A little bit more of astronomy

There are two main movements of the Earth. That rotation and the revolution. The rotation is the movement of the Earth around its axis. An entire rotation takes about 24 hours. At any time of 24 hours there is a half sphere of the earth that receives the light of the Sun and another half which is surrounded by the night. The line that separates the dark half side of the Earth by the half illuminated side and that moves as the Earth rotates is called the circle of illumination. The revolution is the movement that makes the Earth traveling along its orbit around the Sun with a speed of more than 107.000 km / h along an elliptical orbit. The distance between the Earth and the Sun is on average 150 million km; "average" because, since the Earth's orbit is slightly elliptical, this distance varies throughout the year. In (around) January 3 our planet is located to its minimum distance from the Sun, about 147 million kilometres, in the position called perihelion; about six months later, around July 4, the Earth is at its maximum distance from the Sun, about 152 million of kilometers, in the position called aphelion. Variations in the amount of solar energy received from the Earth due to these small changes in distance are very limited and have minimal consequences in the context of much greater seasonal variations in temperature: for example, the Earth is closest to sun when in our own hemisphere, the northern one, is winter.
When the Sun is exactly on the vertical of a given area its rays do a route least equal to the thickness of the atmosphere, when on other hand, are tilted of 30 degrees, they walk into a route twice that minimum, and, when they are inclined of 5 degrees, their journey through the atmosphere is eleven times greater. So, the longer the journey of solar radiation through the atmosphere is, the greater the chance that they will be absorbed, reflected and scattered, and the more the intensity of radiation reaching the earth's surface is reduced.
The Earth, let us not forget, is spherical, so every day the rays of sun at midday vertical fall only on points of the earth's surface that are located in a particular latitude, as we move northward or southward over that latitude, the sun's rays affect the Earth's surface with corners gradually smallers. In addition, the inclination of the sun's rays at a certain latitude varies throughout the year, because the orientation of the Earth: in fact, the sun is not perpendicular to the planeof earth’s orbit (if it were, as now we will see, changes of season would not be) but is 23.27 degrees inclined from the perpendicular. Moreover, since during the movement of revolution of the Earth around the Sun the direction of earth does not change (the axis move parallel to himself, always pointing toward the polar star), the orientation of the latter compared to sunlight change in the various periods of the year.
There is one day in a year where the the northern hemisphere is 23.27 degrees "inclined" towards the Sun; six months later, when the Earth has reached exactly the opposite position along the orbit, the northern hemisphere is 23.27 degrees "inclined" in the opposite direction to the Sun. In the days between these two extreme situations, the Earth's axis is tilted less than 23.27 degrees compared to sunlight.
This periodic change of orientation of the Earth than the sun makes the area on which the rays of sun at midday fall vertically to move annually between 23.27 degrees north of equator and 23.27 degrees south of it. As a result, in many places the maximum height of the Sun in the sky (which is at noon) varies during the year, almost 47 degrees (ie 23.27 degrees + 23.27 degrees). This " annual migration" of the rays of the sun have effects of such migration on the annual cycle of weather conditions, which cause the various seasons. On June 21 the Earth is in a position where its northern hemisphere is directed towards the Sun, with an inclination of 23.27 degrees. At this date, at noon, the rays of the sun are perpendicular to the areas located at 23,27 degrees north latitude, which is a parallel of the Earth called Tropic of Cancer. For those who live in the northern hemisphere on June 21 is the summer solstice. Six months later, on December 21, the Earth is in a position opposite to the previous and rays of sun at midday are perpendicular to the areas located at 23.27 degrees latitude south, which corresponds to a parallel of the Earth called Tropic of Capricorn. For those who live in the northern hemisphere on December 21 is the winter solstice. On the same day in the southern hemisphere occurs the opposite situation, you namely have the summer solstice. Halfway between the two solstices fall equinoxes: in September 22 in the northern hemisphere you have the autumn equinox, in March 21 the spring equinox (the opposite happens in the southern hemisphere). In those days, at noon, the rays of the sun are perpendicular to the equator (0 degrees latitude) and tangent to the poles, because along its orbit the Earth is in a position that the Earth's axis is not inclined either to the Sun, nor in the opposite direction to it.