In the temperate parts of the earth, the seasons change as the earth makes its path around the sun. Near the equator and near the poles, the summer and winter solstice is not quite the same as it is in the temperate zones. So, for the sake of discussion, let's consider the summer solstice where it has the most effect and is most regarded as important.
As the earth orbits the sun, the light and heat from the sun seem to move north and south in varying degrees on the surface of the earth. This movement results from the tilt of the earth in regard to the sun. Since the earth does not stand perpendicular on its axis, at some times of the year, the southern hemisphere gets the most sunlight, and at other times, the northern hemisphere.
Summer is determined by which half of the earth is getting the majority of the heat and light. During June, the northern hemisphere is receiving maximum amounts of sun. In December, the souther hemisphere is the lucky recipient.
Three months prior to the first day of summer in either hemisphere, the sun crosses the equator. To the north of the equator lies a line called the Tropic of Cancer. South of the equator at the same distance away lies the Tropic of Capricorn.
When the sun stands directly above a tropic line, that hemisphere begins its summertime. In both hemispheres, the first day of summer is the longest day of the year. When one hemisphere enters summer with the longest day, the other enters winter with the shortest day of the year.
Between the Tropic of Cancer and the Tropic of Capricorn, the earth receives about the same amount of sunlight each day of the year. The minor changes create wetter and dryer seasons. These are also affected by warm and cool ocean currents that may swirl near a given region.
When the sun stands over a tropic line, it is called a solstice. When it stands directly over the equator, it is called an equinox because the north and south temperate zones receive twelve hours each of daylight and dark on that day. If the sun is moving northward, the equinox is the first day of spring in the northern hemisphere and fall in the southern hemisphere. The reverse is true when it heads south six months later.
The summer solstice occurs when the earth is tilting as much as possible toward the sun for the respective hemisphere. Due to the elliptical orbit of the earth around the sun, the earth is not always at its nearest point to the sun when summer begins. In fact, for the northern hemisphere, the sun is nearly at its farthest distance from the earth in June.
