NOONDAY GUN
Noonday gun is a sundial equipped with a gun. When it is loaded and correctly positioned the cannon explodes at the moment when the sun is at its zenith.
Just like sundial or gnomon and contrary to hourglass and clepsydra noonday gun can only work in the presence of the sun. It must be positioned in the north-south axis and a magnifying glass must be correctly positioned to light the gunpowder at noon.
The noonday gun has sometimes been used as a regulator to set the time of watches. It indicates the true solar time and during the whole year winter time for the regions of the globe that change the time twice a year.
A noonday gun can for example be seen at the museum of history of science in Geneva who also made a video on this object which can be seen on this link.