The length of the day is not the only reason summers are hot and winters cold. Another is the elevation of the Sun above the horizon. When the Sun is near the horizon, not only are the shadows which it casts stretched to greater length, so is its illumination. Any beam of sunlight then spreads out along a greater distance on the ground, diluting the heat given to any area. The noontime Sun in winter is low in the sky, and its heating is less pronounced, while the summer Sun can be almost overhead, heating the ground much more effectively. This is further discussed in the chapter “The Angle of the Sun's Rays".
Babylonian priests, who tracked these regular changes of sunrise and sunset, soon realized that they provided an accurate way of measuring the passage of the seasons. They counted the days between solstices and equinoxes, and from this the first calendar was born. That was a great help to farmers, telling them when to prepare for sowing, when to expect seasonal rain, and in Egypt, when to expect the annual flood of the river Nile, which replenished the land. As will be described in the chapter “The Calendar", other cultures also had their stargazers and developed calendars of their own, probably in much the same way.