The Equinox and Solstice
Equinox and solstice are vital parts of the astronomical calendar which determine the transitions between the seasons.
The equinox and solstice define the transitions between the seasons of the astronomical calendar and are a key part of the Earth's orbit around the sun.
The Equinox in the Northern Hemisphere occur twice a year around 20 March (the spring equinox) and around 22 September (the autumn equinox). They occur between the summer and winter solstices marking the point the sun crosses the equator's path and becomes positioned exactly above the equator between the Northern and Southern Hemisphere. During the equinox, day and night will be around the same length which is evident in the word's origin derived from the Latin equi (meaning 'equal') and nox (meaning 'night').
The spring equinox marks the beginning of Spring and from this day forward the day is longer than the night. Similarly the autumn equinox marks the start of autumn as the night becomes longer than the day.
What is the Equilux?
On the equinox the length of day and night are only nearly equal. This is because the sun appears as a disk in the sky, and the top half rises above the horizon before the centre. As well as this sunlight is refracted by the Earth's atmosphere. The sun therefore, appears to rise before its centre is at the horizon giving more daylight than you might expect (12 hours 10 minutes on the equinox).
The equilux is when day and night are equal and occurs a few days before the spring equinox, and a few days after the autumn equinox.
These occur twice a year and are referred to as the 'summer solstice' and 'winter solstice'. The summer solstice - which occurs around the 21 June in the Northern Hemisphere - is the day of the year with the longest period of daylight while the winter solstice - on or around the 21 December in the Northern Hemisphere - is the day with the shortest period of daylight.
When it is the summer solstice in the Northern Hemisphere, the areas north of the Arctic circle receive sunlight for a full 24 hours, while areas south of the Antarctic circle have a full day of total darkness. This situation is reversed at the winter solstice.
At the summer solstice, the sun reaches its highest point of the year, while at the winter solstice, the noon sun is the lowest it will be all year. During the summer solstice the Northern Hemisphere of Earth is tilted towards the sun resulting in increased sunlight and warmer temperatures, this can also result in continuous daylight in far northern countries such as Iceland and Norway.