Hutton, Charles Mathematical and Philosophical Dictionary 1795

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On a paper, in a dark chamber, let a ray of light be largely refracted into the spectrum ABCDEF, marking upon it the precise boundaries of the several colours, as a, b, c, &c; and across the spectrum draw the perpendicular lines ag, bh, &c. Then it will be found that the spaces, by which the several colours are bounded, viz, BagF containing the red, abhg containing the orange, bcih containing the yellow, &c, will be in exact proportion to the divisions of a musical chord for the notes of an octave; that is, as the intervals of these numbers 1, 8/9, 5/6, 3/4, 2/3, 3/5, 9/16, 1/2.

, belonging to chronology.

Chronological Characters, are characters by which times are distinguished. Of these, some are natural, or astronomical; others are artificial or historical. The natural characters are such as depend on the motions of the stars or luminaries; as eclipses, solstices, equinoxes, the different aspects of planets, &c. And the artificial characters are those that have been invented and established by men; as the solar cycle, the lunar cycle, &c. Historical Chronological Characters are those supported by the testimonies of historians, when they fix the dates of certain events to certain periods.

, the art of measuring and distinguishing time; with the doctrine of dates, epochs, eras, &c.

The measurement of time in the most early periods, was by means of the seasons, or the revolutions of the sun and moon. The succession of Juno's priestesses at Argos served Hellanicus for the regulation of his narrative; while Ephorus reckoned his matters by generations. Even in the histories of Herodotus and Thucydides, there are no regular dates for the events recorded; nor were there any endeavours to establish a fixed era until the time of Ptolomy Philadelphus, who attempted it by comparing and correcting the dates of the olympiads, the kings of Sparta, and the succession of the priestesses of Juno at Argos. Eratosthenes and Apollodorus digested the events related by them, according to the succession of the olympiads and of the Spartan kings.

The chronology of the Latins is still more uncertain. The records of the Romans were destroyed by the Gauls; and Fabius Pictor, the most ancient of their historians, was obliged to borrow the chief part of his information from the Greeks. In other European nations the chronology is still more imperfect, and of a later date: and even in modern times a considerable degree of confusion and inaccuracy has arisen, from the want of attention in the historians to ascertain the dates and epochs with precision.

Hence it is evident, how necessary a proper system of chronology must be for the right understanding of history, and also how difficult it must be to establish such a system. For this purpose, however, several learned men have spent much time, particularly Julius Africanus, Eusebius of Cæsarea, George Cyncelle, John of Antioch, Dennis, Petau, Clavius, Calvisius, Scaliger, Vieta, Newton, Usher, Simson, Marsham, Helvicus, Vossius, Strauchius, Blair, and Playfair.

Such a system is founded, 1st, On astronomical observations, especially of the eclipses of the sun and moon, combined with calculations of the years and eras of different nations. 2d, The testimonies of cre- dible authors. 3d, Such epochs in history as are so well attested and determined, that they have never been controverted. 4th, Ancient medals, coins, monuments, and insoriptions.

The most obvious division of time, as has been observed, is derived from the apparent or real revolutions of the luminaries, the sun and moon. Thus, the apparent revolution of the sun, or the real rotation of the earth on her axis causing the sun to appear to rise and set, constitutes the vicissitudes of day and night, which must be evident to the most barbarous and ignorant nations. The moon, by her revolution about the earth, and her changes, as naturally and obviously forms months; while the great annual course of the sun through the several constellations of the zodiac, points out the larger division of the year.

The Day is divided into hours, minutes, &c; while the month is divided into weeks, and the year into months, having particular names, and a certain number of days.—See a particular account of each of these under the respective words.

Beside the natural divisions of time arising immediately from the revolutions of the heavenly bodies, there are others which are formed from some of the less obvious consequences of these revolutions, and are called cycles, or circles. The most remarkable of these are, 1, The Solar Cycle, or cycle of the sun, a period or revolution of 28 years, in which time the days of the months return to the same days of the week, the sun's place to the same signs and degrees of the ecliptic on the same months and days, and the leap-years begin the same course over again with respect to the days of the week on which the days of the months fall. 2, The Lunar Cycle, or cycle of the moon, commonly called the Golden Number, is a revolution of 19 years; in which time the conjunctions, oppositions, and other aspects of the moon, are on the same days of the months as they were 19 years before, and within an hour and a half of the same time of the day.

The Indiction, or Roman Indiction, is a period of 15 years, used only by the Romans for indicating the times of certain payments made by the subjects to the republic.

The Cycle of Easter, called also the Dionysian Period, is a revolution of 532 years, and is produced by multiplying the solar cycle 28, by the lunar cycle 19.

The Julian Period, is a revolution of 7980 years, and is produced from the continual multiplication of the three numbers 28, 19, 15, of the three former cycles, viz, the solar, lunar, and indiction.

As there are certain fixed points in the heavens, from which astronomers begin their computations, so there are certain points of time, from which historians begin to reckon; and these points or roots of time are called eras or epochs. The most remarkable of these are, those of the Creation, the Greek Olympiads, the building of Rome, the era of Nabonnassar, the death of Alexander, the birth of Christ, the Arabian Hegira, or flight of Mahomet, and the Persian Jesdegird. All which, with some others of less note, have their beginnings fixed by chronologers to the years of the Julian period, to the age of the world, and to the years before and after the birth of Christ.

The testimony of authors is the second principal