The four seasons are another function of the movement of the Earth relative to the Sun. The Earth spins on its axis once every twenty-four hours. This is the movement that gives rise to time zones. The Earth also orbits the Sun. It takes a year to complete its journey round the Sun. The important fact is that the Earth's axis is tilted about twenty-three degrees to the plane of orbit. The effect of this is the seasons experienced on Earth.
Although the Earth spins on its own axis it does not otherwise move relative to the Sun other than to journey round it. Thus at times the Earth seems to be leaning away from the Sun and later in the trip round it is inclined towards it. This affects the parts of the surface of the Earth on which Sun's rays impinge. At times no direct sunlight falls on parts of the earth. At other times sunlight falls continuously on parts of the surface. These effects are seen in the North and South polar regions. The other major effect is the angle at which the Sun's rays strike the earth on either side of the Equator. These solar effects define the five great parallels of latitude. The defining extremities of the regions adjacent to the poles which are alternately each year in total sunlight and total darkness are called the Arctic (northern) and Antarctic (southern) circles. During the period when the southern polar area is in continuous daylight the Sun appears to be overhead at the southern extremity of the limit of the tropical region near the Equator. This parallel of latitude is known as the Tropic of Capricorn. At this time it is mid-summer in the southern hemisphere. The reverse of these effects are seen during the northern summer. The parallel of latitude defining the extreme limit of the northern tropical region is the Tropic of Cancer. All of this is best seen with the help of a picture.
It will be noticed immediately that all of these definitions and terms are related to the northern hemisphere. This is not surprising. When all of the astronomical observation and mathematical work to establish these things was being done much of the southern hemisphere was still unexplored and, except for limited numbers of indigenous inhabitants, no-one lived in the south. The four seasons still occur in the southern hemisphere, they are just reversed in terms of the identifying month compared with the northern half of the world.
only remaining phenomenon concerned with the relationship and
relative movements of the Earth and the Sun is "Twilight".
It is another aspect of a retiree's life in a new place that must be
considered along with time zones and seasons.
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