when the side covered with spots is turned towards us. And as for those which break out all of a sudden with such lustre, it is by no means improbable that they are Suns whose fuel is almost spent, and again supplied by some of their comets falling upon them, and occasioning an uncommon blaze and splendour for some time: which indeed appears to be the greatest use of the cometary part of any system*.

change

Some of the stars, particularly Arcturus, have been Somestars observed to change their places above a minute of a their pla degree with respect to others. But whether this be ces. owing toany real motion in the stars themselves, must require the observations of many ages to determine. If our solar system change its place with regard to absolute space, this must in process of time occasion an apparent change in the distances of the stars from each other: and in such a case, the places of the nearest stars to us being more affected than those which are very remote, their relative positions must seem to alter, though the stars themselves were really immoveable. On the other hand, if our own system be at rest, and any of the stars in real motion, this must vary their positions; and the more so, the nearer they are to us, or the swifter their motions are; or the

* M. Maupertuis, in his Dissertation on the figures of the Celestial Bodies (p. 91-93), is of opinion that some stars, by their prodigious quick rotations on their axes, may not only assume the figures of oblate spheroids, but that by the great centrifugal force arising from such rotations, they may become of the figures of mill-stones; or be reduced to flat circular planes, so thin as to be quite invisible when their edges are turned toward us; as Saturn's ring is in such positions. But when any eccentric planets or comets go round any flat star, in orbits much inclined to its equator, the attraction of the planets or comets in their perihelions must alter the inclination of the axis of that star; on which account it will appear more or less large and luminous, as its broad side is more or less turned toward us. And thus he imagines we may account for the apparent changes of magnitude and lustre in those stars, and likewise for their appearing and disappearing.

The eclip

tic less ob

to the

equator

than for

merly.

more proper the direction of their motion is for our perception.

368. The obliquity of the ecliptic to the equinoclique now tial is found at present to be above the third part of a degree less than Ptolemy found it. And most of the observers after him found it do decrease gradually down to Tycho's time. If it be objected, that we cannot depend on the observations of the ancients, because of the incorrectness of their instruments; we have to answer, that both Tycho and Flamstead are allowed to have been very good observers; and yet we find that Flamstead makes this obliquity 21 minutes of a degree less than Tycho did, about 100 years before him: and as Ptolemy was 1324 years before Tycho, so the gradual decrease answers nearly to the difference of time between these three astronomers. If we consider, that the Earth is not a perfect sphere, but an oblate spheroid, having its axis shorterthan its equatorial diameter; and thatthe Sun and Moon are constantly acting obliquely upon the greater quantity of matter about the equator, pulling it as it were toward a nearer and nearer coincidence with the ecliptic; it will not appear improbable that these actions should gradually diminish the angle between those planes. Nor is it less probable that the mutual attraction of all the planets should have a tendency to bring their orbits to a coincidence; but this change is too small to become sensible in many ages.*

* M. de la Grange has demonstrated, in the most satisfactory manner, that no permanent change can take place in the magnitudes, figures, or inclinations, of the planetary orbits; and that the periodical changes are confined within very narrow limits: the ecliptic therefore, will never coincide with the equator, nor change its inclination above 2 degrees. In short, the solar planetary system oscillates, as it were, round a medium state, from which it never swerves very far. See note subjoined to p. 116.

CHAP. XXI.

Of the Division of Time. A perpetual Table of New
Moons. The Times of the Birth and Death of
CHRIST. A Table of remarkable Æras or Events.

369.

riods.

THE

HE parts of time are, seconds, minutes,
hours, days, years, cycles, ages, and pe-

370. The original standard, or integral measure A year. of time, is a year; which is determined by the revolution of some celestial body in its orbit, viz. the Sun or Moon.

371. The time measured by the Sun's revolution Tropical in the ecliptic, from any equinox or solstice to the same again, is called the solar or tropical year, which contains 365 days, 5 hours, 48 minutes, 57 seconds; and is the only proper or natural year, because it always keeps the same seasons to the same months.

372. The quantity of time measured by the Sun's Sidereal revolution as from any fixed star to the same star year. again, is called the sidereal year; which contains 365 days, 6 hours, 9 minutes, 14 seconds, and is 20 minutes 17 seconds longer than the true solar year.

373. The time measured by twelve revolutions of Lunar the Moon, from the Sun to the Sun again, is called year. the lunar year; it contains 354 days, 8 hours, 48 minutes, 36 seconds; and is therefore 10 days, 21 hours, 0 minutes, 21 seconds shorter than the solar year. This is the foundation of the epact.

374. The civil year is that which is in common Civil use among the different nations of the world; of year. which, some reckon by the lunar, but most by the solar. The civil solar year contains 365 days, for three years running, which are called common years; and then comes in what is called the bissextile or

Lunar year.

Roman

year.

The origi.

Gregorian

or new

year

leap-year, which contains 366 days. This is also
called the Julian year, on account of Julius Cæsar,
who appointed the intercalary day every fourth year,
thinking thereby to make the civil and solar
keep pace together. And this day, being added to
the 23d of February, which in the Roman calendar
was the sixth of the Calends of March, that sixth day
was twice reckoned, or the 23d and 24th were reck-
oned as one day; and was called Bis sextus dies, and
thence came the name bissextile for that year. But
in our common almanacks this day is added at the
end of February.

375. The civil lunar year is also common or intercalary. The common year.consists of 12 lunations, which contain 354 days; at the end of which the year begins again. The intercalary, or embolimic year, is that wherein a month was added to adjust the lunar year to the solar. This method was used by the Jews, who kept their account by the lunar motions. But by intercalating no more than a month of 30 days, which they called Ve-Adar, every third year, they fell 3 days short of the solar year in

that time.

376. The Romans also used the lunar embolimic year at first, as it was settled by Romulus their first king, who made it to consist only of ten months or lunations; which fell 61 days short of the solar year, and so their year became quite vague and unfixed; for which reason they were forced to have a table published by the high-priests, to inform them when the spring and other seasons began. But Julius Casar, as already mentioned, § 374, taking this trou*blesome affair into consideration, reformed the calendar, by making the year to consist of 365 days 6 hours.

377. The year thus settled, is what was used in nal of the Britain till A. D. 1752: but as it is somewhat more than 11 minutes longer than the solar tropical year, the times of the equinoxes go backward, and fall earlier by one day in about 130 years. In the time

style.

of the Nicene council (A. D. 325), which was 1439 years ago, the vernal equinox fell on the 21st of March: and if we divide 1444 by 130, it will quote 11, which is the number of days the equinox has fallen back since the council of Nice. This causing great disturbances, by unfixing the times of the celebration of Easter, and consequently of all the other moveable feasts, pope Gregory the XIII, in the year 1582, ordered ten days to be at once stricken out of that year; and the next day after the fourth of October was called the fifteenth. By this means, the vernal equinox was restored to the 21st of March; and it was endeavoured, by the omission of three intercalary days in 400 years, to make the civil or political year keep pace with the solar for the time to come. This new form of the year is called the Gregorian account, or new style; which is received in all countries where the pope's authority is acknowledged, and ought to be in all places where truth is regarded.

378. The principal division of the year is into Month months, which are of two sorts, namely, astronomical and civil. The astronomical month is the time in which the Moon runs through the zodiac, and is either periodical or synodical. The periodical month is the time spent by the Moon in making one complete revolution from any point of the zodiac to the same again; which is 27 7 43. The synodical month, called a lunation, is the time contained between the Moon's parting with the Sun at a conjunc tion, and returning to him again; which is 29d 12h 44. The civil months are those which are framed · for the uses of civil life; and are different as to their names, number of days, and times of beginning, in several different countries. The first month of the Jewish Year fell, according to the Moon, in our August and September, old style; the second in September and October; and so on. The first month of the Egyptian year began on the 29th of our August. The first month of the Arabic and Turkish