Equinoxes and Solstices

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.

When is the summer solstice in 2019? 

In 2019 it will occur on 21 June at 15:54 GMT (16:54 BST). As the solstice is independent of the specific rotation of the Earth, it can occur even during the middle of the night, as it did in 2016.

What is a solstice?

Our Earth rotates on its axis once each day, producing the cycle of day and night. At the same time, it moves around the Sun on its orbit over the course of a year. The axis of rotation of the Earth is not lined up with the axis of motion around the Sun, instead being tilted slightly at 23.44°. This is sometimes referred to as the ecliptic. 

Ignoring slight wobbles that occur over very long periods of time, this tilt stays fixed in space meaning that during one half of the year the North side of the Earth is tilted slightly towards the Sun while the South is tilted away and for the other half of the year the reverse is true.

At the exact moment that the northern hemisphere is most tilted towards the Sun, the northern hemisphere experiences its summer solstice while the southern hemisphere has its winter solstice. About 6 months later, the northern hemisphere has its winter solstice while the southern hemisphere is at its summer solstice.

What is the summer solstice?

The summer solstice is often referred to as the longest day of the year. On this day, the number of hours of daylight are at their maximum, while the number of hours of night are at their minimum. However, while most people consider the summer solstice to be a day, it is in reality an exact moment in time that falls upon that day.

The summer solstice occurs in June and marks midsummer with the longest day and shortest night.

What is the winter solstice?

The winter solstice occurs in December and marks midwinter with the shortest day and longest night. Find out more about the winter solstice

When is the winter solstice in 2019? 

In 2019 it will occur on 22 December.

When do the solstices occur?

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

2019

21 June, 3.54pm

22 December, 4.19am

 

2020

20 June, 9.44pm

21 December, 10.02am

Yes

All times are UTC (GMT)

 

What is an equinox?

At two points in the year the Sun will illuminate the Northern and Southern Hemispheres equally. These are known as the equinoxes: the autumnal equinox in October and vernal or spring equinox in March. 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.

Diagram of the seasons

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
Creative Commons - Earth lighting equinox

When is the spring Equinox?

The spring equinox occurs in March. In 2019 the spring equinox occured on 20 March at 9.58pm. In 2020 it will also occur on 20 March, but at 03.49am

What is the autumnal equinox?

The autumnal equinox occurs in September, and in the Northern Hemisphere this date marks the end of Summer and beginning of Autumn. 

Find out more about the autumnal equinox

When is the autumnal equinox?

In 2019 it will occur on 23 September at 7.50am

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

2019

20 March, 9.58pm

23 September, 7.50am

 

2020

20 March, 3.50am

22 September, 1.31pm

Yes

All times are UTC (GMT)

Where does the word 'equinox' come from?

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

Why don't the solstices and 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.

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). 

Why isn’t the Summer Solstice the middle of summer?

The tilt of the Earth produces the seasons, including the changes in weather associated with them. When the northern hemisphere is tilted towards the Sun, it receives more direct sunlight and for a longer portion of the day, so the temperature increases. Similarly in winter, the reduction in sunlight hours and more glancing angle to the Sun means the air remains colder.

However, just as with the air in your home, the atmosphere and, more importantly, the oceans take time to heat up. This means that even though the solstice marks the longest or shortest days (and the most direct or indirect sunlight), there is a considerable lag between that and the peak of warm or cold weather. As a result, summer is often deemed to start around the summer solstice, with June, July and August being the “summer months”. However, astronomical summer can either begin on the solstice, or, if using length of days as a guide, be centred on the solstice, depending on the situation.

Celebrations around the World

Since humans began using the Sun as a timekeeper, particularly when it came to the cycle in farming, the summer solstice has been marked with varying degrees of importance.

Perhaps most famously, the ancient monument Stonehenge has for some time been the centre of a ritual celebration. This comes from the fact the stones are lined up to frame the rising of the Sun on the solstice, perhaps suggesting a connection to the day and as a celebration of Sun. However it isn’t clear if marking summer solstice was indeed its purpose. The stones also mark the position of sunset on the winter solstice, and so may instead indicate a place to request the return of the summer months.

In any case, many modern day religions gather at the site to mark the occasion, also being one of the rare times visitors are allowed to walk right up to the stones themselves.

Elsewhere around the world, celebrations range from the biannual appearance of the feathered-serpent shadow on the pyramid at Chichén Itzá in Mexico to a range of floral-themed events in countries like Sweden and Latvia.

In the southern hemisphere, where the summer solstice occurs in December, the day is instead strongly associated with Christmas, having once been the day of its celebration until various calendar changes shifted the dates apart.

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