Obrazy na stronie
PDF
ePub

appears that the autumnal equinox at the creation
was on the 25th of October.

[ocr errors]

Precesssion of the Equinoctial Anticipation
Points in the Heavens.

of the Equi-
noxes on the

Earth.
Motion.

Tine.

Julian
years.

[ocr errors][merged small]
[merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small]

seasons.

249. The anticipation of the equinoxes, and con- The antisequently of the seasons, is by no means owing to the equithe precession of the equinoctial and solstitial points noxes and in the heavens (which can only affect the apparent motions, places and declinations of the fixed stars) but to the difference between the civil and solar year, which is 11 minutes 3 seconds; the civil

year containing 365 days 6 hours, and the solar year 365 days 5 hours 48 minutes 57 seconds. The next following table, page 189, shews the length, and consequently the difference of any number of side. real, civil and solar years, from 1 to 10,000.

250. The above 11 minutes 3 seconds, by which the rea. the civil or Julian year, exceeds the solar, amounts tering the to 11 days in 1433 years: and so much our seasons style. have fallen back with respect to the days of the months, since the time of the Nicene council in A. D. 325; and therefore, in order to bring back all the fasts and festivals to the days then settled, it was requisite to suppres 11 nominal days. And that the same seasons might be kept to the same times of the year for the future, to leave out the Bissex.

PLATE

VI.

tile-day in February at the end of every century of years where the significant figures are not divisible by 4; reckoning them only common years, as the 17th, 18th, and 19th centuries, viz. the years 1700, 1800, 1900, &c. because a day intercalated every fourth year was too much, and retaining the Bissextile-day at the end of those centuries of years which are divisible by 4, as the 16th, 20th and 24th centuries; piz. the years. 1600, 2000, 2400, &c. Otherwise, in length of time, the seasons would be quite reversed with regard to the months of the year; though it would have required near 23,783 years to have brought about such a total change. If the Earth had made exactly 3654 diurnal rotations on its axis, while it revolved from any equinoctial or solstitial point to the same again, the civil and solar years would always have kept pace together, and the style would never have required any alteration.

251. Having already mentioned the cause of the cesssion of precession of the equinoctial points in the heavens,

246, which occasions a slow deviation of the points. Earth's axis from its parallelism, and thereby a

change of the declination of the stars from the equa- . tor, together with a slow apparent motion of the stars forward with respect to the signs of the ecliptic, we shall now explain the phenomena by a dia.

gram. Fig. VI. Let NZSVL be the Earth, SONA its axis pro.

duced to the starry heavens, and terminating in A, the present north pole of the heavens, which is vertical to N, the north pole of the Earth. Let EOQ be the equator, T . Z the tropic of Cancer, and V T v the tropic of Capricorn: VOZ the ecliptic, and BO its axis, both which are immoveable among the stars. But as * the equinoctial points recede in

The pre

noctial

The equinoctial circle intercepts the ecliptic in two opposite points; namely, the first points of the signs Aries and Libra. They are called the equinoctial points, because when the Sun is in either

the ecliptic, the Earth's axis SON is in motion upon the Earth's centre 0, in such a manner as to describe the double cone Non and Sos, round the axis of the ecliptic, BO, in the time that the equinoctial points move quite round the ecliptic, which is 25,920 years, and in that length of time the north pole of the Earth’s axis produced, describes the circle ABC DA, in the starry heavens, round the pole of the ecliptic, which keeps immoveable in the centre of that circle, the Earth's axis being 23 degrees inclined to the axis of the ecliptic, the circle ABC DA, described by the north pole of the Earth's axis produced to A, is 47 degrees in diameter, or double the inclination of the Earth's axis. In consequence of this motion, the point A, which at present is the north pole of the heavens, and near to a star of the second magnitude in the tail of the constellation called the Little Bear, must be deserted by the Earth's axis; which moving backward a degree every 72 years, will be directed toward the star or point B in 6480 years from this time; and in twice that time, or 12960 years, it will be directed toward the star or point C: which will then be the north pole of the heavens, although it is at present 8 degrees south of the zenith of London L. The present position of the equator EOQ will then be changed into eOq, the tropic of Cancer To Z into Vts and the tropic of Capricorn VT vs into t us Z; as is evident by the figure; and the Sun, when in that part of the heavens, where he is now over the terrestrial tropic of Capricorn, and makes the shortest days and longest nights in the northern hemisphere, will then be over the terrestrial tropic of Cancer, and make the days longest and nights shortest. And it will require 12,960 years more, or 25,920 from the pre

of them, he is directly over the terrestrial equator: and then the days and nights are equal.

[ocr errors]

sent time, to bring the north pole N quite round, so as to be directed toward that point of the heavens which is vertical to it at present. And then, and not till then, the same stars, which at present describe the equator, tropics, polar circles, &c. by the Earth's diurnal motion, will describe them over again.

TABLE shewing the Time contained in any Number of Sidereal, Julian,

and Solar Years, from 1 to 10000.

[graphic]
[ocr errors]
[ocr errors]
« PoprzedniaDalej »