tca785 dimmer
It's a strange thing to need.But nevertheless here's how I would do it:
A pic 16F84 can have 13 available outputs.Imagine that you have the full-moon face printed on a disk 15cm. in diameter.Behind it put some semitransparent white plastic material.Then behind the disk you make 13 independent compartments opaque to each other.Then put inside 13 white LEDs.
Each LED must have individual resistor so that when all of them are ON,the light will be uniform.
This way not only will you have a very exact representation of the illumination but it will be much more beautiful,too.
The program is very simple (provided it is not the first one you make).The quartz will provide the necessary accuracy and the problem is that you'll have to start the device at a certain moon phase and back the power up so that a power failure does not occur.