When it comes to astronomical events, the solstices and equinoxes are some of the most important to human cultures around the world and throughout history. But what exactly are these events, and why do they happen when they do?
What is a solstice?
Technically speaking, the solstice is the moment when the north or south pole of the Earth is either closest to or furthest away from the sun. Solstices come around twice a year, in June and December.
Because the Earth spins on an angle, the northern hemisphere is facing towards the sun for half the year, and the southern hemisphere for the other half.
This has some important implications for us here on Earth: days are longer and warmer when the hemisphere we’re on is more tilted towards the sun, and shorter and colder when we’re tilted away. Hence, the seasons are reversed on opposite hemispheres.
At the June solstice, the north pole is at its closest point to the sun during our yearly orbit, and the south pole is at its furthest. This is the summer solstice in the northern hemisphere, and the winter solstice in the southern hemisphere. At the December solstice, the opposite is true.
Whichever side of the Earth you’re on, the summer solstice falls on the longest day of the year and the winter solstice on the shortest. However, areas that are closest to the equator experience the least variation in seasons and day length through the year.
What is an equinox?
Once you’ve got your head around the solstices, the equinoxes are pretty straightforward.
The word equinox comes from the Latin for “equal night”. These events fall exactly between the solstices – in spring and autumn. Accordingly, some people mark the beginning of these seasons at the equinox, rather than at the start of a calendar month.
Tracking the seasons through time and space
Many human societies have special ways to mark the solstices and equinoxes. One of the best-known examples, at least within the English-speaking world, is the large stone circle monument at Stonehenge in England. The monument is about 4,500 years old and constructed so that the avenue leading to the circle’s centre aligns with the spots on the horizon where the sun rises at the summer solstice and sets at the winter solstice.
A 5,200-year-old passage tomb monument in Newgrange, Ireland, consists of a large mound containing a long passage leading to a burial chamber. Around the time of the winter solstice, the sun shines through a tiny window at the front of the mound as it rises, sending a beam of light down the passage and into the tomb.
Not to be left out, the spring and autumn equinoxes shine at the Angkor Wat temple in Cambodia. On these days, the sun rises directly above the main tower of the temple.
And the Cahokia Mounds historic site in what’s now the US state of Illinois includes a reconstructed “woodhenge” – a monument of wooden posts that align with the rising sun at the solstices and equinoxes. The woodhenge is thought to have been a calendar for the settlement of up to 20,000 people who lived at Cahokia in the 11th-12th century BCE.