As its mains voltages (240VAC here in the UK!!!!) i'd be inclined to buy a pre-designed kit with a pre-made circuit board for the dimmer.
I know Maplins do a voltage controlled dimmer module and i think they're about a tenner so get one of those to fade the lighting.
Next.... Creating the control voltage. Well, ... this can be done in several ways, PIC (like Stevez sugested) would be ideal, but as your a new member i'm going to assume you dont have a PIC programmer, so its a hardware solution you need...
:?: :?: :?: :?: <ten minuites later> :?: :?: :?: :?:
What about a up/down counter IC (8-bit) and a (8-bit) DAC device?
The clock input to the device (clock frequency) could be set so that the counter moves up one step 0.04 seconds:
(90*60)seconds / 255 steps = 0.0472Hz
-- which is equal to 1 pulse every (1/0.0472) 21.2 seconds.
A simple 555 timer set as astable can do that easy - R1=300k, R2=3k C1=100uF. This would give 99% duty cycle, with a frequency of 0.0472Hz (on time is approx. 20.964 secs / off time is approx. 211.8mS)
Now place the counters' output value into the DAC to get a DC ramp voltage that will change as the counter changes. Feed this (via an op-amp to adjust offsets, and add a bit of gain etc) to the input of the voltage controlled dimmer module.
When the counter reaches its maximum value, we need to disable the clock.
When night comes, we then need to flick the count down switch somehow to tell the counter to count downwards, then restart the clock. The counter will count down, the DAC output will ramp down and the dimmer module will dim down.
Now who would like to draw the schmatics!
dimmer module is a fiver! bargain! (goto the shop, they will have them in stock or be able to get them)
**broken link removed**
** input is 0->10Vdc ... this is great, we can simply use a 5V full-scale DAC with an opamp connected to the output with a gain of 2.. easy!