Towards the shortest day of the year

December 21 is considered the "first day of winter" because on this day the height of the sun above the horizon at noon is the lowest of the year, but why is this so and does it affect our lives?

December 21 is considered the "first day of the winter” means that on this day the height of the sun above the horizon at noon is the lowest of the year, but why is this so and does it affect our lives?

To understand this, you only need to glance at the globe closest to you and you will be able to prove that the axis along its length the earth rotates at an angle of about 23.5 degrees from the perpendicular to the table. This angle also exists in reality, but instead of a table on which the globe stands, the plane to which we compare the inclination is the "whip plane", a sort of imaginary table on which the planets move. In the earth's sky, we will see the sun, the moon and the other planets on the Melka plane (the "signs" are the constellations we see on the Melka plane).

Although in school they teach that the sun rises in the east, this is not entirely true, in different seasons the sun rises with slight angle changes from the east to the north or the south. Now imagine two different situations, one in which the sun reaches a great height above the horizon, for this purpose it rises in a relatively northern point because the path the sun travels * in the daytime sky is long and the duration of the day is also long, this situation occurs in the summer. In winter, the height of the sun above the horizon is lower, its sunrise point is further south and the time it spends above the horizon is much shorter. Although, sometimes a second is added to this or that year because of the interaction between the Earth and the Moon, but overall - the rate of rotation of the Earth around its axis is almost unchanged.

Now imagine a real winter day, one where when you get home you immediately go to the stove to warm your palms a little. How will you put your hands? Turn them right perpendicular to the oven! In this way, you will feel the most of the heat of the oven and a pleasant calmness will spread throughout your body. This situation is similar to summer - when the sun is high in the sky, its rays are concentrated in a relatively small area.

In winter, the sun is low and close to the horizon, then the same amount of sunlight spreads over a larger area, and we are less warm. Of course there are other factors that change the degree of heat we feel, but the height of the sun above the horizon is the most significant. For this reason, when we have winter in the northern hemisphere, summer in the southern hemisphere, and vice versa.

In the ancient world they knew about the changing place of the sun's sunrise and the connection to the seasons, perhaps the most famous place is the site stonehenge in England. At Stonehenge, huge stones are arranged in circles so that if you stand in the center of the circle and look towards a certain giant "jam" you can see the sun shining through it on that special day. In this illustration from the 18th century you can see the sun rising in slightly different places on two consecutive days, with the help of the stones in the place its builders knew when the seasons would change and perhaps also when to celebrate their holidays. In other places in the world there are stone circles, although less impressive, which were probably used for similar purposes, but we do not know this for sure. It is possible that a site called Rujum al Hiri (Ghost Wheel) located in the Golan Heights was such a huge calendar, but we are not sure.

For thousands of years we have not needed giant stones to predict the seasons, we update the calendar at the level of seconds here and there and since the 60s weather satellites allow forecasters to look down on us and predict when it will rain and when sandstorms will cover the sky. In this image taken from the EUMETSAT satellites it is possible to distinguish not only cloud systems on the surface of the earth or the arrangement of the countries on the surface of the globe.

If we look a little deeper, we can notice that the northern and southern halves of the earth are not lit equally. The southern (lower) hemisphere is brighter, the entire south pole is even lit! Travelers to the South Pole in the near future should prepare in advance for a long, long day that will last for months! At this time in the North Pole the sun does not rise at all. In the picture you can also notice that the earth is lit "from below", the sun does not move up or down during the year, but take a second look at the globe and you will immediately notice - when the earth revolves around the sun, its axis sometimes points the northern hemisphere towards the sun and sometimes the southern one.

Before the agricultural era, there was no real need to know the seasons, later the understanding came that through the movement of the sun in the sky we can predict the seasons and today, we look down at our earth and know whether it is worth taking an umbrella or a short shirt. We may not yet be able to make it rain where and when we want it, and only a tiny drop of the sunlight that reaches the earth is used by us to generate electricity, but look - what a long way the human race has come.

* Of course the sun does not move around the earth but the earth is the one that rotates around its axis.

Amir Barnett is an activist at the Ilan Ramon Center for Youth Seeking Physics