If you live in a place where sunrise and sunset varies a lot throughout the year, you know that setting up an automatic lighting system in your home (or garden) is a pain. DIYer Paulo Renato's solution was to hook up his lighting system to an Arduino that taps into an astronomical clock.
The Arduino uses latitude and longitude to help decide when to click on the lights, so it's incredibly accurate. Otherwise, the build just includes an on/off switch, a Tiny RTC, a time adjustment button and a few other parts. It's a bit overengineered if you live in a place where daylight doesn't vary that much, but it can help automate your lights accurately.
Arduino Astronomical Clock for Automatic Light Control| Paulo Renato via Hack a Day