For LDR it require periodic maintenance. If dust has been accumulated it might no work. But using RTC i can say that it will be fine for working but problem will be degrade in angle and we cant keep feed back for control the action .
0AlphaOmega:
Unless you really want to calc the sun's position, as suggested below, track the sun.
I would think that 4 LDRs would be cheaper than a RTC, the code smaller and as pointed out, accuracy is of no concern, it just tracks. Simpler still if you use a N/S mountasyou only need 2 LDRs, and your single actuator. "darkness" is used to reset to the morning sun.IDK, but tracking may also provide more power from reflected light in times of cloud cover as the sensor, being a heliotrope would point to the brightest part of the sky, not where it thinks the sun is.
[edit]in fact, at Nick Gammon pointed out, you don't need a constant sun location if using a simple N/S mount and single actuator! All you need is (usable) sunrise and day length (1 byte would do, since you will not get 15hrs of usable sun) You may also have mountain/hill/building shadow to contend with . You may also need to adjust for apparent sun speed. You then move your actuator, starting at the correct time, at the correct speed for any day. Personally LDRs would be my choice - simples![/edit]