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.

The ecliptic and celestial equator

During the course of a year the Earth completes one orbit around the Sun. To us on Earth we see this as the Sun moving against the background of stars through the year, along an imaginary line which we call the ecliptic. This defines the plane in which the Earth and most of the other planets orbit around the Sun.

The celestial equator is the projection of the Earth's equator onto the sky. As the Sun moves in its apparent track along the ecliptic it is for half the year seen to be above the equator (northern summer) and half the year below the equator (northern winter). The Sun will therefore appear to cross the equator twice in a year.

The equinoxes

At the times when the Sun is crossing the celestial equator day and night are of nearly equal length at all latitudes and so we call these dates the equinoxes (which means 'equal night'). In March, as the Sun is moving northwards along the ecliptic, this is called the vernal equinox and in September as the Sun is moving southwards we refer to it as the autumnal equinox. The equinoxes are also the points on the celestial sphere where the ecliptic and equator cross and the vernal equinox is used as the zero point in measuring star co-ordinates.

Why do the equinoxes not always occur on the same days each year?

The Earth takes approximately 365¼ days to go around the Sun. This is the reason we have a leap year every four years, to add another day to our calendar 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 table below shows the dates and times of both the vernal (spring) and autumnal equinoxes:

 

Year

Vernal equinox

Autumnal equinox

Leap year

2016

20 March, 04:30

22 September, 14:21

Yes

2017

20 March, 10:28

22 September, 20:02

 

2018

20 March, 16:15

23 September, 01:54

 

2019

20 March, 21:58

23 September, 07:50

 

2020

20 March, 03:50

22 September, 13:31

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 and these occur in mid-summer and mid-winter. 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

2015

21 June, 16:38

22 December, 04:48

 

2016

20 June, 22:34

21 December, 10:44

Yes

2017

21 June, 04:24

21 December, 16:28

 

2018

21 June, 10:07

21 December, 22:23

 

2019

21 June, 15:54

22 December, 04:19

 

2020

20 June, 21:44

21 December, 10:02

Yes

all times are UTC (GMT)

 

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

Midsummer Day is 24 June (each year) 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). 

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