A year on Earth can be split into four as we complete our orbit of the Sun. Each of these are marked by an equinox or solstice.

What are the equinoxes?

The Earth’s axis is tilted by 23.4 degrees and so the plane of the Earth’s equator is tilted with respect to the plane of the Earth’s orbit around the Sun – sometimes referred to as the ecliptic.

Image of equinox diagram

This means that during the Northern Hemisphere summer, the Sun illuminates the Northern Hemisphere more than the Southern Hemisphere and during the winter, the Sun illuminates the Southern Hemisphere more than the Northern Hemisphere.

But at two points in the year the Sun will illuminate the Northern and Southern Hemispheres equally – these are known as the equinoxes. It’s the moment in which the plane of Earth's equator passes through the centre of the Sun's disk or the moment that the Sun passes the celestial equator from the Northern to the Southern Hemisphere or vice a versa.

On these dates, there are approximately equal hours of daylight and darkness.

The ecliptic: the plane in which the Earth and most of the other planets orbit around the Sun over a year.

The celestial equator: an imaginary projection of the Earth's equator onto the sky.

Image of equinox diagram

What is the vernal (or Spring) equinox and when does it occur?

The vernal equinox occurs in March, and in the Northern Hemisphere this date marks the end of Winter and beginning of Spring when the days will start getting longer and the nights shorter.

In 2018 it will occur on 20 March at 4.15pm

What is the autumnal equinox and when does it occur?

The autumnal equinox occurs in September, and in the Northern Hemisphere this date marks the end of Summer and beginning of Autumn when the days will start getting shorter and the nights longer.

Find out more about the autumnal equinox

In 2017 it will occur on 22 September at 8.02pm

Image of autumn leaves

Where does the word equinox come from?

The word equinox comes from the Latin aequinoctium meaning 'equal night'.

Why don't the equinoxes occur on the same days annually?

The Earth takes approximately 365¼ days to go around the Sun. This is why we have a leap year every four years to add another day to our calendar; and so that there is not a gradual drift of date through the seasons.

For the same reason the precise time of the equinoxes are not the same each year, and generally will occur about six hours later each year, with a jump of a day (backwards) on leap years.

When do the equinoxes occur?

The table below shows the dates and times of both the vernal (spring) and autumnal equinoxes:

Year

Vernal equinox

Autumnal equinox

Leap year

2017

20 March, 10.28am

22 September, 8.02pm

 

2018

20 March, 4.15pm

23 September, 1.54am

 

2019

20 March, 9.58pm

23 September, 7.50am

 

2020

20 March, 3.50am

22 September, 1.31pm

Yes

All times are UTC (GMT)

The solstices

The times when the Sun is at its furthest from the celestial equator are called the summer and winter solstices. These occur at midsummer and midwinter.

The world 'solstice' comes from the Latin solstitium meaning 'Sun stands still', because the apparent movement of the Sun's path north or south stops before changing direction.

Year

Summer

Winter

Leap year

2017

21 June, 4.24am

21 December, 4.28pm

 

2018

21 June, 10.07am

21 December, 10.23pm

 

2019

21 June, 3.54pm

22 December, 4.19am

 

2020

20 June, 9.44pm

21 December, 10.02am

Yes

All times are UTC (GMT)

The path of the sun

Kelly Barfoot explains how the solstices inspired her photo, 'Solargraph – Newton’s Apple Tree'. Her image was shortlisted in the Insight Astronomy Photographer of the Year Award 2017

Find out more about the largest international competition of its kind

Image of path of sun with Newton's apple tree
Solargraph – Newton’s Apple Tree (c) Kelly Barfoot | Our Sun, Insight Astronomy Photographer of the Year Award 2017

“This is a solargraph image of Sir Isaac Newton’s apple tree at Woolsthorpe Manor, Lincolnshire, where many of his great discoveries were made.

The aim was to capture the path of the sun each day between the winter and summer solstices. As the photographic exposure of six months was so long, the result was not processed using usual dark room methods, instead it was processed digitally." - Kelly Barfoot

What is the difference between Midsummer Day and the summer solstice?

Midsummer Day occurs annually on 24 June and is one of the four Quarter Days in the UK Legal Calendar. The other Quarter Days are Lady Day (25 March), Michaelmas (29 September) and Christmas Day (25 December).

The Royal Observatory is open daily from 10am.

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