20 December 2011 – At 23.38 UTC (GMT) tomorrow, 21 December, the solstice will occur when the poles of the Earth align with the Sun. In the Northern hemisphere, the North pole will point directly away from the Sun (the winter solstice), while in the Southern hemisphere, the South pole will point directly towards the Sun (the summer solstice).
At the solstices the Sun is at its furthest from the celestial equator (the projection of the Earth’s equator onto the sky). The world ‘solstice’ comes from the Latin solstitium meaning ‘Sun stands still’ because the apparent movement of the Sun’s path north or south stops before changing direction. At the winter solstice, the apparent position of the Sun reaches its most southerly point against the background stars.
The winter solstice also marks the shortest day (and longest night), but not the earliest sunrise or latest sunset. The earliest sunset occurred on around 12 December 2010 (15:51 in London) and the latest sunrise will occur around 30 December (08:06 in London).
The reason for this is the slight variation in the length of ‘natural’ days throughout the year, resulting from a combination of the Earth’s elliptical orbit around the Sun and the tilt of the planet’s rotation axis. For clocks to work all days need to have an equal length, which is therefore fixed at the average length of a natural day (hence the ‘mean’ in Greenwich Mean Time). This has a knock-on effect on sunrise/sunset times, and the earliest sunrise occurs several days before the longest day and the latest several days (about 9) after the shortest.
Opinion is divided over whether the solstice marks the start of winter or the middle of winter, or whether winter actually starts on 1 December (as reckoned by most meteorologists).
Under the early Julian Calendar, the winter solstice occurred on 25 December. When the more accurate Gregorian calendar was introduced in 1582, the solstice slipped to the 21st, but the Christian celebration of the birth of Christ has continued to be held on 25 December. This date is also associated with the Roman Saturnalia festival, and ‘Dies Natalis Solis Invicti’ (the birthday of the unconquered sun), as well as Nordic pagan festivals.
For an alternative way to celebrate the solstice, why not join us for our 25-minute Yuletide stargazing sessions, daily at 16.30 and 17.00 on 20-23 December? These offer a unique opportunity to look through our historic 28-inch diameter refracting telescope, and to view one of the more striking double stars.
Places are limited and tickets are £6 per person. You can book online or call 020 8312 6608 between 10.00-16.00. (Please note: If adverse weather conditions prevent viewing through the telescopes, an alternative programme will be offered. We are not able to offer refunds under such circumstances.)
Whichever way you choose to celebrate this time of year, we hope you will enjoy yourselves and have a very happy festive season!
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