at once the centre of our system and the source of light and heat; it seems therefore not only more natural, but certainly more convenient for the ordinary purposes of life, that the "working day" should be regulated by the Sun.
Now the true solar, or apparent day, which is measured by two successive passages of the sun over the meridian of any place on the earth's surface, is a variable quantity ; but
there is a regular succession of its variations, which in a certain period, termed a year, come to an end, to be commenced
In order, therefore, to obtain a convenient and equable measure of time, astronomers assume a mean solar day, the length of which is equal to the average of all the apparent solar days in the year.
The interval by which the apparent is in excess or falls short of the mean day, constitutes the equation of time; this difference is greatest about the 3rd of November, and four times in a year viz. April 15, June 15, September 15, and December 24, it vanishes, or is exceedingly small.
We next notice two methods of commencing the day, which unless fully understood, must be productive of very considerable error in working the various questions which arise in Nautical Astronomy.
1. The astronomical day begins at noon, and its minor divisions are reckoned from that instant, or Oh Om Os to 24", continuously.
2. The civil day, which is the one used in reference to the ordinary transactions of life, commences at midnight and precedes the astronomical day by 12 hours, but its minor divisions are not counted successively to 24 hours; the interval from midnight to noon being styled A.m., and that from noon to midnight again P.M., each reckoned to 12 hours.
The distinction between the astronomical and civil day being clearly understood, it will at once be evident that from noon to midnight, the day of the month and the hour of the day are the same in both methods of reckoning; but from midnight to noon they differ ; in illustration, turn to the Nautical Almanac for the year 1854, page 564; at the top of the first column, under January, is found 1d 2h 7 O in perigee; this is in astronomical time, and coincides with January 1st, 2h 7m P.M. civil time. The next phenomen on on the same page is marked thus, in astronomical time, id 23h 53m f o l, which expressed in civil time would be January 2nd, 11' 53m A.M.
So in like manner, a morning observation of a celestial object made under any meridian, say March 20, (civil time) at 8h 40m A.M. would be represented astronomically thus, March 198 206 40m; while another observation made on the same day, but in the evening, as thus, March 20 (civil time) at 46 20m P.M. is expressed in astronomical time, March 200 4h 20m; and the application of the longitude in time, (by addition if westerly, by subtraction if easterly,) to the given or estimated time of observation, will be the corresponding Greenwich time.
Since all the elements of the heavenly bodies given in the Nautical Almanac bear reference to the astronomical day, its relation to the civil mode of reckoning must never be lost sight of; for unless the true Greenwich astronomical date be known at the time an observation is made to determine latitude, longitude, or variation of the compass, &c. it is impossible that the solution of the question can be correct.
The sidereal day, to which we have already referred, consists of 236 56 4.098 mean solar time. In the determination of certain elements by means of a Planet, the Moon, or a Fixed Star, the operation is somewhat shortened by a knowledge of sidereal time.
We have made these remarks solely to impress on Navigators the importance of making themselves perfectly acquainted with time, which can be done by a careful perusal of the “Epitomes," and it cannot be too strongly urged that it is also especially desirable that every one using the Nautical Almanac should make himself thoroughly acquainted with its contents, to aecomplish which it is necessary to peruse very carefully the “Explanations” at the end of that book,