The reason is because when Period = 0, Sun.X = 0 too. What you need is decrease the amount you are multiplying cox(Period) by.
This should fix it. And I've used the size of the layout in the maths to ensure it'll always be the same, even if you change layout sizes, as it was originally 3200x900 for and it's 1600x900 in this example.
https://dl.dropboxusercontent.com/u/108 ... ement.capx