What is the Reason for the Season? Equinox, Solstice & Everything Else

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It is the start of Spring in the Nothern Hemisphere and Autumn or Fall in the Southern Hemisphere. But how does it all happen?

 

Satellite views of Earth on the solstices and equinoxes. From left to right, a June solstice, a September equinox, a December solstice, a March equinox, via NASA Earth Observatory.

Satellite views of Earth on the solstices and equinoxes. From left to right, a June solstice, a September equinox, a December solstice, a March equinox, via NASA Earth Observatory.

What are meteorological seasons?

These are easier to define and are based entirely on how meteorologists and climatologists break the seasons into groupings of three months based on the annual temperature cycle and our calendar. Meteorological spring occurs during March, April, and May; meteorological summer during June, July, and August; meteorological autumn (or fall if you prefer) includes September, October, and November; and meteorological winter includes December, January, and February.

What are astronomical seasons?

Astronomical seasons are defined by significant points in Earth’s orbit around the Sun, such as solstices and equinoxes, marking the changes between seasons like spring, summer, autumn, and winter. These dates can shift slightly due to leap years, reflecting the Earth’s elliptical orbit.

What is a solstice?

As the Earth rotates on an imaginary pole called its axis, we get our daily cycle of night and day. The Earth also moves around the Sun over the course of the year but the axis of rotation of the Earth is not lined up to the axis of its orbit around the Sun. In fact, it is tilted at 23.44°, with respect to the Sun and this tilt is called the eliptic. This means that for half the year, the north side of our planet is tilted toward the Sun and the other half is tilted away, while the opposite is true for the rest of the year (the south side is tilted toward the Sun and north tilted away). A solstice occurs when the Sun is at its highest or lowest point in the sky at noon, marked by the longest and shortest days.

The world ‘solstice’ comes from the Latin solstitium meaning ‘Sun stands still’, because it seems that the movement of the Sun’s path, whether north or south, stops before changing direction.

Image from the Royal Museum Greenwich depicting earth's orbit around the Sun, the solstices and equinoxes as well as astrological signs.

Image from the Royal Museum Greenwich depicting earth’s orbit around the Sun, the solstices and equinoxes, as well as astrological signs.

 

What are summer and winter solstices?

Earth as it orbits around the Sun and the June and December Solstice

Earth as it orbits around the Sun and the June and December Solstice. NASA

In the northern hemisphere, we get the summer solstice at the exact moment when it is most tilted toward the Sun. This occurs in June (between the 20th and 22nd) and this is also when the northern hemisphere experiences the longest day and shortest night of the year. After this, days will get progressively shorter, but the northern hemishphere will also receive more sunlight and heat during the day. And in the Arctic circle there will be 24 straight hours of sunlight.

At the same time, below the equator the southern hemisphere experiences its winter solstice, at which time it is tilted away from the Sun and has the shortest day of the year. Six months later, the northern hemisphere has its winter solstice in December (around the 22nd) because now this part of the Earth is tilted away from the Sun and the southern hemisphere will have its summer solstice (its longest day of the year), as it now tilts toward the Sun.

In astronomical terms the summer solstice marks the start of the summer season, while the winter solstice marks the start of winter.

A diagram of Earth's season due to its axial tilt by NASA

A diagram of Earth’s season due to its axial tilt by NASA

What are equinoxes?

During the year, as the Earth orbits around the Sun it also reaches positions when the Sun is directly above the equator (from Latin aequi meaning equal and nox meaning night). Night and day are of equal duration. In the northern hemisphere, the vernal or spring equinox (start of astronomical spring) is on or around March 21, and the autumnal equinox (start of astronomical autumn) is on or around September 22. In the southern hemisphere these seasons are reversed but begin on the same dates.

Four views of the Earth showing the solstices and equinoxes

Four views of the Earth showing the solstices and equinoxes. NASA

 

The featured picture is a composite of hourly images taken of the Sun above Bursa, Turkey on key days from solstice to equinox to solstice. The bottom Sun band was taken during the north's winter solstice in 2007 December, when the Sun could not rise very high in the sky nor stay above the horizon very long. This lack of Sun caused winter. The top Sun band was taken during the northern summer solstice in 2008 June, when the Sun rose highest in the sky and stayed above the horizon for more than 12 hours. This abundance of Sun caused summer. The middle band was taken during an equinox in 2008 March, but it is the same sun band that Earthlings see today, the day of the most recent equinox. Image Credit & Copyright: Tunç Tezel (TWAN)/ NASA

The featured picture is a composite of hourly images taken of the Sun above Bursa, Turkey on key days from solstice to equinox to solstice. The bottom Sun band was taken during the north’s winter solstice in 2007 December, when the Sun could not rise very high in the sky nor stay above the horizon very long. This lack of Sun caused winter. The top Sun band was taken during the northern summer solstice in 2008 June, when the Sun rose highest in the sky and stayed above the horizon for more than 12 hours. This abundance of Sun caused summer. The middle band was taken during an equinox in 2008 March, but it is the same sun band that Earthlings see today, the day of the most recent equinox. Image Credit & Copyright: Tunç Tezel (TWAN)

What are some solar traditions that have continued to today?

Before we had meteorology and clocks though, humans used the Sun to mark daily and annual time. In particular, the solstices  and equinoxes were important for agriculture. The spring equinox signified birth, renewal, growth and the arrival of the planting season. The summer solstice marking a productive crop season and the fertility of the Earth. Almost all civilizations across the globe have some sort of ceremony associated with the summer solstice and in the southern hemisphere it was even associated with Christmas, before the calendar dates were shifted. In some northern countries, such as the UK, Midsummer’s Day occurs on June 24 (the exact dates of midsummer vary between different cultures).

Read about May 1 – Arrival of the Sun

Similarly, the autumn equinox is the time to harvest, give thanks for the harvest and to prepare for the long winter months. The winter solstice is associated with death and rebirth, heralding the arrival of long, dark winter nights, a time when nature seems to be in repose and fresh crops and meat are not available. Again many celebrations and rituals have been, and are, still associated with them. Suffice it to say, these four important solar events marked important times of the year for people before Christianity, which later adapted many of the rituals into its own celebrations, particularly Christmas, Easter and Halloween. Many ancient and current festivals still continue around the world.

 

 

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I am a Chartered Environmentalist from the Royal Society for the Environment, UK and co-owner of DoLocal Digital Marketing Agency Ltd, with a Master of Environmental Management from Yale University, an MBA in Finance, and a Bachelor of Science in Physics and Mathematics. I am passionate about science, history and environment and love to create content on these topics.