The motion of the Sky and the Earth, the Astronomical Seasons
Since ancient times among many civilizations in different parts of the world the observation of the sky and of its cyclical phenomena has provided the basis for the development of the calendars as shared systems of time units organization with the aim to plan agriculture and hunting, divination and prognostication, as well as several other recurring chores and events of the mankind.
Image: the 15th century astronomical clock in Prague (Czech Republic), the third-oldest in the world and the oldest one still working (copyright: Andrew Shiva, Godot13 User, via Wikimedia Commons, CC BY-SA 4.0 license).
The positions and motions of the Earth, Sun, Moon, and stars gradually shaped the life of the different folks worldwide, marking the alternance of the seasons and the cycles of the human activities throughout the year, including the main agricultural and pastoral dates, the traditional religious and civil feasts and celebrations, thus contributing also to define the heritage and folklore of any local community.
From our Earth perspective the two most important astronomical objects are the Sun and the Moon, and in fact their cycles quickly became important in the building and understanding of the first calendars. The key astronomical astronomical events on which the calendars are based properly include:
- the day, which is based on one complete rotation of the Earth on its axis, and represents the time from one sunrise to the next sunrise;
- the year, which is based on the revolution (one complete orbit) of the Earth around the Sun, and corresponds to approximately 365.24 days;
- the month, which is based on the revolution (one complete orbit) of the Moon around the Earth, and lasts approximately 29.53 days.