What are Equinoxes and Solstices?

Last Updated January 10, 2019 by . First Published in 2008.

1,069 words, estimated reading time 4 minutes.

Introduction to Astronomy Series
  1. Astronomy for Beginners
  2. What are Right Ascension (RA) and Declination (Dec)?
  3. What is Angular Size in Astronomy?
  4. Magnitude Scale and Distance Modulus in Astronomy
  5. Sidereal Time, Civil Time and Solar Time
  6. What are Equinoxes and Solstices?
  7. How Do We Measure Distance in Space Using Parallax and Parsecs
  8. Brightness, Luminosity and Flux of Stars Explained
  9. Kepler's Laws of Planetary Motion Explained
  10. What Are Lagrange Points?
  11. Glossary of Astronomy & Photographic Terms
  12. Astronomical Constants - Useful Constants for Astronomy
What are Equinoxes and Solstices?

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.

An equinox is a moment in which the plane of Earth's equator passes through the centre of the Sun, a time when both hemispheres of the Earth are illuminated equally and day and night are the same lengths. Because of the geometry involved, the equinoxes are the only times when the solar terminator is perpendicular to the equator, and at the equator, the Sun is exactly overhead at midday. The Sun appears to travel the line of the equator.

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 start to become longer than nights, and moving the Northern hemisphere into Spring and Summer. When the Autumnal Equinox occurs, the days start to 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 in time 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 are used to determine the start of seasons. The March Equinox indicates the end of Winter and the start of Spring, the June Solstice marks the beginning of Summer, the September Equinox marks the start of Autumn and the December Solstice the start of Winter.

Earth orbits the Sun at a slant, which is why equinoxes and solstices happen.
Earth orbits the Sun at a slant, which is why equinoxes and solstices happen.

Date and Time (UTC) of Equinoxes and Solstices

The date of the equinoxes can shift each year but always fall around or on 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 the beginning of spring in the Southern Hemisphere.

Summer solstice marks the longest day in the year, whilst 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.

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

Precession of the Equinoxes
Precession of the Equinoxes

The precession of the equinoxes refers to the change of the Earth's rotational axis with respect to the stars in the galaxy. The precession of the equinoxes is caused by the rotational axis of the Earth changing over a period of 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.

Precession of the Equinoxes over 26,800 year cycle
Precession of the Equinoxes over 26,800 year cycle

Currently, 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 a period of approximately 25,765 years, the so-called great Platonic year, during which time the equinox covers a full 360°.

Even in the last 50 years the North Celestial Pole has moved, albeit a very small amount.
Even in the last 50 years the North Celestial Pole has moved, albeit a very small amount.

The North Celestial Pole for the year 2000 is marked with a large capital X, while for the year 1950 it is marked with a small x, showing the effect of precession over a 50 year period. The ragged circle of faint stars down and to the left of Polaris is called the "Engagement Ring," with of course 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 another. The First Point of Aries, which is actually in Pisces, defines the zero-point for Right Ascension. It is also the location of the vernal equinox.

First Point of Aries
First Point of Aries

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 it will cross into the next zodiacal constellation, Aquarius, in about the year 2600. Following its journey along the Ecliptic, it will return to Aries once 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 own 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 12 pm (midday) on 1 January 2000, and J1900 refers to the instant of 12 pm on 1 January 1900.

Celebrating the Equinoxes and Solstices

Marking the Sun's progression in the sky during the year has been a staple of most, if not all, cultures around the world. The Sun's position in the sky at a certain time of day at specific times in the year have determined the position and orientation of massive monuments and constructions.

The most famous of these is Stonehenge which appears to be lined up in a way 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.


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