The Chinese also record an observation, made between the years 2514 and 2436 B.C., of five planets being in conjunction.
The Chaldeans appear to have observed the motions of the moon, and an observation in 2227 B.C. is recorded; but these old dates are probably fictitious.
It is impossible to regard without surprise the general attention given to astronomical investigation in those early days compared with what we find now. Yet if we attempt to build up for ourselves any idea as to the problems of which the ancients attempted the solution, it is difficult if not impossible to do it; we cannot realize the blank which the heavens presented to them, so many great men have lived between their time and our own, by whose labours we, even if unconsciously, have profited. The first idea seems to have been to observe which stars were rising or setting at seed or harvest time, to divide the heavens into Moon Stations, and then to mark astronomically their monthly and yearly festivals.
If one looks into the old records we find that all the labours of man which had to be performed in the country or elsewhere were determined, by the rising or setting of the stars. All the exertions of the navigator and the agriculturist were thus regulated. Of the planets in those early times we hear little, except from the Chinese annals which record conjunctions.
This was before man began to use the sun as a standpoint, and hence it is that there are so many references in the ancient writers to the rising and setting of the most striking star cluster—the Pleiades, and the most striking constellation—Orion. It is known that the year, in later times at all events, began in Egypt when the brightest star in the heavens, Sirius, the dog-star, rose with the sun, this day being called the 1st of the month Thoth,[[1]] which was the commencement of the Sothiac period of 1461 years.
It would appear that observations of culminations, that is, of the highest points reached by the stars, were not made till long after horizon observations were in full vigour; and here it is a question whether pyramids and the like were not the first astronomical instruments constructed by man, because for great nicety in such observations—a nicety, let us say, sufficient to determine astronomically by means of culminations the time for holding a festival—a fixed instrument of some kind was essential. The rich mine recently opened up by Mr. Haliburton and Mr. Ernest de Bunsen concerning the survival in all nations—in our own one takes the name the Feast of All Souls’—of ancient festivals governed by the midnight culmination of the Pleiades will doubtless ere long call general attention to this earliest form of accurate astronomical observation, and the determination by Professor Piazzi Smyth of the fact that in 2170 B.C., when the Pleiades culminated at midnight at the vernal equinox, the passages in the north and south faces of the pyramid of Gizeh were directed, the southern one to this culmination, and the northern one to the then pole star, α Draconis, at its transit, about 4° from the pole.
Hence one may regard the pyramid as the next astronomical instrument to the horizon. While then it is possible that such culmination observations soon replaced in some measure that class of observations which heretofore had been made on the horizon, another teaching of horizon observations became apparent. By and by travellers observed that as they travelled northwards the stars that were just visible on the southern horizon, when culminating, gradually disappeared below it. These observations were at once seized on, and Anaximander accounted for them by supposing that the earth was a cylinder.[[2]] The idea of a sphere did not come till later; when it did come then came the circle as an astronomical instrument. For let us consider that a person on the earth stands, say, at the equator; then he will just be able to see along his north and south horizon the stars pointed to by the axis of the globe: if now he is transported northwards, his horizon will change with him; he will no longer be able to see the southern stars, but the northern ones will gradually rise above his horizon till he gets to the north pole, when the north pole star, instead of being on his horizon, as was the case when he was at the equator, will be over his head. So by moving from the equator to the pole (or a quarter of the distance round the earth) the stars have moved from the horizon to the point overhead, or the zenith, that is also a quarter of a circle. So it appears that if an observer moves to such a distance that the stars appear to move over a certain division of a circle with reference to the horizon, he must have moved over an equal division on the earth’s surface. Then, as now, the circle in the Western world was divided into 360°, so that the observer in moving 1° by the stars would have moved over 1
360 of the distance round the earth, on the assumption that the earth is a globe; and if the distance over which the observer has moved be multiplied by 360, the result will be the distance round the earth.
Fig. 2.—The Zodiac of Denderah.
Now let us see how Posidonius a long time afterwards (he was born about 135 years B.C.) applied this conception. He observed that at Rhodes the star Canopus grazed the horizon at culmination, while at Alexandria it rose above it 7½°. Now 7½° is 1
48 of the whole circle; so he found that from the latitude of Rhodes to that of Alexandria was 1
48 of the circumference of the earth. He then estimated the distance, getting 5,000 stadia as the result; and this multiplied by 48 gave him 240,000 stadia, his measure of the circumference of the earth.