Autumnal Equinox 2026

Today is a very busy teaching day, so I just have time to post a quick note to say that the Autumnal Equinox (in the Northern Hemisphere) takes place in the early hours of tomorrow morning, Wednesday 23rd September 2026, at 01:05 Irish Time (00:05 UTC). I don’t intend to be awake then, so I’m posting this now. Of course the Equinox will be later today rather than tomorrow for some of you, depending on your time zone. Of course for those of you in the Southern Hemisphere it will be the Spring (or Vernal) Equinox. Although the term `equinox’ refers to a situation in which day and night are of equal length, which implies that it’s a day rather than a specific time, the astronomical equinox is more accurately defined by a specific event, i.e. when the plane defined by Earth’s equator passes through the centre of the Sun’s disk (or, if you prefer, when the centre of the Sun passes through the plane defined by Earth’s equator). Day and night are not necessarily exactly equal on the equinox,…