And so from then onwards Daniel understood that the point of this grueling sundial project was not merely to plot the curves, but to understand why each curve was shaped as it was. To put it another way, Isaac wanted to be able to walk up to a blank wall on a cloudy day, stab a gnomon into it, and draw all of the curves simply by knowing where the shadow would pass. This was the same thing as knowing where the sun would be in the sky, and that was the same as knowing where the earth was in its circuit around the sun, and in its daily rotation.