The shortest day of the year in the Northern Hemisphere occurs during the December solstice, an astronomical phenomenon that takes place between December 20 and 22, according to the Gregorian calendar. In 2024, the solstice will be recorded on December 21, at 09:21 (Lisbon time). This event marks the beginning of winter in the Northern Hemisphere and summer in the Southern Hemisphere.
The December solstice corresponds to the moment in which the Sun reaches its lowest decline in relation to the Celestial Equator in the Northern Hemisphere, resulting in the day with the fewest hours of sunlight and the longest night of the year.
What is the solstice?
The word "solstício" derives from the Latin solstitium , formed by solis (Sun) and sistere (stop), referring to the apparent "station" of the Sun in the sky. During this phenomenon, the position of the Sun at birth and at the bottom of the Sun appears to remain constant, marking an inversion in the apparent movement of its trajectory.
This phenomenon is due to the inclination of the Earth's surface (approximately 23.5 degrees in relation to the orbital plane), which causes the solar rays to affect the different regions of the planet unequally throughout the year.
Characteristics of the December solstice in the Northern Hemisphere
Shortest day of the year: The December solstice marks the smallest number of hours of daylight, which varies according to latitude. The further north it is, the shorter the day will be and the longer the night will be.
Minimum height of the Sun in the sky: At mid-solar day, the Sun reaches its lowest position on the horizon throughout the year.
Beginning of Winter: This phenomenon marks the official start of the coldest season in the Northern Hemisphere, even as the peak of cold weather occurs weeks later due to the effect of thermal lag.
Symmetry as the Southern Hemisphere
The Southern Hemisphere, the December solstice, presents a phenomenon opposite to the Northern Hemisphere. While in the North we live the shortest day of the year at the beginning of Winter, in the South it is the longest day of the year, marking the official start of Summer. This difference results from the axial inclination of the Earth, which distributes sunlight unevenly in both hemispheres during the year.
Cultural and historical importance
Since ancient times, the December solstice has been celebrated by various cultures as a moment of renewal and rebirth, given the progressive return of sunlight in the days that follow.
Saturnália (Ancient Rome): A festival dedicated to the deus Saturn, associated with the cycle of colheitas and the rebirth of the Sun.
Yule (Germanic words): Festivities in honor of the Sun and hope do not return for two longer days.
Natal: Many birth traditions are rooted in solstice celebrations, representing the light that conquers the threes.
Astronomy and science
The December solstice, or Tropic of Capricorn, at 23.5° south latitude, receives the solar rays perpendicularly. This astronomical alignment results in the maximum inclination of the Earth, like the Northern Hemisphere tilted for the length of the Sun.
Climate impact: Or seasonal mismatch
Although the December solstice marks the beginning of winter, lower temperatures usually occur a few weeks later. This phenomenon, known as a seasonal imbalance, is due to the thermal capacity of the oceans and the atmosphere, which takes longer to cool in response to lower solar radiation.
Curiosities about the December solstice
The duration of the day on the solstice varies depending on the location. In Lisbon, the daylight hours are about 9 hours, while in the Arctic regions there may be no daylight.
After the solstice, the days gradually begin to become longer until the Spring equinox, in March.
Locations like Stonehenge, in the United Kingdom, and Machu Picchu, in Peru, are architectural landmarks that celebrate the solstices.
The December solstice is an astronomical and cultural framework that symbolizes the transition in the cycle of nature. This day, with its long nights and short daylight, inspires the celebration of the return of light and the beginning of a new annual cycle.
Clocks and solstices
Solar Clocks
Solar watches are widely used to indicate the position of the Sun in the sky, and some are designed to mark the solstices:
Analytical solar clocks: Use the position of the Sun throughout the year, as specific points for the summer and winter solstices.
Gnomons vertical or horizontal: The shadow cast by a gnomon can be used to identify the solstice when it reaches its maximum or minimum position along a specific line.
Astronomical Clocks
Complex astronomical clocks, such as those seen in some cathedrals and historical buildings, incorporate solar calendars that indicate the dates of important astronomical events, such as solstices.
Orloj - Prague
Orloj - Prague Astronomical Clock: Displays solar, lunar and seasonal data, as well as time.
Strasbourg Astronomical Clock: Presents representations of solar motion in relation to stations.
Megalithic Sun Clocks and Alinhamentos
Newgrange and Stonehenge
Monuments such as Stonehenge, in the United Kingdom, or Newgrange, in Ireland, have specific elements such as solstices and function as primitive "solar clocks".
Modern Clocks with Astronomical Complications
Watches such as the Vacheron Constantin Les Cabinotiers Celestia, can also present astronomical complications that indicate solstices and equinoxes:
Watches with annual or perpetual calendars: Some advanced watch models consider detailed astronomical scales.
Solar complications: Some things indicate solar decline, a complication designated by the equation of time, as well as other phenomena linked to the cycles of the Sun.
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