Hi all guys and welcome to my YouTube channel. In this video lesson we will talk about astronomical seasons. As we know, the Earth performs a counterclockwise revolution around the Sun describing an elliptical orbit and since it is an elliptical orbit the Sun will occupy one of the two foci. Therefore the Earth-Sun distance will not always be constant; in fact, during the motion of revolution we distinguish the perihelion which represents the minimum distance from the Earth to the Sun which corresponds to about 147 million kilometers and which occurs around January 3 and the aphelion whose distance is approximately 152 million kilometers which instead is the maximum Earth Sun distance and occurs around July 4th. Observing the motion of revolution, it would be natural to say that when the Earth is closer to the Sun it will be hotter while when it is further away it will be cold. However, we know very well that in our hemisphere it is winter when we are closer to the Sun and summer when we are further away. How can this apparently contradictory phenomenon be explained? The reason is due to the fact that the seasons in addition to the revolution motion of the Earth around the Sun are the consequence of two very important factors: the first reason is due to the inclination of the earth's axis on the orbital plane called ecliptic, the second reason is that during the entire journey that the Earth makes around the Sun, the Earth's axis is constantly parallel to itself and always points towards the North Star. As mentioned, the earth's axis is not perpendicular to the plane of the ecliptic but inclined by 66 degrees and 33 minutes and therefore the normal to the plane of the ecliptic forms an angle of 23 degrees and 27 minutes with the earth's axis . If the axis of rotation were perfectly perpendicular to the orbital plane there would be no astronomical seasons as the exposure to heat and light in a given portion of the planet would be constant throughout the year. At the equator we will always have maximum insolation while the poles would always be cold and we would no longer speak of tropics and polar circles in reality this does not happen as during the year in all places on the earth's surface periodic changes in conditions are observed both lighting and heating. To evaluate the astronomical seasons we must consider with what incidence the solar rays add to the earth's surface and to do this we take as a reference some parallels which are the Equator which is the maximum circumference equidistant from the poles and which divide the earth into two hemispheres: the Boreal one. also called northern hemisphere and the southern hemisphere also called southern hemisphere. Furthermore, the parallels of the Tropic of Cancer of Capricorn and of Arctic and Antarctic Polar Circles are taken into consideration. March 21 is the day of the Spring Equinox equinox from the Latin equi noctium which means equal night. The circle of illumination passes tangent to the earth's poles and consequently all parallels are cut exactly in half for all points on the earth. The duration of the night is the same as that of the day. When the sun is at this point it culminates at the zenith of the Equator forming an angle of 90 degrees; the angles with respect to the two tropics of Cancer and Capricorn are both 66 degrees and 33 minutes those of the Polar circles of 23 degrees and 27 minutes. From this moment of the year the day begins to lengthen for all points of the northern hemisphere and on this day for the north pole begins the day whose duration will last for six months. June 21 is the day of the summer solstice; solstice from the Latin solstitium which means Sun that rests. On this day the circle of illumination passes tangent to the two Polar Circles, the Arctic and Antarctic, leaving the northern cap completely illuminated and causing the total darkening of the southern one. At noon on June 21, the sun's rays are at the zenith that is 90 degrees on the Tropic of Cancer while they will be 66 degrees and 33 minutes at the equator and 43 degrees and 06 minutes in the tropic of Capricorn. At the same time, the sun reaches its maximum height all year round for the regions located in the northern hemisphere. For all points of the northern hemisphere , the day has the longest duration and solar rays arrive with a greater incidence, while the points of the earth located south of the equator remain for a longer stretch in the darkened part. On September 23, the day of the autumn equinox as on March 21, the circle of illumination passes tangent to the earth's poles; all parallels are cut exactly in half and for all points of the earth the duration of the night is equal to that of the day. Also in this case the sun culminates at the zenith of the terrestrial equator forming an angle of 90 degrees.The angles of the two tropics of cancer of capricorn will both be 66 degrees and 33 minutes those of the polar circles of 23 degrees and 27 minutes from this day of he year begins to lengthen for all points of the southern hemisphere and on this very day for the south pole begins whose duration will last for six months. December 21 is the day of the winter solstice in this case the exact opposite occurs compared to June 21, the illumination circle always passes tangent to two polar circles, the Arctic and Antarctic, but the southern cap is completely illuminated, causing the total darkening of the northern one. At noon on December 21, the sun's rays are the zenith on the tropic of capricorn while they will be 66 degrees and 33 minutes at the equator and 43 degrees and 06 minutes at the tropic of Cancer At the same time the Sun reaches its maximum height of the whole year for regions located in the southern hemisphere in all points of the southern hemisphere the di has the longest duration and solar rays will arrive with greater incidence. Instead, the points of the Earth located north of the equator remain for a longer stretch in the darkened part. The polar circles and the tropics divide the earth into five astronomical zones characterized by very different lighting conditions. We distinguish a torrid zone, two temperate zones: the Austral, the Boreal and the polar caps. These are macro areas that do not coincide with the real climatic conditions which are instead influenced by the elements of time and climate such as precipitation, humidity, temperature and pressure and by climate factors such as latitude, altitude, various sea currents and vegetation.