Some of the ſtars, particularly Arcturus, have been obſerved to change their places above a minute of a degree with reſect to others. But whether this be owing to any real motion in the ſtars themſelves, muſt require the obſervations of many ages to determine. If our ſolar ſyſtem changeth 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 ſwifter their motions are, or the more proper the direction of their motion is for our perception.
The obliquity of the ecliptic to the equinoctial 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 to 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 inſtraments; we have to anſwer, that both Tycho and Flamſteed are allowed to have been very good obſervers; and yet we find that Flamſteed makes this obliquity 212 minutes of a degree leſs than Tycho did about 100 years before him: and as Prolemy 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 ſpheriod, having its axis ſhorter than its equatorial diameter; and that the ſun and moon are conſtantly acting obliquely upon the greater quantity of matter about the equator, pulling it, as it were, towards a nearer and nearer co-incidence 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 attractions of all the planets ſhould have a tendency to bring their orbits to a coincidence: bat this change is too small to become ſenſible in many ages.
Chap. XVIII. 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.
The parts of time are Seconds, Minutes, Hours, Days, Years, Cycles, Ages, and Periods.
The original ſtandard, or integral meaſure of time, is a year; which is determined by the revolution of ſome celeſtial body in its orbit, viz., the ſun or moon.
The time meaſured by the ſun's revolution 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.
The quantity of time meaſured by the ſun's revolution, as from any fixed ſtar to the ſame ſtar again, is called the ſydereal year; which contains 365 days 6 hours 9 minutes 1412 ſeconds; and is 20 minutes 1712 ſeconds longer than the true folar year.
The time meaſured by twelve revolutions of the moon, from the ſun to the ſun again, is called 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.
The civil year is that which is in common uſe among the different nations of the world; of 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 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 year keep pace together. And this day, being added to the 23d of February, which in the Roman kalendar was the ſixth of the kalends of March, that ſixth day was twice reckoned, or the 23d and 24th were reckoned 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.
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. Bur by intercalating no more than a month of 30 days, which they called Ve-Adar, every third year, they fell 314 days ſhort of the ſolar year in that time.
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-prieſt, to inform them when the Spring and other ſeaſons began.
or be reduced to flat circular planes, ſo thin as to be quite inviſible when their edges are turned towards us; as Saturn's ring is in ſuch poſitions. But when excentric 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 towards 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.
