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 time span between the June solstice and the September equinox (northern hemisphere astronomical summer / southern hemisphere astronomical winter) currently is about 93.66 days and is getting longer as aphelion likewise advances by about a day every 70 years.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 time span between the December solstice and the March equinox (northern hemisphere astronomical winter / southern hemisphere astronomical summer) currently is about 88.99 days and is getting shorter as perihelion advances by about a day every 70 years. The astronomical seasons are not equal in length. So how come it doesn't fall middle of the year, 365/4 = 91 days into the year. Like many things political, shifting the start of the year to the first of January was a bit arbitrary. The calendar used by many nations are based on the calendar reformations mandated by Julius Caesar. The solstices occur a bit over a week before the first of January and the first of July. The March equinox advances by about 5 hours from the previous year's March equinox in non leap year years but retreats by about 19 hours from the previous year's March equinox in the case of a leap year.ĪFAIK the summer and and winter solstices are based on longest day and longest night doesn't fall on 1st of Jan and July 1st because we consider the day to begin and end when the middle of the sun is over the horizon but when it stars to rise and after if has completely set. The reason for using March 21 simplifies the calculation of the date of Easter. Christian churches use March 21 as the ecclesiastic equinox. In slight contradiction to the accepted answer, the Gregorian calendar is designed to have the March equinox occur on March 20, plus or minus a day, not March 21.
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