“Let them be for days” Gen. i. 14. says Scripture, not to produce them but to rule them; because day and night are older than the creation of the luminaries and it is this that the psalm declares to us.  “The sun to rule by day…the moon and stars to rule by night.” Ps. cxxxvi. 8, 9.  How does the sun rule by day?  Because carrying everywhere light with it, it is no sooner risen above the horizon than it drives away darkness and brings us day.  Thus we might, without self deception, define day as air lighted by the sun, or as the space of time that the sun passes in our hemisphere.  The functions of the sun and moon serve further to mark years.  The moon, after having twelve times run her course, forms a year which sometimes needs an intercalary month to make it exactly agree with the seasons.  Such was formerly the year of the Hebrews and of the early Greeks. The Syrians and Macedonians had also an intercalary thirteenth month to accommodate the lunar to the solar cycle.  Solon is credited with the introduction of the system into Greece about 594 b.c.  But the Julian calendar improved upon this mode of adjustment.  As to the solar year, it is the time that the sun, having started from a certain sign, takes to return to it in its normal progress.