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. 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. These occur twice a year and are referred to as the 'summer solstice' and 'winter solstice'.
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. The Sun, therefore, appears to rise before its centre at the horizon, giving more daylight than you might expect (12 hours 10 minutes on the equinox). As well as this sunlight is refracted by the Earth's atmosphere. This is because the Sun appears as a disk in the sky, and the top half rises above the horizon before the centre. On the equinox, the length of day and night are only nearly equal. Similarly, the autumn equinox marks the start of autumn as the night becomes longer than the day. The spring equinox marks the beginning of spring and from this day forward the day is longer than the night. 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').
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. The Equinox in the Northern Hemisphere occurs twice a year around 20 March (the spring equinox) and around 22 September (the autumn equinox). 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.
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