Precession of the Equinoxes

The oldest system of time reckoning using cyclical astronomical events was fixing the beginning of the year. While it is based on the proportions of days to years, it transfers easily to rotations per orbital circuit by simple division. In its expanded form of 1461 days in proportion to 4 years, a day represents one rotation however a year doesn't represent one orbital circuit insofar as a year formatted as 366 days (Leap year) is just as valid as a year with 365 days.

The first annual appearance of a star to one side of the Sun is due to the orbital motion of the Earth as the horizon acts as a type of sunshade with the horizon acting to allow everything to the left of the sun in the evening and the right side of the Sun in the morning. Only on a solar eclipse does the central Sun display the solar system in its entirety where stars and planets display their position either side of the Sun from our orbital vantage point -

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Regulus which appears to the left of the Sun will eventually be seen to the right as the Earth moves along its orbital circuit and this astronomical event is the baseline for the calendar format, the proportion of rotations to orbital circuits, the precession of the equinoxes and many other primary and secondary topics.