Greenwich Park is the home of the Prime Meridian Line. It is where GMT (Greenwich Mean Time) starts and thus, poetically, where time begins. The Prime Meridian Line used to be a line drawn on the earth (not around the globe though as I would imagine) but now, it is represented by a green laser beam. This line can be seen clearly in the night sky.
The park also hosts the National Maritime Museum and the Royal Observatory. As it sits on top of a hill, the place is perfect for sky watchers and star gazers. But apart from science, Greenwich Park also offers spaces for sport, dining (including picnics) and simply relaxing.