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 http://www.usno.navy.mil/USNO/astronomical-applications/data-services/earth-seasons

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

Related links

Why do mornings still get darker after the winter solstice? (21/12/2014)
BBC News
Winter Solstice 2014: Why Google has given the shortest day of the year a doodle (21/12/2014)
The Independent
Everything you need to know: December solstice 2014 (18/12/2014)
EarthSky
Winter Solstice Brings Shortest Day of 2011 This Week (21/12/2011)
Space.com article explaining the December 2011 winter solstice.
Winter Solstice 2011: Facts on Shortest Day of the Year (22/12/2011)
National Geographic article with some facts about the shortest day.
Sky News: Winter solstice sees total lunar eclipse (21/12/2010)
BBC Radio 4 Today programme: Lunar eclipse coincides with winter solstice (21/12/2010)
Audio commentary from Dr David Whitehouse describing the first lunar eclipse to coincide with the winter solstice since 1638.
BBC News: Winter solstice at Maeshowe on Orkney" (17/12/2010)
An article from the BBC News for NE Scotland, Orkney and Shetland about the significance of the solstice to Neolithic people.
Shropshire Star: UFO sighting linked to shortest day (07/01/2009)
An article from the Shropshire Star about the shortest day being linked to an increase in UFO activity.
BBC News: Hundreds celebrate shortest day (21/12/2008)
An article from BBC News about the celebrations at Stonehenge in 2008.
Table 1: Solstice times (UTC) from 2010-2020
2010 June 21 11:28 December 21 23:38
2011 June 21 17:16 December 22 05:30
2012 June 20 23:09 December 21 11:11
2013 June 21 05:04 December 21 17:11
2014 June 21 10:51 December 21 23:03
2015 June 21 16:38 December 22 04:48
2016 June 20 22:34 December 21 10:44
2017 June 21 04:24 December 21 16:28
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