ASTRONIUM, a genus of plants of the class Dicecia, and order Pentandria. See BOTANY. (w) from cia-reov, a awn and vopeag, a law, is that science which explains the nature and motions of the planets, comets, and fixed stars ; the various phenomena which these bodies exhibit ; and the laws by which their motions are regulated.
In attempting to give a complete view of this interest ing science, we shall divide the subject into three parts ; namely, 1. DESCRIPTIVE AsTaorlomy, containing a filll account of the phenomena of the planets, comets, and fixed stars. 2. PHYSICAL AsTaoxomv, containing the theory of the planetary motions. 3. PRACTICAL ASTRO womv, containing the description and use of astronomi cal instruments, and the nature of astronomical calcula tions. But before entering on these subjects, we shall endeavour, in a short historical sketch, to trace the va rious steps of the human mind, by which astronomy has advanced from a few rude observations, to be the most interesting and perfect of the sciences.
Ix the earliest ages of society, the heavenly bodies must have been observed under the impulse of neces sity and superstition, before they were examined as ob jects of curiosity. The sun and moon, the first subjects of idolatrous worship, were regarded as emblems of the Great Being by whom the universe was created ; and must therefore have attracted a degree of attention which they never could have excited when considered only as inanimate portions of his works. Even in the occupations of hunting and fishing, by which the savage provided for his wants, he must have recognised his de pendence on the luminaries of heaven ; and in the night ly excursions to which he was impelled either by hunger or revenge, he must have watched with anxiety for the returning moon to light him to his prey. In the pro gress of civilization, however, when man begins to draw his subsistence from less precarious resources, he is in cited to the contemplation of the stars by new and more powerful motives. Even the rudest operations by which he cultivates the soil are dependent on the varying mo tions of the sun ; and he is thus led to mark the approach of the seasons, and to fix in his rude calendar the period of their return. The observations which were thus made under the powerful impulse of necessity, might, in a temperate climate, and beneath a serene sky, be continued when that impulse was withdrawn, and astro nomy might then be studied as a science, when man felt the pleasure which springs from the exercise of his fa culties. We accordingly find, in those remote ages when history and tradition begin to throw their mingled and confused light over the affairs of the world, that the rising and setting of the principal stars had been carefully observed, and that the phases and eclipses of the moon, and her occultation of the stars, had been fre quently recorded. By observing the times when the principal 'Stars were eclipsed by the light of the sun at the evening twilight, and when they emerged from his beams before the dawn of day, the progress of the sun was traced through the heavens, and the course of the moon and planets was carefully marked by their approach to the fixed stars, which are situated in their path.
The revolution which the sun appeared to perform in the starry firmament, embraced all the variety of the seasons ; and a narrow zone which encircled his path comprehended the orbits or all the other planets. In order, therefore, to distinguish the seasons, and deter mine the positions and motions of these wandering bo dies, this zone, which the ancients called the zodiac, was divided into twelve constellations or signs, which were descriptive, either of the motion of the sun, or of those operations in agriculture, and those changes of weather, which marked the entrance of the sun into these arbitrary divisions.
Leaving the regions of conjecture and fable, in which the early history of astronomy is lost, and in which the mind is bewildered with the opposing testimonies of cre dulous historians, we shall proceed to detail the progress of the Chaldeans in the study of the heavens. Vie are informed by the peripatetic philosopher Simplicius, ou the authority of Porphyry, that when Babylon was taken by Alexander the Great, Callisthenes collected the as tronomical observations of the Chaldeans for 1903 years, and transmitted them to Aristotle at the desire of the Macedonian king. This interesting fact receives no support from the authority of contemporary authors, and has therefore been rejected by several astronomers ; but we conceive that the account given by Simplicius is not destitute of probability, and that we are scarcely enti tled to question the express testimony of history, when it is not contradicted by indisputable evidence. We know for certain, that three eclipses of the moon were accurately observed at Babylon in the years 719 and 720 before the Christian sera; and it is highly probable, that, in the temperate climate and unclouded atmosphere of Chaldea, these were not their earliest attempts in prac tical astronomy. Ptolomy, who made use of these eclip ses for determining the mean motion of the moon, has recorded other four lunar eclipses, the last of which was observed at Babylon about 367 years before the Chris tian xra. The astronomical knowledge of the Chal deans is more unequivocally marked in their lunisolar periods, which must have been deduced from a great number of accurate observations. Their period of eclip ses, which they called Saroa, consisted of 223 inflations, or 6585 days, 8 hours, at the end of which the moon re turned to the very same position with regard to the sun, and to her own node and perigee. The eclipses, there fore, which were observed during one period, enabled them to predict those which were to take place in the period that succeeded, and all the other phenomena which result from the combined motions of the two naries. The accurate tables of Dclambre and Mason, make this period 6585 days, 7 hours, 42 minutes, and 31 seconds ; so that the error of the Chaldean astrono mers amounted only to 17' 29".