The earth is not stationary, It slowly revolves on its axis around the sun. The part of the earth nearest to the sun gets the fullest benefits of the heat from the sun and thus summer happens. Fall or autumn happens when the earth is slowly moving away from the sun. . Winter comes when the sun has moved farthest away from the sun and spring begins when the earth is moving closer to the sun as it rotates.
According to Sci Jinks, the distance from the sun and earth at its warmest period is still not close, it being 91,400,000 miles away; at it coldest it is 94,500,000. That fact is not as simple as it seems however, since the earth is divided into the Northern and Southern Hemispheres.
Someone living in the United States would naturally assume that summer therefore for them would be at the time when the sun is closest to the earth, but not so. Summer is actually winter according to the distance chart and that is so because the closest distance from the sun to the earth at this time is in the Southern Hemisphere.
Whichever part of the sun is tilted toward the earth is the warmest and denotes summer time. It just happens then that the Northern and Southern hemispheres are at opposite poles. Were the sun rotating on a straight up and down – perpendicular – rather than a tilted axis, the seasons would probably be different, but since the axis is more like a slant mark than than the numeral one, the seasons are as they are.
What tilted the axis, that point on the map known as the north and south pole? Researching that question brings up more questions than answers. But since supposedly it is the axis and not the actual rotation of the sun that is contributory to the season, the question of what tilted the axis is a reasonable one. Some websites claim it was a huge cataclysmic accident several trillion years ago that caused this tilt of the earth, other disclaim it. This article will henceforth bypass the mention of it.
The next question, by comparison, seems more logical. What causes the seasons to suddenly change their nature and either fast forward to summer from spring, or to spring from winter. Of course these changes are atmospheric changes and belongs more to the specialty known as meteorology, an earth science but not particularly of the seasons.
Soltices and equinoxies come into play when studying the seasons. Understanding them is to know a little more about the seasons and their probabilities and possibilities. Yet they are simply terms descriptive of the relationship between the sun and the earth.
The solstice is the demarcation line between summer and winter. They occur each year in June, 20 or 21 and December 21 or 22. It is the points when the Axis is tilted at its farthest point toward or away from the sun. Daylight is affected as well. Summer days are longer than the shorter winter days. The vernal equinox is the corresponding term for the beginning of spring and fall, with the months of March and Septmeber completing the four season cycle.