PERIOD
, in Astronomy, the time in which a star or planet makes one revolution, or returns again to the same point in the heavens.
The sun's, or properly the earth's tropical period, is 365 days 5 hours 48 minutes 45 seconds 30 thirds. That of the moon is 27 days 7 hours 43 minutes. That of the other planets as below.
There is a wonderful harmony between the distances of the planets from the sun, and their Periods round him; the great law of which is, that the squares of the Periodic times are always proportional to the cubes of their mean distances from the sun.
The Periods, both tropical and sydereal, with the proportions of the mean distances of the several planets are as follow:
| Planets | Tropical Periods | Sydereal Periods | Proport. Dists. |
| Mercury | 87d 23h 14′ | 87d 23h 16′ | 36710 |
| Venus | 224 16 42 | 224 16 49 | 72333 |
| Earth | 365 5 49 | 365 6 9 | 100000 |
| Mars | 686 22 18 | 686 23 31 | 152369 |
| Jupiter | 4330 8 58 | 4332 8 51 | 520110 |
| Saturn | 10749 7 22 | 10761 14 37 | 953800 |
| Georgian or Herschel | 30456 1 41 | 1908180 |
As to the comets, the Periods of very few of them are known. There is one however of between 75 and 76 years, which appeared for the last time in 1759; another was supposed to have its Period of 129 years, which was expected to appear in 1789 or 1790, but it did not; and the comet which appeared in 1680 it is thought has its Period of 575 years.
Period, in Chronology, denotes an epoch, or interval of time, by which the years are reckoned; or a series of years by which time is measured, in different nations. Such are the Calippic and Metonic Periods, two different corrections of the Greek calendar, the Julian Period, invented by Joseph Scaliger; the Victorian Period, &c.
Calippic Period. See Calippic Period.
Constantinopolitan Period, is that used by the Greeks, and is the same as the Julian Period, which see.
Chaldaic Period. See Saros.
Dionysian Period. See Victorian Period.
Hipparchus's Period, is a series or cycle of 304 solar years, returning in a constant round, and restoring the new and full moons to the same day of the sólar year; as Hipparchus thought.
This Period arises by multiplying the Calippic Period by 4. Hipparchus assumed the quantity of the solar year to be 365d. 5h. 55m. 12 sec. and hence he concluded, that in 304 years Calippus's Period would err a whole day. He therefore multiplied the Period by 4, and from the product cast away an entire day. But even this does not restore the new and full moons to the same day throughout the whole Period: but they are sometimes anticipated 1d. 8h. 23 m. 29 sec. 20 thirds.
Julian Period, so called as being adapted to the Julian year, is a series of 7980 Julian years; arising from the multiplications of the cycles of the sun, moon, and indiction together, or the numbers 28, 19, 15; commencing on the 1st. day of January in the 764th Julian year before the creation, and therefore is not yet completed. This comprehends all other cycles, Periods and epochs, with the times of all memorable actions and histories; and therefore it is not only the most general, but the most useful of all Periods in Chronology.
As every year of the Julian Period has its particular solar, lunar, and indiction cycles, and no two years in it can have all these three cyeles the same, every year of this Period becomes accurately distinguished from another.
This Period was invented by Joseph Scaliger, as containing all the other epochs, to facilitate the reduction of the years of one given epoch to those of another. It agrees with the Constantinopolitan Period, used by the Greeks, except in this, that the cycles of the sun, moon, and indiction, are reckoned differently; and also in that the first year of the Constantinopolitan Period differs from that of the Julian Period.
To find the year answering to any given year of the Julian Period, and vice versa; see Epoch.
Metonic Period. See Cycle of the Moon.
Victorian Period, an interval of 532 Julian years; at the end of which, the new and full moons return again on the same day of the Julian year, according to the opinion of the inventor Victorinus, or Victorius, who lived in the time of pope Hilary.
Some ascribe this Period to Dionysius Exiguus, and hence they call it the Dionysian Period: others again call it the Great Paschal Cycle, because it was invented for computing the time of Easter.
The Victorian Period is produced by multiplying the solar cycle 28 by the lunar cycle 19, the product being 532. But neither does this restore the new and full moons to the same day throughout its whole duration, by 1d. 16h. 58m. 59s. 40 thirds.
Period, in Arithmetic, is a distinction made by a point, or a comma, after every 6th place, or figure; and is used in numeration, for the readier distinguishing and naming the several figures or places, which are thus distinguished into Periods of six figures each. See Numeration.
Period is also used in Arithmetic, in the extraction| of roots, to point off, or separate the figures of the given number into Periods, or parcels, of as many figures each as are expressed by the degree of the root to be extracted, viz, of two places each for the square root, three places for the cube root, and so on.
