The September equinox

In 2019 September 23 is the date of the September equinox and is also the first day of autumn (or the first day of spring if you’re one of my readers in the Southern Hemisphere). In this post I’ll talk about the equinoxes and discus the commonly held view that they are the two days in the year when all places on Earth have exactly 12 hours of daylight and 12 hours of darkness.

What is an equinox?

The origin of the word equinox comes from two Latin words aequus (equal) and nox (night). This definition suggests that at an equinox the length of the day and night are equal. However, the precise astronomical definition of an equinox is slightly different.

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The diagram above shows the Earth going around the Sun in its orbit.

• At the December solstice (point A in the diagram) the North Pole is tilted further away from the Sun than at any other time of the year, and the South Pole is tilted nearest the Sun.  In the Northern Hemisphere the period of darkness is longest compared with the period of daylight, and in the Southern Hemisphere the reverse applies.
• At the June solstice (point C) it is exactly the opposite of the winter solstice – the North Pole is now tilted nearest to the Sun. So, the Northern Hemisphere experiences the longest period of daylight.
• There are two times a year (B and D in the diagram) when the neither the North Pole nor the South Pole are tilted towards the Sun and these times are the equinoxes.  If we take two places with the same latitude, one of which is north of the equator and the other one south of the equator and roughly the same longitude (for example Tokyo and Adelaide, South Australia) they will both have the same amount of daylight at the equinox.

On what date do the equinoxes occur?

The diagram also shows that the Earth moves in an elliptical orbit around the Sun. This means that it has further to travel in its orbit between the March equinox and the September equinox than in the return leg of its journey from September to March. The two equinoxes are therefore not exactly half a year apart. The time between the March equinox and the following September equinox is around 186 days, whereas the time between the September equinox and the following March equinox is only 179 days.

The tables below give the times of the two equinoxes for the years from 2016 to 2021 for three locations:

• London (Time zone GMT – or more precisely UTC)
• Honolulu (UTC -10 hours)
• Tokyo (UTC +9 hours).

As you can see, the September equinox occurs on 22 or 23 September and the March equinox  occurs on March 19, 20 or 21.

Are there are exactly 12 hours of daylight at the equinox?

The first thing we need to think about when we answer this question is what do we mean by the word ‘daylight‘? Do we consider twilight, the time just after sunrise or just before sunset when it is not completely dark, to be daylight? Or do we consider daylight as being the time when the Sun is above the horizon? If we use the definition of ‘daylight’ as being the interval between sunrise and sunset, then for two reasons there are actually slightly more than 12 hours of daylight at the equinox everywhere in the world.

• The first reason is that the definition of sunrise is that it is the time when the first light from the Sun’s rays reaches above the horizon, not when the centre of the Sun is above the horizon. The diagram below shows the path of the Sun’s disc around sunrise at the equinox in London.  In the early morning, the time when the half of the Sun is above the horizon and half below the horizon is 6:48 am, shown as B in the diagram, but sunrise is about a minute before this time.

Similarly, in the early evening the time when half of the Sun is above the horizon and half below the horizon is 6:56 pm, B in the diagram, but sunset – when the last light from the Sun’s rays is above the horizon – is about a minute after this time.

• The second reason is that when the Sun is just below the horizon the Earth’s atmosphere bends the Sun’s rays, causing them to appear just above the horizon. This bending of light is known as refraction and has the effect of slightly extending the hours of daylight.

Taken together, these two effects mean that there are slightly more than 12 hours of daylight at the equinox. The table below shows the amount of daylight for dates around the equinox in London and Wellington.  The date on which there almost exactly 12 hours of daylight and 12 hours of darkness in London is 26 September, three days after the equinox. In Wellington it is 20 September, three days before the equinox.

Note: If we measure the hours of daylight to the nearest second rather than rounding to the nearest minute, then in London 25 September 2019 has 12 hours 1 minute and 59 seconds of daylight and 26 September 2019 has 11 hours 58 minutes and 4 seconds of daylight. So, September 26 is the date on which the amount of daylight is closest to 12 hours.

When does Autumn begin?

Although I started this post by saying that September 23 is the first day of the Northern Hemisphere autumn, in the UK there is an alternative definition of autumn used by meteorologists and widely used by the public as well. Meteorological autumn consist of the months of September, October and November and so starts on September 1 and finishes on November 30. Similarly, meteorological winter is December/January/February, spring is March/April/May and summer June/July/August.  To many people, it makes more sense for the middle of December with its low temperatures and short hours of daylight to be in winter than in autumn!

The table below shows the meteorological and astronomical seasons for 2019/20 in the Northern Hemisphere

 

• Astronomical autumn starts on the September equinox and finishes the day before the December solstice
• Astronomical winter starts on the December solstice and finishes the day before the March equinox
• Astronomical spring starts on the March equinox and finishes the day before the June solstice
• Astronomical summer starts on the June solstice and finishes the day before the September equinox

Because the dates of the solstices and equinox vary from year to year then so do the dates of the astronomical seasons.

 

And finally…

I hope you have enjoyed this post. To read more from Explaining Science click on the Explaining Science home page .

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Note

The term Greenwich Mean Time is no longer used by astronomers.    Instead, they use two different times which agree with each other to within 1 second.

• Universal Time, often abbreviated to UT1, is the mean solar time, the time determined by the rising and setting of the Sun at the Greenwich Meridian, zero degrees longitude.
• Co-ordinated Universal Time, usually abbreviated to UTC, is the time measured by atomic clocks and is kept to within 1 second of UT1 by the addition of leap seconds.

In common use, Greenwich Mean Time (GMT) is  taken to be the same as UTC, which is the approach I have taken for this post. However, it can also be taken to mean UT1. Owing to the ambiguity of whether UTC or UT1 is meant, and because timekeeping laws usually refer to UTC, the term GMT is normally avoided in precise writing.