Keeping Track of Time

By analog trade and skill, one can track time by charting a traceable region of sky and maintaining the image as a photograph, drawing, chart, et cetera. After a certain number of documented depictions, an overlap occurs. That overlap is the sign of a full cycle, and that cycle is what signifies one measureable unit of time. 

As an example if one were to track a rectangular region of night sky with a telescope for ninety days - with the nintieth day being the day of the repeated image - then one full cycled unit would be of measure.