equinox and solstice

Great resource by Greenwich Royal Observatory at equinoxes-and-solstices. A year on Earth can be split into four as we orbit the Sun. Each fourth is 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. [aka The Zodiac?] 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 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, it is called the vernal equinox and in September as the Sun is moving southwards, 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.

Upcoming equinoxes

2017: 20 March, 10:28 and 22 September, 20:02
2018: 20 March, 16:15 and 23 September, 01:54
2019: 20 March, 21:58 and 23 September, 07:50
2020: 20 March, 03:50 and 22 September, 13:31
All times are UTC (which stands for Coordinated Universal Time, even though the letters are in the wrong order!). UTC is effectively the same as Greenwich Mean Time (GMT). But GMT is a time zone and UTC is a time standard: see GMT versus UTC - Timeanddate.com

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 word '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.

Upcoming solstices

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

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).

My edits in the above.
See also pagan-sabbats and ancient-meaning-of-christmas and
Teach Astronomy - Solstices and Equinoxes, a one-minute YouTube video