The earth revolving around the sun in an ellipse, the position of the major axis of the orbit would indicate something in regard to eras in astronomy extending not only beyond the historical period, but so far back in the past that imagination is almost at fault. The position of the major axis of the orbit depends on the direct motion of the perigee and the precession of the equinoxes conjointly, the annual motions respectively being 11´´.8 and 50´´.1, the two combined motions being 61´´.9 annually. A tropical revolution is made in 209.84 years. This being a constant quantity, we may ascertain when the line of the major axis coincided with the line of the equinoxes. This occurrence took place about 4,000 or 4,090 years before the year 0. In the year 6,483 the major axis will again coincide with the line of the equinoxes, but then the solar perigee will coincide with the vernal equinox. So, it will be seen that the period of revolution is 20,966 years. But in the progress of this revolution there must have been a time when the major axis was perpendicular to the line of the equinoxes. A simple calculation will show that the eventful year was 1250; and so important is this event considered, that La Place, the immortal author of the _Méchanique Céleste_, proposed to make the vernal equinox of this year the initial day of the year 1 of our era. Again, at the solstices the sun is at the greatest distance from the equator; consequently the declination of the sun is equal to the obliquity of the ecliptic. The length of a shadow cast at noonday from the stile of an ordinary sun-dial would accurately determine the precise time on which this position occurs. Though wanting in accuracy, such a measurement is of interest, from the fact that there are recorded observations of this kind that were taken in the city of Layang, in China, 1100 years before our present era is dated. This observation gives the zenith distance of the sun at the moment of the observation. Half the sum of the zenith distances gives the latitude, and half their difference gives the obliquity of the ecliptic at the period. Now the law of the variation of the ecliptic is well known, and modern computation has verified both the moment of taking the observation and the latitude of the place. Eclipses were the foundation of the whole of Chinese chronology, and recorded observations prove the civilization of that strange race for 4700 years. Horology, with astronomy, was not neglected even as early as 3102 years before Christ, as the following will show. The cycles of Jupiter and Saturn are very unequal, the latter being a period of 918 years; the mean motion of the two planets was determined by the Indians in that part of the respective orbits where Saturn’s motion was the slowest and Jupiter’s the most rapid. This observed event must have been 3102 years before, and 1491 after the year 0; but the record shows that the observation was taken before the last-named date.