Solstice and Equinox

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.

This means that during the Northern Hemisphere summer, the Sun illuminates the Northern Hemisphere more than the Southern Hemisphere. During this time we receive more hours of sunlight and the longest day is known as the summer solstice. During the winter, the Sun illuminates the Southern Hemisphere more than the Northern Hemisphere because the North Pole is titled away from the Sun. During this time we receive fewer hours of sunlight and the shortest day (and longest night) is known as the winter solstice.

But 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 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.

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.

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.

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.

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

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.