Here's why:

* Understanding celestial cycles: The creation of calendars relies on the observation and understanding of celestial cycles, primarily the movement of the Sun and Moon.

* Tracking seasons and time: Early calendars were developed to track the changing seasons, which are directly influenced by the Earth's position relative to the Sun.

* Predicting astronomical events: Astronomers were able to predict eclipses, solstices, and equinoxes, which were crucial for calendar development.

While other earth science areas like geology and meteorology were also developing at the time, it was the study of the stars and planets that directly informed the creation of the first calendars.