The equinox moon was captured over Cromer yesterday. It is the time when the nights and the day are of almost exactly equal length. It’s the beginning of autumn, and a moment on our way towards the winter solstice, the longest night, that marks the beginning of that season.
An equinox is the moment in which the plane of Earth’s equator passes through the center of the Sun’s disk, which occurs twice each year, around 20 March and 23 September.
On an equinox, day and night are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction.
When Julius Caesar established the Julian calendar in 45 BC, he set 25 March as the date of the spring equinox. Because the Julian year is longer than the tropical year by about 11.3 minutes on average the calendar “drifted” with respect to the two equinoxes — such that in AD 300 the spring equinox occurred on about 21 March, and by AD 1500 it had drifted backwards to 11 March.