The Lunar Calendar
Astronomy gives us a calendar based on the Moon.
The Lunar Calendar is a calendar that is based on the numerous cycles of the appearance of the illuminated portion of the moon that is visible to viewers on earth, otherwise known as the lunar phase. Since the Moon orbits the Earth, the lunar phase will change cyclically based on the position in relation to everything else. Although there are multiple variations of the Lunar Calendar, the only one that is purely lunar based is the Islamic (Hijri) Calendar. This calendar always has 12 months. The other calendars, which may be more popular and well-known are actually lunisolar, which has basically become synonymous to lunar in this day of age. The Chinese, Hebrew, and Hindu calendars are reflective of this type, as their months are variable.
Also check out our info on the Catholic Calendar