What does it mean that the “greater light,” and the “lesser light” and the “stars” of Genesis 1 were “given for signs, seasons, days and years?” First and foremost, these celestial bodies are not accidents of some physical process but that they were purposely established for use by mankind. They are points of reference for every generation that has looked up into the heavens. These would be used for days, months, seasons and years; more specifically for time markers. Furthermore, these lights have a significance to them beyond just marking time: they are for signs which implies– communication!
The Sun and the Moon and the planets are ‘fixed’ in the heavens, that is to say, they are faithful in their orbits and run continuously in their circuits without interruption. For example, we can precisely measure the length of the solar year: 365.242199 days. Likewise, we can measure the synodic orbit of the moon at 29.530589 days per month. Neither of these two numbers are perfect multiples of each other (about 12.36826664717 to 1) so about every two or three years another month needs to be added to keep the seasons from rotating throughout the year.
Every date mentioned in the Biblical text, for example, uses this system of dating, along with perhaps the day number of the week. For example, Genesis 7 tells us that “11 In the six hundredth year of Noah’s life, on the seventeenth day of the second month—on that day all the springs of the great deep burst forth, and the floodgates of the heavens were opened.”
This is a specific, traceable day, provided that we have a continuous chronology from start to finish. Of course this is only the beginning of a wonderful study of the people of Israel that is tied to creation through time. Their use of the seven day week is intriguing, because it doesn’t match very well with any of the celestial bodies. However, the number seven seems to pop up at different times and places: many prescribed events are done for seven days or weeks(Pentecost), and sometimes it is hidden from plain sight: dividing the lunar year of 354 days in half nets you 177 days. (More to come on this fascinating number!)