To understand the solstices and equinoxes we must first understand the how the seasons happen. To do this we must look some basic concepts about our wonderful planet’s structure.
The Earth is round but slightly tilted on its axis. The axis is the invisible line that runs from the north to the south pole.
The Earth rotates around its this inclined axis once every 24 hours and the side of the Earth that faces the Sun during this rotation becomes bright or day, while its other side becomes dark or night. In other words, the rotation of the Earth on its axis causes the sequence of the day and the night.
The Earth revolves around the Sun once every 365 days or one year. The rotation of the Earth around the Sun causes the sequence of the four seasons – summer, spring, autumn and winter.
The Earth consists of two hemispheres – the northern hemisphere and the southern hemisphere. The hemispheres are divided equally at the centre by an invisible line of latitude known as the equator. The equator is an invisible line of latitude that encircles the planet horizontally or east to west. The lines that circumnavigate the earth vertically or north to south are known as lines of longitude. You can’t see them, but they are used by people like pilots to get an exact location on the planet and are measured in degrees.
As the Earth revolves around the Sun, the location where the sun is overhead determines the seasons. The most important lines of latitude when measuring the seasons and ultimately, the equinoxes and solstices, are the equator and two other lines of latitude called the ‘Tropic of Cancer’ and the ‘Tropic of Capricorn’.
The Tropic of Cancer is also known as the Northern Tropic as it located in the northern hemisphere. It is the line of latitude that runs parallel to the equator at around 23 degrees north. The sun shining over this tropic heralds the beginning of summer in the northern hemisphere and the beginning of winter in the southern hemisphere.
In the northern hemisphere the Sun shines vertically over the Tropic of Cancer on June 2oth-22nd, which is known as the ‘Summer Solstice’ in the northern hemisphere and ‘Winter Solstice’ in the southern hemisphere. The word Solstice comes from the Greek word ‘Solstitium’ which means ‘Sun standing still’ as the sun appears to stand still for a time. At this time of the year, the northern hemisphere is tilted towards the sun and so has summer with longer days and shorter nights, while the southern hemisphere, is tilted away from the sun and has winter with shorter days and longer nights.
The Tropic of Capricorn, also known as the Southern Tropic, lies around 23 degrees south of the equator. It is the farthest southern latitude at which the sun can reach or appear directly overhead. The sun appearing directly over this latitude signals the beginning of winter in the northern hemisphere because it is tilted away from the sun. But it’s the beginning of summer in the southern hemisphere as its leaning towards the sun. The sun is directly over the Tropic of Capricorn on December 20th-23rd, which is the winter solstice in the northern hemisphere and summer solstice in the southern hemisphere. Hence, at this time of the year the southern hemisphere has summer with longer days and shorter days, while the northern hemisphere has winter with shorter days and longer days.
As the earth continues its journey around the sun, the Sun’s rays fall vertically on the equator twice a year. These are known as the equinoxes. There are two equinoxes every year – one in March – the Spring equinox, and the other in September – The Autumn equinox.
On March 19th-21st, the Sun’s rays fall vertically on the equator, making the days and nights of approximately equal length all over the Earth. Since the days and the nights are of equal length, the season is neither too hot nor too cold. At this time, it is spring in the north and autumn in the south. This position is called the ‘Spring Equinox’. After this date the northern hemisphere starts leaning towards the Sun, so the days will start getting longer and the nights shorter.
Similarly, around September 21st-24th, the Sun’s rays are again directly over the equator which illuminate northern and southern hemispheres equally, making the days and nights of approximately equal length all over the Earth. After this date the position of the Earth, revolving round the Sun, is such that the northern hemisphere starts tilting away from the Sun and the southern hemisphere tilts gradually towards the Sun. So, in the northern hemisphere the days start getting shorter and the nights longer. This position is called the ‘Autumnal Equinox’ in the northern hemisphere. Conversely, it is spring in the southern hemisphere.
In ancient Ireland, they didn’t have dates, times, knowledge of latitude lines or axial tilts. Their only constant was the Sun itself which they studied and mapped with such precision that allowed them to organise the sowing and harvesting of crops and ultimately their survival. This was the reason why solstices and equinoxes were sacred in ancient times and the cause for much celebration.