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 is the celestial equator and the ecliptic?

The ecliptic

During the course of a year the Earth completes one orbit around the Sun. This is seen on Earth as the Sun moving against the background of stars, around an imaginary circle which we call the ecliptic. This defines the plane in which the Earth and most of the other planets orbit around the Sun over a year.

The celestial equator

This is an imaginary projection of the Earth's equator onto the sky. As the Sun moves on its apparent track along the ecliptic, it can be seen for half a year above the equator (northern summer); and for half a year below the equator (northern winter). So the Sun seems to cross the equator twice 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. So we call these dates the equinoxes, which means 'equal night'.

The vernal equinox occurs in March, as the Sun moves northwards along the ecliptic. In September the autumnal equinox takes place as the Sun moves southwards.

The equinoxes are also points on the celestial sphere where the ecliptic and equator cross. The vernal equinox is used as the zero point in measuring star co-ordinates.

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.

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)

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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