Watchers of the Heavens

The shortest shadows during the day define the north direction

    Suppose you watch the Sun rise and set day after day. Using as markers features on the horizon--trees, houses, etc.--you soon realize that the points where Sun rises and sets are not always the same, but shift week after week. On the other hand, the direction of south where the Sun is "highest" above the horizon does not change, and neither does that of north, the direction of the shortest shadow (in most countries). Because those directions are fixed, it is best to choose as the "true" east and west those directions which are perpendicular to north-south. Only twice each year are sunrise and sunset exactly in those directions, but they help measure and understand what happens in the rest of the year.

Seasons of the Year     

    Even in Babylon the year has seasons--winters are cool, summers dry and very hot. As already noted, twice a year, halfway between summer and winter, the Sun rises exactly in the east (as defined above), and sets exactly in the west (well, nearly exactly, in both cases). We now know that on the days when this happen, day and night are very nearly equal in length, and that time of year is therefore called "equinox." One equinox happens in the fall ("autumnal equinox") and one in the spring ("vernal equinox," "ver" is Latin for spring).

    As fall advances towards winter, the location of sunrise moves south, as does the location of sunset. The steepness of the curve traced by the Sun does not change, nor does the rate ("speed") with which the Sun appears to move along it, but the length of the curve changes, it becomes shorter. Around December 21 --the "winter solstice" halfway between the equinox dates (typically, September 23 and March 21) sunrise and sunset are as far south as they can go (at any one location). As a result, the Sun has its shortest path for the year, the day is at its shortest and night is at its longest. Other days of that season are short, too, which is one reason for the colder weather in winter.

The apparent path of the Sun across the sky. In summer, the Sun's path is longest, and so are the days. In winter, the Sun's path is shortest, and so are the days.

    After that the points of sunrise and sunset migrate northward again, and days get longer. This migration continues past equinox (when it is at its fastest), and the Sun crosses the horizon furthest northwards around June 21, the "summer solstice" (celebrated in some cultures as "midsummer day"), longest day of the year with the shortest night. After that days get shorter again as sunset and sunrise migrate south again. The long days of summer, of course, match the warmer summer weather. The reason for this behavior will be described in section #2 and section #3.

Elevation of the Sun     

    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 section #3a

    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 section #6, other cultures also had their stargazers and developed calendars of their own, probably in much the same way.