Interfacing LM19 temperature sensor with Arduino
The ADC has a built in capacitor at it's input. So, you should allow the capacitor to charge slowly to the actual value. If there is high resistance (LDR), it will take more time to charge.
Since, you are switching the ADC to different channels, you should give sufficient time for the ADC sampling capacitor to charge.   

Provide sufficient delay after choosing the channel - OR read the ADC several times, and consider the last couple of values once the values are stable. You can read once, give a delay, and do a second read.