The Signs once arranged, the next step was an easy one. Each sign was equivalent to 30 degrees of longitude. A third of a sign, a "decan," was 10 degrees of longitude, corresponding to the "week" of ten days used in Egypt and in Greece. This change from the constellations to the Signs cannot have taken place very early. The place of the spring equinox travels backwards amongst the stars at the rate of very little more than a degree in 72 years. When the change was made the spring equinox was somewhere in the constellation _Aries_, the Ram, and therefore Aries was then adopted as the first Sign, and must always remain such, since the Signs move amongst the stars with the equinox. [Illustration: POSITION OF SPRING EQUINOX, B.C. 2700.] We cannot fix when this change was made within a few years, but it cannot have been _before_ the time when the sun at the spring equinox was situated just below _Hamal_, the brightest star of the Ram. This was about 700 B.C. The equal division of the zodiac must have taken place not earlier than this, and with it, the Bull must have been deposed from the position it had always held up to that time, of leader of the zodiac. It is probable that some direct method of determining the equinox itself was introduced much about the same time. This new system involved nothing short of a revolution in astronomy, but the Babylonian Creation story implies that this revolution had already taken place when it was composed, and that the equal division of the zodiac was already in force. It is possible that the sixth and seventh lines of the poem indicate that the Babylonians had already noticed a peculiar fact, viz. that just as the moon passes through all the signs in a month, whilst the sun passes through only one sign in that time; so the sun passes through all the signs in a year, whilst Jupiter passes through but one sign. _Nibir_ was the special Babylonian name of the planet Jupiter when on the meridian; and Merodach, as the deity of that planet, is thus represented as pacing out the bounds of the zodiacal Signs by his movement in the course of the year. The planet also marks out the third part of a sign, _i. e._ ten degrees; for during one-third of each year it appears to retrograde, moving from east to west amongst the stars instead of from west to east. During this retrogression it covers the breadth of one "decan" = ten degrees. [Illustration: POSITION OF SPRING EQUINOX, A.D. 1900.] The Babylonian Creation epic is therefore quite late, for it introduces astronomical ideas not current earlier than 700 B.C. in Babylonia or anywhere else. This new development of astronomy enables us also to roughly date the origin of the different orders of systematic astrology.