The Earth is constantly moving. Once each year it completes one revolution in its orbit around the sun, and once every 24 hours it completes one rotation on its own axis.
The Earth’s rotation on its axis is what causes day and night. The different periods of the year that are known as seasons—spring, summer, fall, and
winter—are caused by the tilt of the Earth’s axis and the revolution of the Earth around the sun. In most places, the different seasons bring changes in the
length of daylight, in temperature, and in weather.
Seasonal changes occur because the Earth’s axis is tilted at an angle of 23.5 degrees to the plane of its orbit around the sun. Because of this tilt, the Earth’s
North and South Poles are each turned toward the sun for part of the year and away from the sun for the rest of the year. When the North Pole is tilted toward the sun, it
is spring and summer in the Northern Hemisphere. The sun shines longer each day than it does in fall and winter, and it takes its highest and longest path across the sky
on the first day of summer. For this reason, this is also the longest day of the year.
The seasons north and south of the equator are always the opposite of each other. During the time one pole is tilted toward the sun, the other pole is tilted away from it.
Thus, while it is spring and summer in the Northern Hemisphere, it is fall and winter in the Southern Hemisphere. In winter, the days are shorter, the sun is low in the
sky, and the sun’s radiation is spread over a larger area. During fall and winter at either the North or South Pole, there is almost continuous night.
Some regions of the Earth experience fewer seasonal changes than others. This is especially true in areas near the equator, which receive about the same amount of sunlight
all year long and have little or no change in temperature throughout the year. These regions generally have only a rainy season and a dry season.
The Astronomical Seasons
Seasonal changes in temperature and weather characterize what are known as the climatic seasons. The beginnings and ends of these climatic seasons vary depending upon the
place and the year. Scientists, however, have worked out specific dates for when what are known as the astronomical seasons begin and end each year.
The summer solstice, which occurs on June 20 or 21, is the date that summer begins in the Northern Hemisphere. This is the day of the year when the Northern
Hemisphere experiences the most hours of daylight. December 21 or 22 marks the winter solstice, the beginning of winter in the Northern Hemisphere and the day with
the fewest daylight hours. The beginning of spring, the vernal equinox, occurs on March 20 or 21. On this day, the sun appears directly above the equator, and the
hours of daylight and darkness are equal. The same is true for the beginning of autumn, the autumnal equinox, which occurs on September 22 or 23.
The path of the Earth’s orbit around the sun is not a perfect circle. As a result, the planet’s distance from the sun changes. The Earth is nearest the sun about
January 1 and farthest away about July 1. The difference in distance is not enough to affect the seasons. But the dates marking the start of the astronomical seasons may
differ from year to year because our calendar requires the addition of a full day every four years. The year with the extra day is known as leap year.
Colin A. Ronan
Fellow of the Royal Astronomical Society
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