The following answer is obtained by our Internet Research Algorithm : People observed the position of the rising and setting sun and the date when it began to rise and set in a different direction on the horizon. They noticed a coincidence or congruence between the observed solar cycle and the lunar cycles (28 days, 13 “moons” in a year, approximate 364/5 day combined cycle). They noticed that prominent stars were in the same positions on the horizon around the solstice times as well as the equinox times, which corroborated the idea of the 365 day revolving cycle. Much like you can observe where the sun rises and sets, you can observe the rising and setting of the stars and the constellations they create. At the summer solstice, one would expect certain constellations to reside in the same location in the sky. If you pay close enough attention (which those that studied the sky's did, back then), you could determine over the course of a dozen years or so that every 4 “years” you need to add a day in order to put the constellations in the right spot. This is what leads to the understanding of 365.25 days in a year, even at an early stage of human scientific development.