Great! I was hoping that to be the case. I’m using the 555 because I have several of them ‘just sitting there’ and have never used one. I don’t have the LMx39 to play with.
The idea is sort of like when the mailman opens the mailbox once a day to put your mail in, the LDR will turn the system on so the Arduino can do its thing. Then the box gets closed for 24 hours. Sometime after the box gets closed, the Arduino will finish its job and shut things down, to preserve the battery.