This is kinda cool to me. This is the mars calender that will most likely be used by martian colonists. Each zodiac sign represents the position of mars as viewed from the sun, so at midnight during that zodiac month, that constellation should be straight overhead. If you put a penny on the diamond of a year of the outer circle (mars calender) and then put another penny on the 1st of January in the inner circle (earth calender) and then move them together one diamond at a time counterclockwise, you can determine the alignment of earth and mars at a certain time of year and the corresponding calender dates.