In Focus: The Equinox
- Justin
- Sep 21
- 3 min read
Updated: Oct 7
The 22nd September marks two events this year.
Firstly, it's the 20th anniversary of Pavel Bahtinov's invention of the Bahtinov mask – which you can read about in my previous blog post here.
It is also the Autumn Equinox today, marking (in the Northern hemisphere) the beginning of the Autumn season.
In this blog post I'll be looking at what the equinoxes are, and how they define the seasons.
What is the Equinox?
"Equinox" comes from the Latin words "æqui" meaning equal, and "nox" meaning night. "Equal night" is exactly what the equinox means: it is the point in the year when, almost everywhere on the planet, the length of the nighttime is roughly equal to the length of the daytime.
There are two equinoxes per year, with the Vernal Equinox marking the start of Spring (around March 21st) and the Autumnal Equinox marking the start of Autumn (around September 23rd). During the 6 months between the Autumn and Spring Equinoxes, the nights are longer than the days, and during the rest of the year the days are longer than the nights.
In the Southern hemisphere this is the other way around, with the March Equinox marking the start of Autumn and the September Equinox being the start of Spring.
The Equinoxes and Solstices
As the below diagram illustrates, Earth's axis is tilted approximately 23.5°, and it is this tilt that causes the seasons on Earth [1].
The Summer and Winter solstices are the points in Earth's orbit when Earth's axial tilt points towards the sun, so that one pole is closer to the sun than the other pole.
The equinoxes occur when the Earth has reached the point in its orbit around the Sun where it is neither tilted towards or away from the Sun (effectively it's tilted sideways), so that both poles are the same distance from the sun. This is what makes the days and nights an equal length at the equinox.
The equinoxes, together with the summer and winter solstices, mark the start of each season and have been used as a way to measure time for millenia. Stonehenge, for example, is thought to be around 4,500 years old and was built to align with the sunrise at the Summer Solstice and sunset of the Winter Solstice (around the 21st December and 21st June each year). [3]
You'll notice that the dates given above are approximate. This is not due to uncertainty, but is because the exact date of the equinoxes and solstices shift slightly each year. This is because each year is not exactly 365 days long, so on the same date next year the Earth will not be at quite the same point in its orbit as it is this year. [4]
The Autumn Equinox and the Harvest Moon
The Autumn Equinox marks the start of longer nights for astronomers, but in addition it defines when the Harvest Moon occurs each year. This is the full Moon closest to the date of the Autumn Equinox, and is known as the "Harvest Moon" because at this time of year the moon rises at around sunset for several nights in a row, which historically gave farmers the crucial extra light to work by when bringing in the harvest. [5]
This year's Harvest moon will be on the 7th October.
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