"God made two great lights - the greater light to govern the day and the lesser light to govern the night. He also made the stars. God set them in the expanse of the sky to give light on the earth, to govern the day and the night, and to separate light from darkness. And God saw that it was good. And there was evening, and there was morning - the fourth day." NIV translation
Throughout the Bible, the number four is associated with trials. On this day, God created the sun and the moon and gave them a threefold purpose (to give light, govern, and separate). We looked at light in the last passage. They govern in that, for much of time, what we did as people was determined by the time of day. (People used to work only during hours of sunlight because there were no other lights such as electric or gas powered lights) To this day, many farmers still plant and harvest by the phases of the moon. Their third purpose was to separate light from darkness. Light and darkness have nothing to do with each other and in fact cannot exist together at the same place and time. God is light and, as Christians, we walk with Him in that light but many times we take a wrong turn and end up in the dark. That usually happens when we are faced with trials and temptations (symbolized by the number 4).