The Moon Stick - fun with an astronomical slide rule
Analog astronomical computation has a long history. This isn’t as sophisticated as the 2200 year old Antikythera mechanism, but it’s still pretty cool and works when the power is off. The Moon Stick is an analog computer with 6 triangular slides that can calculate the phases of the moon for any date up to 8000 years in the past or future. The 4 digits of the year are set sequentially, followed by the month. The moon phases are read off across from the days of the month. This image shows the moon phases for December 2014.