This Sunday is the biggest day of 2020, but also the first official day of the summer, as on Saturday night shortly after midnight (at 00:43 Cyprus time) the summer solstice took place. On the other hand, today is the first day of winter in the southern hemisphere. From now on, in the northern hemisphere the days will gradually decrease, at first imperceptibly and from August more noticeably, while the nights will increase accordingly.
The Sun is "standing" today in its northernmost point in the sky, as one observes it from the northern hemisphere of our planet, where Greece is located. But at the upper end of our hemisphere, in northern Scandinavia above the Arctic Circle, today is not considered the beginning but rather the middle of summer, as during the summer solstice there is the phenomenon of the "midnight sun".
Sunsets and equinoxes, which astronomically determine the duration of the seasons, occur because the days of the year do not have the same duration. The reason is that the Earth orbits the Sun and its axis of rotation is almost 23,4 degrees. If there was no such inclination, there would be no seasons and the day would always last 12 hours, while the Sun would follow the same "path" above the Earth every day.
However, due to the inclination of the axis of rotation, in the first half of the year (from spring to autumn equinox) the northern hemisphere "leans" towards the Sun, culminating in the summer solstice which occurs between 20 and 22 June, while the other half year the southern hemisphere "tilts" more towards the Sun, culminating in the winter solstice.
According to the astrophysicist Fiori-Anastasia Metallinou of the National Observatory of Athens, in the 5th c. e.g. The astronomer Metonas, observing from the hill of Pnyx in Athens the movement of the Sun during the year, calculated with great accuracy the duration of the seasons. The instrument he used was called a sunflower, whose base is preserved and can be seen on the hill of Pnyx.
Metonas had observed that during the summer solstice the Sun rose above Lycabettus, as he observed it from his ancient observatory in Pnyx. Six months later, during the winter solstice, he was observed rising from the top of Hymettus. According to the Attic calendar, the year began in ancient Athens after the summer solstice. It had 12 months, dedicated to each god, with each month beginning with the New Moon.
On the other hand, Eratosthenes, an important mathematician and geographer of the 3rd c. BC, had the idea, using the position of the Sun in the summer solstice, to conduct an experiment in two different cities in Egypt and calculate the perimeter of the Earth.
Source: KYPE