Seasons

Seasons and the Sun

The Sun appears to move eastward around the ecliptic once each year. This yearly motion results from the Earth’s revolution about the Sun. The Sun also moves north and south of the celestial equator. On the date of the summer solstice (June 21), the Sun is north of the celestial equator and has a declination of 23.5 degrees while, on the date of the winter solstice (December 21), the Sun is south of the celestial equator and has a declination of -23.5 degrees (see Figure 6.1).

      Figure 6.1 The North-South Motion of the Sun

The Sun moves north and south because the Earth’s rotational axis is inclined to its orbital plane (see Figure 6.2).

Figure 6.2 Inclination of the Earth’s Axis

This north-south motion causes the Sun’s rising and setting azimuths, the length of the day, and the Sun’s noon altitude to change throughout the year. When the Sun is at the summer solstice, it rises farthest north of east, days are longer than nights, and the noon altitude of the Sun is greatest. At the winter solstice the Sun rises farthest south of east, days are shorter than nights, and the Sun’s noon altitude is the smallest of any time in the year.

Because of the Sun’s north-south motion, the angle that the Sun’s rays strike the Earth changes throughout the year. During the summer the rays come in more vertically than they do in the winter. Consequently, in the summer the energy in a cylindrical bundle of rays goes into heating a smaller area than it does in the winter (see Figure 6.3). Thus, the warming effect of the Sun’s rays is greater in the summer than in the winter. It is this difference in heating efficiency that results in the seasons.

If the Earth’s rotational axis were not inclined to its orbital plane, the Sun would rise at the east point and set at the west point each day. Each day the Sun would remain above the horizon for 12 hours and below it for 12 hours. We would not have seasons as we know them. It is the 23.5 degree inclination of the Earth’s axis of rotation that produces the seasons.

Figure 6.3 Seasonal Difference in Heating Efficiency