But what is the shortest day of the year?
According to popular tradition is Saint Lucia, according to the science on December 21. But there is another possibility…
The saying goes: “Saint Lucia is the shortest day that there is.” In fact, December 13, precisely the date dedicated to the Christian martyr, the sun sets earlier than any other day of the year.
Without all but the winter solstice falls on December 21, then logically (and astronomical calculations of solids) it should be the shortest day of the year. And indeed it is. Then how do you explain this apparent paradox and who is right: the popular tradition, or math?
In reality both. The apparent dualism is in fact solved: it reflects the combined effect of the revolution of the Earth around the Sun (which does not make a perfectly circular orbit but slightly ecliptic) and the inclination of its axis of rotation (which is shifted by 23 about degrees with respect to the orbital plane).
So the Earth, when it is close to perihelion, the point of closest approach to the Sun, which happens January 4, runs faster around our star, but the speed of rotation on itself remains unchanged and the poles are tilted by 23 degrees with respect perpendicular to the orbit. Jumping tedious mathematical considerations, suffice to say that the result produced by these circumstances results in an “advance” of the position of the sun in the sky that accumulates during the year, reaching a maximum of sixteen and a half minutes November 1 and after that returns to decrease. But in Saint Lucia this effect is still present and, therefore, three minutes before the sun goes down compared to December 21.
That’s why you have the tangible feeling that 13 is the shortest day of the year. But the winter solstice, although the sunset occurs later and then “after it gets dark,” the daylight hours are lower, because the sunrise is delayed, much more than the 13. And it is then 21, to all intents and purposes, the true shortest day of the year. So in the first fortnight of December, the sun disappears from the horizon earlier in the day, but from the middle of the month rises later and later, until January 4, when the Earth is at perihelion.
In astronomy, to avoid confusion, we use the sidereal day as the unit of time, i.e. the time it takes the Earth to complete a rotation with respect to the fixed stars of heaven, because this period is constant, while the solar day lasts seven-eight seconds more perihelion and in less extreme when the Earth is farthest from the Sun.