Solstices and Equinoxes

Our planet follows a path around the sun. This is Earth's orbit. Earth also rotates around its own axis. If this axis were at a right angle to the Earth’s orbit, sunlight would always fall perpendicular to the equator. In that case, any point on the Earth’s surface, regardless of its distance from the equator – its latitude — would receive exactly the same amount of daylight over the course of a year.

Day and night would be the same length everywhere. But the Earth’s axis is not perpendicular to its orbit. Instead, it is tilted at roughly 23 degrees. And so, the amount of sunlight falling at different latitudes varies throughout the year. It’s late December.

The South Pole is tilted towards the Sun, and the North Pole away from it. The southern hemisphere receives plenty of sunlight. The days here are long, and the nights are short. The northern hemisphere is shadier. The days are shorter, and the nights longer.

Around the 22nd of December, sunlight falls at a right angle on the latitude of about 23 degrees south, on the Tropic of Capricorn. That day is the longest day in the southern hemisphere, and the shortest one in the northern hemisphere. This date, and the position of the Earth in relation to the Sun, are known as the December solstice. On this day, traditionally, winter begins in the northern hemisphere, and summer begins in the southern hemisphere. A quarter of a year after the December solstice, around the 20th of March, both the North Pole and the South Pole are the same distance from the Sun.

On this day, sunlight falls at a right angle at the equator, and the northern and southern hemispheres receive the same amount of daylight. All latitudes experience about 12 hours of daylight and 12 hours of nighttime. This is known as the March equinox. The March equinox traditionally marks the beginning of spring in the northern hemisphere, and the beginning of autumn in the southern hemisphere. Now, the days start getting longer than the nights north of the equator, and shorter south of the equator.

About three months after the March equinox, the North Pole is tilted towards the Sun, and the South Pole away from it. Around the 22nd of June, sunlight falls at a right angle on a latitude of about 23 degrees north — on the Tropic of Cancer. On this day, the northern hemisphere receives the most daylight. It’s the longest day of the year. At the same time, the southern hemisphere is in shadow, and experiences the longest night of the year.

This day is known as the June solstice, and traditionally marks the beginning of summer north of the equator, and the beginning of winter south of the equator. Three months after the June solstice, the Earth passes through one more special point on its path around the Sun. Around the 23rd of September, Earth is once again aligned with the Sun, so the Sun appears directly above the equator. Just like during the March equinox, day and night are of equal length at all latitudes, both north and south of the equator. This is the September equinox.

Solstices and equinoxes come with a certain regularity every year. They bring about important changes in the length of day and night, and mark changing seasons in many places around the world. Since ancient times, different cultures have tracked and celebrated the arrival of solstices and equinoxes. Even today, many traditional celebrations and holidays take place around the time of equinoxes or solstices. Do you celebrate any of these?