This instrument has two uses and was inspired by similar instruments made during the 17th and 18th centuries.

One side of the postal card shows a lunar calendar that gives the lunar phase and the lunar age (days that have passed since the last new moon) for any given day of the year. It consists of a graduated scale fixed on the outer and is divided up to represent the 12 months and 365 days of the year. On the inside a rotating disc showing the lunar phases for the period of one year.

On the other side we can find an instrument that allows us to determine the time high and low tide will take place. It is composed of an outer, graduated circle divided into fixed 24 solar hours and a revolving disc graduated for lunar time (24 hours). Is fitted with an index that allows us to read the lunar age on a scale dived into 29,5 parts. At its center is an engraving that acts as a mirror with a hole that shows the moon during its different phases.