Historically, the common cyclic phenomena of the sky, such as the Sun's daily rising and setting, played an important role in the development of the geocentric and heliocentric concepts. Many of these phenomena were not discovered in any literal sense, but had been observed and known long before anyone wrote about them. What is new and has changed over time is an explanation in either the geocentric or heliocentric conceptual scheme of why, for example, the Sun rises and sets. Rather than trace their historical development, let us consider only our present understanding of these cyclic phenomena.
The different seasons are caused by the tilt of the Earth's axis of rotation relative to its orbital plane and by the Earth's revolution once a year about the Sun. As seen from Earth, the Sun appears to move eastward about 1o each day relative to the stars. Because of this, stars rise about 4m earlier each night than they did the previous night. Consequently, by the end of a month, stars are rising approximately 2h earlier than they did in the previous month. At the end of a year, the nightly change adds up to 24h, and the annual cycle of the heavens begins again.
Over time most ancient peoples were able to identify the Sun's yearly path through the constellations, a path called the ecliptic. Earth's geographic equator projected onto the celestial sphere produces an imaginary line called the celestial equator. The celestial equator intersects the ecliptic at two points, the vernal equinox and the autumnal equinox. The Moon and planets move almost entirely along a narrow band of sky, the zodiac, which is 16o wide and is centered on the ecliptic. The zodiac is divided into 12 constellation divisions, or signs, through which the Sun passes in successive months.
The Earth's axis of rotation is tilted 23.5o to its orbital plane, which is consequently the angle between the ecliptic and the celestial equator. In the Northern Hemisphere we incline away from the Sun in December and toward it in June. Consequently, the amount of sunlight falling on the surface of either hemisphere varies depending on whether the hemisphere is inclined toward or away from the Sun.
In the Northern Hemisphere spring begins in March at the time of the vernal equinox, when the Sun crosses the celestial equator from south to north. Summer starts in June at the time of the summer solstice, when the Sun reaches its maximum distance of 23.5o north of the celestial equator. The next season, autumn, begins in September at the time of the autumnal equinox, when the Sun crosses the celestial equator from north to south. Finally, winter commences in December, when the Sun reaches its maximum angular distance of 23.5o south of the celestial equator at the time of the winter solstice. In the Southern Hemisphere, the seasons are reversed; for example, Christmas occurs there during the warm summer months.