Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/589

Rh Some of the tars, particularly Arcturus, have been oberved to change their places above a minute of a degree with reect to others. But whether this be owing to any real motion in the tars themelves, mut require the obervations of many ages to determine. If our olar ytem changeth its place, with regard to abolute pace, this mut in proces of time occaion an apparent change in the ditances of the tars from each other: and in uch a cae, the places of the nearet tars to us being more affected than thoe which are very remote, their relative poitions mut eem to alter, though the tars themelves were really immoveable. On the other hand, if our own ytem be at ret, and any of the tars in real motion, this mut vary their poitions; and the more o, the nearer they are to us, or wifter 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 preent to be above the third part of a degree les than Ptolemy found it. And mot of the obervers after him found it to decreae gradually down to Tycho's time. If it be objected, that we cannot depend on the obervations of the ancients, becaue of the incorrectnes of their intraments; we have to anwer, that both Tycho and Flamteed are allowed to have been very good obervers; and yet we find that Flamteed makes this obliquity 2$1⁄2$ minutes of a degree les than Tycho did about 100 years before him: and as Prolemy was 1324 years before Tycho, o the gradual decreae anwers nearly to the difference of time between thee three atronomers. If we conider, that the earth is not a perfect phere, but an oblate pheriod, having its axis horter than its equatorial diameter; and that the un and moon are contantly 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 thee actions hould gradually diminih the angle between thoe planes. Nor is it les probable that the mutual attractions of all the planets hould have a tendency to bring their orbits to a coincidence: bat this change is too small to become enible in many ages.

parts of time are Seconds, Minutes, Hours, Days, Years, Cycles, Ages, and Periods.

The original tandard, or integral meaure of time, is a year; which is determined by the revolution of ome celetial body in its orbit, viz., the un or moon.

The time meaured by the un's revolution in the ecliptic, from any equinox or oltice to the ame again, is called the Solar or Tropical Year, which contains 365 days, 5 hours, 48 minutes, 57 econds; and is the only proper or natural year, becaue it always keeps the ame easons to the ame months.

The quantity of time meaured by the un's revolution, as from any fixed tar to the ame tar again, is called the ydereal year; which contains 365 days 6 hours 9 minutes 14$1⁄2$ econds; and is 20 minutes 17$1⁄2$ econds longer than the true folar year.

The time meaured by twelve revolutions of the moon, from the un to the un again, is called the lunar year; it contains 354 days 8 hours 48 minutes 36 econds; and is therefore 10 days 21 hours 0 minutes 21 econds horter than the olar year. This is the foundation of the epact.

The civil year is that which is in common ue among the different nations of the world; of which, ome reckon by the lunar, but mot by the olar. The civil olar year contains 365 days, for three years running, which are called common years; and then comes in what is called the biextile or leap-year,which contains 366 days. This is alo called the Julian year, on account of Julius Cæar, who appointed the intercalary-day every fourth year, thinking thereby to make the civil and olar year keep pace together. And this day, being added to the 23d of February, which in the Roman kalendar was the ixth of the kalends of March, that ixth day was twice reckoned, or the 23d and 24th were reckoned as one day, and was called bis extus dies; and thence came the name biextile for that year. But in our common almanacks this day is added at the end of February.

The civil lunar year is alo common or intercalary. The common year conits 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 adjut the lunar year to the olar. This method was ued 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 3$1⁄4$ days hort of the olar year in that time.

The Romans alo ued the lunar embolimic year at firt, as it was ettled by Romulus their firt king, who made it to conit only of ten months or lunations, which fell 61 days hort of the olar year, and o their year became quite vague and unfixed; for which reaon, they were forced to have a table publihed by the high-priet, to inform them when the Spring and other eaons began.