We have the idea that in the Northern Hemisphere the shortest days are in December. But this perception only applies to the period of sun exposure; in fact, across the entire planet, if measured from one solar noon to the next, the longest days are those of the last month of the year.
This Sunday, the day of the Winter Solstice, it wasn’t the shortest day of the year; just the one that had fewer hours of sunshine, in the case of the Northern Hemisphere.
In fact, December 21st is one of the longest days of the year, if we measure the interval between the moment the Sun reaches its highest point in the sky one day and the moment it reaches that point again the next day — and this is true regardless of where on the globe you live, notes .
In current usage, we can speak of a day as a period of light. But it is also possible measure a day in relation to the Sun or stars. Astronomers use the term solar day to describe a day in relation to the Sun.
A solar day is time that passes between two solar noons — two “local noons”, when the Sun is at its highest point. It is the interval between successive days as marked by the Sun’s passage through the maximum point in our sky.
If we look at the day this way, the longest days of the year happen every year, around the December solstice…no matter where you live on the planet.
It is worth remembering that clocks do not measure the actual length of a daywhen this is defined from solar noon to solar noon. To measure this type of day, it would be necessary a sundialwhich indicates the exact moment of local solar noon, when the Sun reaches its highest point in the sky daily.
When measured from one solar noon to the next, days are always longer than 24 hours around the solstices, and shorter than 24 hours around the equinoxes.
Why are the days longest in December?
The days are now at their longest, maximum, across the Earth, because we are closest to the Sun on the December solstice than on the June solstice. The Earth’s perihelion — the closest point to the Sun — always occurs at the beginning of January.
So, when we are closer to the Sun, the planet moves in its orbit a little faster than average. This means that the Earth travels, every day, a slightly greater distance than usual.
The result is that the Earth has to rotate a little more on its axis so that the Sun returns to its midday position. This effect lengthens the solar day by about eight seconds. In contrast, at aphelion, when the Earth moves slower in its orbit, the solar day is about seven seconds shorter.
There is yet another effectwhich occurs on both the Winter and Summer Solstice, and which increases the solar day by 21 secondsdue to the way in which, during the solstices, the Sun moves mainly east in relation to the stars: when the Sun rises and rises in the sky, It takes a little longer to reach noon compared to noon the previous day.
On the winter solstice, the combination of these two phenomena increases the solar day by about 29 seconds. Half a minute may not seem like a long time, but this difference accumulates.
Thus, two weeks before the December solstice, noon arrives at about seven minutes earlyby the clock, than on the solstice — and two weeks after the December solstice, noon arrives about seven minutes later, by the clock, than on the solstice itself.
