CTuesday, December 21 will be the winter solstice in the northern hemisphere. At noon, the height of the Sun above the horizon will be the smallest for the whole year. In the days that follow, the Sun will rise barely higher in the sky and this explains the origin of the word: sol (for “Sun”) and stop (to stop “). It will also be the shortest day of the year. The following days will grow very slowly, which is what we really need in the dark times we are living in now. But this lengthening of the day is not done symmetrically between rising and setting: the Sun was already setting later and later since December 18 and it will continue to rise later and later until January 6.
In space, the Earth turns on itself in a day like a top, around an axis which passes through its poles, an axis which is not entirely perpendicular to the plane of the Earth’s orbit around the Sun and which points to the North Star. Our winter solstice occurs when the direction of this axis is closest to the direction of the Sun.
Many still think that winter is the time of the year when we are furthest from the Sun, but that is of course absurd: when the northern hemisphere is winter, the southern hemisphere is summer. On the contrary, the Earth will be closest to the Sun on January 4. If it is colder in France in December than in June, it is because the sunlight hits us more grazing. In our latitudes, we receive during the winter only about 40% of the solar energy received in midsummer.
Understanding your place in the Universe
All of this has been well known for a long time, but it unfortunately seems that many of our citizens are unaware of it. Could there be a regression in the scientific knowledge of the public, even this elementary? Since the beginning of the XIXe century, high school math programs included a part called “cosmography.” The aim was to describe the main characteristics of the Solar System: the seasons, eclipses, the Sun and the planets, comets, etc. A famous mathematician, now deceased, explained to me that when he had taken the oral test of the aggregation of mathematics in 1948, the jury had asked him to present the theory of the phases of the Moon, which would put in great difficulty more than one candidate today.
Mathematics teachers did not like this teaching which they considered to be outside their discipline. However, they were lessons in geometry in space – literally – which allowed the students to better understand their place in the Universe. But the arrival of “modern mathematics”, which emphasized abstraction and rejected any link with the natural sciences, put an end to the teaching of cosmography in 1968, without anyone commenting on it. emotion. It must be said, to justify the abandonment of these subjects in mathematics lessons, that astronomy has gradually transformed into astrophysics, which relates more to physics.
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The unfortunate eclipse of cosmography