What do we mean by the shortest day?

The shortest day, winter solstice and midwinter are the colloquial terms used to describe the 24 hours around an annual astronomical event which occurs around the 22nd December. The shortest day marks the point when the days start to get longer and the nights shorter, and has profound cultural meaning around the world and throughout history. The cultural significance varies, but generally refers to a time of rebirth and renewal and is celebrated with festivals and rituals.

The astronomical event of the solstice occurs when the angle of the Sun to an observer is at its greatest, for an observer in a given hemisphere. For the Northern hemisphere this will be when the Sun is at it's southern most position, and for an observer in the Southern hemisphere the northern most, see Figure 1.

The opposite of the winter solstice is the summer solstice and occurs around the 22nd June, and marks the point when the days are longest and nights shortest.

Timings for the summer and winter solstices are given in Table 1, summarised from the US Naval Observatory page at https://aa.usno.navy.mil/data/docs/EarthSeasons.php

Winter Solstice
Figure 1: Winter solstice as seen by observer in Northern Hemisphere.

Related links

Table 1: Solstice times (UTC) from 2010-2020
2018 June 21 10:07 December 21 22:22
2019 June 21 15:54 December 21 04:19
2020 June 21 21:43 December 21 10:02
2021 June 21 03:32 December 21 15:59
2022 June 21 09:14 December 21 21:48