in Astronomy, the time taken up by a star or planet in making a revolution round the sun; or the duration of its course till it return to the same part of its orbit. See PLANET.

The different periods and mean distances of the several planets are as follows:

| Planet | Days | h' | m' | s" | Mean Dist. | |----------|------|----|----|----|------------| | Herschel | 30737| 18 | | | 1908352 | | Saturn | 10759| 1 | 51 | 11 | 954072 | | Jupiter | 4332 | 14 | 27 | 10 | 520279 | | Mars | 686 | 23 | 30 | 35 | 152369 | | Earth | 365 | 6 | 9 | 12 | 100000 | | Venus | 224 | 16 | 49 | 10 | 72333 | | Mercury | 87 | 23 | 15 | 43 | 38710 |

There is a wonderful harmony between the distances of the planets from the sun, and their periods round him; the great law whereof is, that the squares of the periodical times of the primary planet, are to each other as the cubes of their distances from the sun: and likewise, the squares of the periodical times of the secondaries of any planet are to each other as the cubes of their distances from that primary. This harmony among the planets is one of the greatest confirmations of the Copernican hypothesis. See ASTRONOMY, p. 100 and 101.

For the periods of the moon, see MOON, ASTRONOMY Index.

The periods of several comets are now pretty well ascertained. See ASTRONOMY, No 306.

in Chronology, denotes a revolution of a certain number of years, or a series of years, whereby, in different nations, and on different occasions, time is measured; such are the following.

Calippic PERIOD, a system of seventy-six years. See CALIPPIC, and ASTRONOMY, No 11, &c.

Dionysian PERIOD, or Victorian Period, a system of 532 lunar-solar and Julian years; which being elapsed, the characters of the moon fall again upon the same day and feria, and revolve in the same order, according to the opinion of the ancients.

This period is otherwise called the great paschal cycle, because the Christian church first used it to find the true time of the pascha or Easter. The sum of these years arises by multiplying together the cycles of the sun and moon.

Hipparchus's PERIOD, is a series of 304 solar years, returning in a constant round, and restoring the new and full moons to the same day of the solar year, according to the sentiment of Hipparchus. This period arises by multiplying the Calippic period by four.—Hipparchus assumed the quantity of the solar year to be 365 days 5 hours 50' 12"; and hence concluded, that in 104 years Calippus's period would err a whole day. He therefore multiplied the period by four, 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 1 day 8 hours 23' 29" 20".

See ASTRONOMY, No 14.

Julian PERIOD. See JULIAN.

Grammar, denotes a small compass of discourse, containing a perfect sentence, and distinguished at the end by a point, or full stop, thus (.); and in members or divisions marked by commas, colons, &c.

Father Buffier observes two difficulties in the use of the period, or point; i.e. in distinguishing it from the colon, or double point; and in determining justly the end of a period, or perfect sentence. It is remarked that the supernumerary members of a period, separated from the rest by colons and semicolons, usually commence with a conjunction; yet it is true these same conjunctions sometimes rather begin new periods than supernumerary members of old ones. It is the sense of things, and the author's own discretion, that must make the proper distinction which of the two in effect it is. No rules will be of any service, unless this be admitted as one, that when what follows the conjunction is of as much extent as what precedes it, it is usually a new period; otherwise not.

The second difficulty arises hence, that the sense appears perfect in several short detached phrases, wherein it does not seem there should be periods; a thing frequent in free discourse: as, We are all in suspense: make your proposals immediately: you will be to blame for detaining us longer. Where it is evident, that simple phrases have perfect senses like periods, and ought to be marked accordingly; but that the shortness of the discourse making them easily comprehended, the pointing is neglected.

De Colonia defines period a short but perfect sentence, consisting of certain parts or members, depending one on another, and connected together by some common vinculum. The celebrated definition of Aristotle is, a period is a discourse which has a beginning, a middle, and an end, all visible at one view. Rhetoricians consider period, which treats of the structure of sentences, as one of the four parts of composition. The periods allowed in oratory are three: A period of two members, called by the Greeks diocolos, and by the Latins binembris; a period of three members, tricolos, trimembris; and a period of four, quadrimembris, tetracolos.

See PUNCTUATION.

numbers, is a distinction made by a point or comma, after every sixth place, or figure; and is used in numeration, for the readier distinguishing and naming the several figures or places; which see under NUMERATION.

Medicine, is applied to certain diseases which have intervals and returns, to denote an entire course. course or circle of such disease; or its progress from any state through all the rest till it return to the same again.

Galen describes period as a time composed of an intention and remission; whence it is usually divided into two parts, the paroxysm or exacerbation, and remission.

In intermittent fevers, the periods are usually stated and regular; in other diseases, as the epilepsy, gout, &c., they are vague or irregular.

Oratory. See there, No. 47.