Its been done before, just three sensors and basically the motor turns to the left/right based on the left/right sensors, until the sensor in the middle reads higher.
Couldn't you just let it sense that more light is coming from east, and it'll turn to it? I suppose it'd be more efficient to have it reset itself at night, but why not make it easy?
For facing east why not just add a Solid state compass to the mix, similar to the compass' you get in mobile phones/gps etcYou'd need three sensors to know when to stop, otherwise you'll be alternating left and right without knowing when to stop.
Better answering the question then him thinking he's being ignored, causing him to bump/spam this issue?
Also, the actual position of the sun, doesn't always equate to the highest amount of solar activity, cloud cover etc sensing where it is brightest would be better, otherwise you're defeating its purpose.
Tubs and shields are not needed in the hackaday project I linked, I think such things would over-complicate the problem.
Thinking about it, you probably would only need two sensors if you can accurately place them at the front of the device, minus one value from the other, a positive number move to the left, and a negative number move to the right, zero don't move.
I'd imagine you'd be using move power to run it than you'd be generating from the panels, not sure.