This Tuesday, December 21 will be the winter solstice in the Northern Hemisphere. At noon, the height of the Sun above the horizon will be the lowest of the whole year. In the following days, the Sun will rise only slightly higher in the sky and this explains the origin of the word: sol (for “Sun”) and sistere (for “stop”). This will also be the shortest day of the year. The following days will lengthen very slowly, which is much needed in the dark times we are living in. But this lengthening of the day is not symmetrical between sunrise and sunset: the Sun has already been setting later and later since December 18 and will continue to rise later and later until January 6.
In space, the Earth turns on itself in one day like a top, around an axis that passes through its poles, an axis that is not quite perpendicular to the plane of the Earth’s orbit around the Sun and that points to the North Star. Our winter solstice occurs when the direction of this axis is closest to the direction of the Sun.
Many people still think that winter is the time of year when we are farthest from the Sun, but this is of course absurd: when the Northern Hemisphere is in winter, the Southern Hemisphere is in summer. On the contrary, the Earth will be closest to the Sun on January 4th. If it is colder in France in December than in June, it is because the Sun’s light hits us more sharply. In our latitudes, we only receive during winter about 40% of the solar energy received during the summer.
Understanding your place in the Universe
All this has been well known for a long time, but unfortunately it seems that many of our fellow citizens are unaware of it. Is there a regression in the public’s scientific knowledge, even if it is only elementary? Since the beginning of the 19th century, high school mathematics programs have included a section called “cosmography”. It was about describing 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 took the oral test for the agrégation in mathematics in 1948, the jury asked him to present the theory of the phases of the Moon, which would cause great difficulty to many candidates today.
The mathematics teachers did not appreciate this teaching, which they considered to be outside their discipline. However, these were lessons in spatial geometry – literally – that allowed students to better understand their place in the universe. But the arrival of “modern mathematics”, which privileges abstraction and rejects any link with natural sciences, put an end to the teaching of cosmography as early as 1968, without anyone being bothered by it. It must be said, to justify the abandonment of these subjects in mathematics courses, that astronomy was gradually transformed into astrophysics, which is more similar to physics.
Today, school geometry programs ignore astronomy. What a shame to disconnect mathematics and nature in this way! Didn’t Galileo say that nature is written in mathematical language? What a shame to forget the etymology of the word “geometry”, which reminds us that it was first and foremost about measuring the Earth. Let’s hope that the two hours of science instruction, which are now part of the core curriculum of the general high school curriculum, will allow students to (carefully!) look up at the Sun and understand its apparent movement in the sky.