God had said back in verse 3, “…Let there be light…” without talking about different kinds of light.  The verse simply says He divided the light from the darkness. 

In these next verses, God is talking about: 1.Signs, 2.Seasons, 3.Days, and 4.Years.  All of those involve some sort of light phenomenon, change, or occurrence.  Verses 14-18.  God says "...let there be lights...let them be for signs, seasons, days and years.  And let them...give light upon the earth..and God made two great lights..."  One to rule the day and one to rule the night.  We begin to have tides.  The moon begins to pull the oceans back and forth.

Perhaps the signs could be talking about the North star.  Or stars that come into view of earth at different times in the orbit of earth around the sun.  Or heavenly bodies that would be seen as a result of rotation.  Or explosions of novas.  Shooting stars?  When you talk about signs, seasons, days, and years, you are talking about systems for people to determine and organize their time.  When you are talking about dividing the day from the night--you are talking about rotation.  When you are talking about seasons and years, you are talking about the earth’s orbit around the sun.   All of those things are concerned with the passing of time.  

The word “Signs” might be about fixing navigational guides, or eclipses, or comets.  It sounds like God is organizing how the stars, sun, moon and other heavenly objects reflect light--to affect and guide life on earth.