What Are The Equinoxes and Solstices About?

Each year the changing of the seasons is defined by equinox and solstices. What are they and why do they happen? Find out in this guide.

In the Northern Hemisphere, before the Spring Equinox, the North Pole is tipped away from the Sun; the days are shorter than nights. After the equinox, the north pole is angled toward the Sun so that days become longer than nights, moving the Northern hemisphere into Spring and Summer. When the Autumnal Equinox occurs, the days become shorter and the nights longer. For the Southern Hemisphere, these effects are reversed.

At the times when the Sun is crossing the celestial equator, day and night are of nearly equal length at all latitudes, so we call these dates the equinoxes, which means 'equal night'.

A Solstice is a point between equinoxes when the day or night is the longest, the point at which the days start to get longer or shorter.

Equinoxes and Solstices play a crucial role in marking the start of seasons. The March Equinox, for instance, signals the end of Winter and the onset of Spring, while the June Solstice heralds the beginning of Summer. Similarly, the September Equinox marks the start of Autumn, and the December Solstice ushers in Winter.

Date and Time (UTC) of Equinoxes and Solstices

The date of equinoxes can shift each year but always fall around 20 March and 22 September.

The March equinox, sometimes called the vernal equinox, signals the start of Spring in the Northern Hemisphere and the beginning of Autumn in the Southern Hemisphere. In September, the opposite occurs. The autumnal equinox marks the beginning of Autumn in the Northern Hemisphere and Spring in the Southern Hemisphere.

The summer solstice marks the year's longest day, while the winter solstice marks the shortest day.

Precession of the Equinoxes

We all know that the North Star (or Pole Star) is Polaris, but the true North Celestial Pole is slightly above Polaris, and it is moving.

Hipparchus first discovered that the celestial pole was moving, and it was noted in his observations from 147 BC to 127 BC. Back then, the celestial pole was closer to Thurban than Polaris.

The precession of the equinoxes refers to the change of the Earth's rotational axis concerning the stars in the galaxy. The precession of the equinoxes is caused by the rotational axis of the Earth changing over about 25,765 years, centred around the Ecliptic north pole, with an angular radius of about 23.4° the angle known as the obliquity of the Ecliptic.

This annual motion is about 50.3 arcseconds per year (1 degree every 71.6 years). The process is very slow but cumulative. A complete precession cycle covers approximately 25,765 years, the so-called great Platonic year, during which the equinox covers a full 360°.

The ragged circle of faint stars down and to the left of Polaris is called the "Engagement Ring," with Polaris as its shining stone.

First Point of Aries

The First Point of Aries is a position in the sky where the Ecliptic and the Celestial Equator cross each other. It is a crucial point in astronomy, defining the zero-point for Right Ascension and marking the location of the vernal equinox.

Due to the effect of precession, the First Point of Aries crossed into the neighbouring constellation of Pisces in about 70 BCE. It has taken about 2,000 years to cross Pisces and will cross into the next zodiacal constellation, Aquarius, in about the year 2600. Following its journey along the Ecliptic, it will return to Aries again in about 23,000 years.

Because the First Point of Aries is relative to the Ecliptic and the Celestial Equator, the First Point of Aries is fixed relative to the Earth. It acts as the zero-point for calculating coordinates on the Celestial Sphere; its coordinates are always fixed, regardless of its motion. They are, of course, zero hours Right Ascension and zero degrees Declination.

Due to the floating nature of the First point of Aries, coordinates for Right Ascension and Declination need to be adjusted over time to compensate. These adjustments are referred to as Epochs. When referring to celestial coordinates, an Epoch is often quoted along with, for example, J2000 refers to the instant of noon (midday) on 1 January 2000, and J1900 refers to the instant of noon on 1 January 1900.

Celebrating the Equinoxes and Solstices

Celestial events, such as the equinoxes and solstices, have been significant across cultures worldwide. The Sun's position in the sky at specific times of the year has influenced the design and orientation of monumental structures in various civilizations.

The most famous of these is Stonehenge, which is lined up so that the Sun rises between key markers during the equinox and solstices.

Another lesser-known monument is the Temple of Kukulcán in Chichén Itzá, a step pyramid in Mexico. At the spring and autumn equinoxes, the great pyramid of Kukulcán visually represents day and night as the Sun, serpent-like, as it slithers down the staircase and reveals the serpent-like pattern upon the ancient ruins.