Translations of Encyclopedia about Geology

Seasons

Seasons divide the tropical year (solar year) into four periods, which are determined by the apparent solar orbit in the sky.

Their occurrence, however, is regulated by the orbit of the earth around the sun and the tilt of the polar axis against the plane of the earth's orbit. That means, that the equatorial plane (perpendicular to the connecting line of the two poles) is tilted 23,5 degrees against the plane of the earth. This angle does not change as earth orbits around the sun. What changes during the course of the year is the angle at which the solar rays reach the earth. In wintertime, the sunlight always reaches a given area following a more slanted path and it disperses more widely. In summertime, the opposite happens: the sunlight comes down more vertically, encompassing a smaller area, thus producing more warmth.

These changes are gradual, not abrupt. In addition, the difference in elevation, movement of the air masses, and ocean currents are also important factors concerning the climatic conditions.

The patterns in the animal behaviour such as, for example, annual migration, or in the case of flora, the onset of the flowering season, are certain signs that the season is changing.

Spring, summer, autumn, and winter are prominent mainly in the temperate zones of the earth. At the south pole and the north pole, there are only two seasons: polar night and polar day. The seasons may be characterised also by a dry season or a rainy season. The greatest temperature differences between summer and winter occur in Siberia. Winter temperatures may fall to minus 75 degrees Celsius, while the summer temperatures may reach 40 degrees Celsius.

Although the sun rises every day in the east and sets in the west, the location of the sunrise and the sunset is not always the same. When both earth poles are equidistant from the sun, the day and the night are of equal length of time. In the northern hemisphere, this happens on the 21st of March, which is the first day of spring. On that day, the sun rises exactly in the east and sets exactly in the west.

When the north pole reaches its shortest distance from the sun, the date is the 21st of June, which in the northern hemisphere means the beginning of summer. The day of the summer solstice is also the longest day of the year.

Autumn begins on the 23rd of September, when the day and the night are equally long (autumnal equinox). Again, that day the sun rises exactly in the east and sets exactly in the west.

Winter solstice on the 21st of December marks the beginning of winter. It is the shortest day and the longest night of the year. This information is valid for the northern hemisphere, because the opposite happens in the southern hemisphere. When we have spring, it is autumn there, and our summer is their winter.

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