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Page:Popular Science Monthly Volume 18.djvu/781

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have been a very difficult matter to ascertain with sufficient correctness. But to mark and fix the time of the sun's apparent revolution through the heavens among the stars was a matter of so great difficulty that it was not exactly ascertained even at the time of the reformation of the calendar in 1582; yet so uniform is the motion of the earth in its orbit that the results of modern experiments render it next to absolutely certain that the time of orbital revolution has never varied even the fraction of a second. In the infancy of astronomy, many ingenious expedients were adopted to ascertain this and other matters connected with the times and motions of the planets and other heavenly bodies, one of which may be mentioned even at the risk of tediousness. To ascertain the exact time of the revolution of the concave of the heavens, two vessels were placed over each other, the upper filled with water, the lower empty. At the moment of the appearing of a certain star above the horizon, the water was permitted to flow from the upper into the lower vessel, and the flow was continued until the same star appeared the next night, when the flow was stopped. The whole concave of the heavens had then made one revolution. The water which had flowed out during this time was then divided into twelve equal parts, and smaller vessels were made each to hold just one of those parts, and on the following evening they repeated the operation, filling successively six of those vessels, and noting carefully what stars rose above the horizon during the time required to fill each of them. Each group of stars which rose during the time of filling one small vessel was called a station or house of the sun. They then postponed operations upon the other half of the heavens for six months, when they repeated it, and thus divided the path of the sun through the whole heavens into twelve divisions, to most of which they gave the names of certain animals: hence the term zodiac, the propriety of which could have been seen only by the fertile fancies of the childhood of the race. The whole ancient method of dividing and naming the constellations is to us utterly absurd, and is really a hindrance to a knowledge of the stars. Fanciful forms of snakes and dogs and lions and bulls and wagons and scorpions convey to us no idea but one of confusion and perplexity, and they are tolerated for the same reasons that we tolerate our bungling orthography: we are loath to break away from the associations of antiquity; we are loath to sever the giant strides of Science, in its strength and manhood, from the feeble totterings of its infancy.

The time required by the sun to pass through one of these groups or signs is nearly equal to a lunar month; the time required to pass through three of them was called a season, as we have it now. All this was done by the Chaldeans or Egyptians, centuries before Greece or Rome had inhabitants or a name.

The Greeks divided the year into twelve lunar months, but, as this lunar year differed from the true year by about eleven days, they cor-