'Let us not wonder,' says he, that we are still ignorant of the law of the motion of comets, whose appearance is so rare, that we can neither tell the beginning nor the end of the revolution of these bodies, which descend to us from immense distances. It is not fifteen hundred years since the stars have been numbered in Greece, and names given to the constellations. The day will come, when, by the continued study of successive ages, things which are now hidden will appear with certainty, and posterity will wonder they have escaped our notice.'

In the same school, they taught that the planets were inhabited, and that the fixed stars were suns, the centres of other planetary systems. These philosophic views would from their grandeur have obtained the suffrages of antiquity, had they not been clogged with systematic opinions, and, at the same time, been wanting of that proof, which has since been obtained, by the agreement of observations.

[To be continued.]

Astronomical Occurrences

In JANUARY 1816.

Of the Obliquity of the Ecliptic-Equinoctial Points. By a long series of remarks, the shepherds of Asia were able, in the early periods of astronomical observations, to trace out the Sun's path in the heavens, he being, always, in the opposite point to that which comes to the meridian at midnight, with equal but opposite declination. Thus they could tell the stars among which the Sun then was, though they could not see them. They discovered that this path was a great circle of the heavens, afterwards called the ecliptic, which cuts the equator in two opposite points, dividing and being divided. by it into two equal parts. They farther observed, that when the Sun was in either of these points of intersection, his circle of diurnal revolution coin

cided with the equator, and, therefore, the days and nights were equal. On this account the equator was called the Equinoctial Line, and the points in which it cuts the ecliptic were called the Equinoctial Points, and the Sun was then said to be in the equinoxes. One of these was called the Vernal, and, the other, the Autumnal Equinox.

It was a very important problem in practical astronomy to determine the exact moment of the Sun's occupying these stations; for it was natural to compare the course of the year from that moment. This has, therefore, been a leading problem in the astronomy of all nations. It is susceptible of a considerable degree of precision without the aid of nice instruments. It is only necessary to observe the Sun's declination at twelve o'clock two or three days before and after the equinoctial day; and on two following days of this number his declination must have changed from north to south, or vice versa. If his declination on one day was observed to be 21 minutes north and, on the next five minutes south, it follows, supposing his apparent motion to be equable, that his declination was nothing at 23 minutes past 7 in the morning of the second day, for, as his motion has been equal to 21 + 5 = 26 minutes in 24 hours, we say, by the common Rule of Three, as 26′ : 241 :: 21': 19h 23m; but, as we have before shown, that the astronomical day commences at 12 o'clock at noon, 19h 23m bring us to 23 minutes past 7 in the second morning. Knowing, then, the precise moments, and knowing likewise the rate of the Sun's motion in the ecliptic, it is easy to ascertain the exact point of the ecliptic in which the equator is intersected.

By observations made at Alexandria between the years 161 and 127 before the Christian æra, Hipparchus, to whom we have already referred, found that the point of the autumnal equinox was about six degrees to the east of the star named Spica Virginis.

By considerable research he was enabled to find that observations of the same kind had been made about 150 years before this period, from which it appeared that the point of the autumnal equinox was about 2° east of that star; hence, he assumed that the equinoctial points are not fixed in the heavens, but move to the westward about a degree in 75 5 years: this motion is called the precession of the equinoxes, because, by it, the time and place of the Sun's equinoctial station precedes the usual calculation, a circumstance that is fully confirmed by all subsequent observations. In 1750, the autumnal equinox was observed to be 20° 21′ westward of Spica Virginis. Now, supposing the motion to have been uniform during this long period, it follows that the annual precession is about 50"; that is, if the celestial equator cut the ecliptic in a particular point on any day of this year, it will, on the same day in the following year, cut it in a point 503" to the west of it, and the Sun will come to the equinox 20′ 23′′ before he has completed his round of the heavens. Hence, the equinoctial, or, as it is sometimes called, the tropical year, is so much shorter than the revolution of the Sun, or sidereal year.

It will, perhaps, be asked, How the Sun can return to the same equinox at the end of every year, without returning to the same spot in the heavens, or to the same fixed star? In answer to this, it must be observed, that the equator is an imaginary eircle, which, being equidistant from the poles, divides the celestial sphere into equal portions. Now, if the poles were stationary, that is, if they always coincided with the same points in the heavens, then the equator would pass constantly over the same fixed stars, and would constantly cut the ecliptic at the same points, for the ecliptic is invariable, that is, it passes always over the same stars. It has, however, been observed, that the poles of the equator are subject to a constant but small motion, so that, if at one time any one of the two points in the bea

vens which do not revolve with the daily motion of the rest of the sphere, and which, for that reason, are called the poles, be near a certain star, some years after it will be found near some other star; in other words, the polar star is not always the same, and it is found, that the path of either of the poles is a circle, the pole of which coincides with the pole of the ecliptic, and that the pole of the equator will move along so slowly as to take 25,791 years to accomplish the whole revolution.

Now, as the ecliptic is a fixed circle in the heavens, but the equator, which, being equidistant from the poles, moves with the poles, and must therefore be constantly changing its intersection with the ecliptic at the rate, as we have observed, of about 50′′ more to the westward than it did before; hence, the Sun's arrival at the equinoctial point precedes its arrival at the same fixed spot of the heavens every year by 20' 23" of time.

The obliquity of the ecliptic for the year 1816, varies from 23° 27′ 42′′,2 to 23° 27' 52",3. Although we have, in our former volumes, explained, very particularly, the subjects and tables which are necessarily introduced in this volume; yet, as we may expect an accession of new readers every year, among whom there will probably be some young persons to whom astronomical terms are not familiar, we shall again explain what we mean by the

Equation of Time.

According to common signification, the day and night mean respectively the time of the Sun's remaining above the horizon, and the time of its remaining below it; this is called the artificial day. The natural day, as opposed to this, is the time employed by the Earth in turning on its axis, or, which is the same thing, the apparent motion of the Sun round the Earth, from one meridian to the same again. Natural, as well as artificial days, are of unequal lengths, which is made quite evident by means of well-going clocks, watches, or chronometers;

for, by observing the Sun's transit over the meridian each day, it will be found that the Sun's centre comes to the meridian sometimes before, and sometimes after, the lapse of 24 hours, as shown by the going of a well-regulated timekeeper. There are, however, in the course of every year, certain days in which the apparent motion of the Sun takes up precisely 24 hours in making its apparent revolution: these are denominated mean natural days, viz., those that are between the longest and the shortest being precisely equal to 24 hours by the clock. Now, the equation of time is the adjustment of the difference of time shown by a well-regulated clock and a true sun-dial.

A good clock, watch, &c., measures that equable time which the rotation of the Earth on his axis exhibits; but the sun-dial measures time by the apparent motion of the Sun, which is different at different periods; and this variation arises, as we have fully explained in the preceding volumes, from the Earth's orbit being an ellipse, from the Earth's axis being inclined to the plane of the ecliptic, and from the precession of the equinoxes.

To adjust a watch, clock, &c., to true time in the month of January, the following table will show what is to be added on each 5th day of the month :—

Hence, the explanation of the table is, that, on the 1st of January, when the time shown on the dial is 12 o'clock, the watch or clock, if it show true. time, must be 3 m. 35 sec. after 12; and, on the 26th, the clock must, if it be true, be 12 m. 47 sec. before the dial.