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Happy solstice! Today’s #GeoAdvent treat is especially for the day that’s in it. This is my (non physical geographer!) explanation of solstices. #LoveGeography
A #solstice is a point in the earth’s yearly jaunt around the sun where the sun appears to reach its most northerly or southerly point from the equator. https://video.nationalgeographic.com/video/101-video-shorts/00000160-7547-df2f-a17d-77d7ae7a0000
On earth, we more commonly know this as the shortest or longest day of the year. The shortest day of the year, as we’re experiencing today in Ireland, is where the northern hemisphere of the earth is tilted farthest from the sun.
Here is a good educational video from @pbsteachers that explains to learners about the earth’s orbit: https://www.pbslearningmedia.org/resource/buac18-35-sci-ess-earthinorbit/earth-in-orbit/
The solstice we’re experiencing in Ireland today is the winter solstice, marking the beginning of astronomical winter. It also marks the new solar year. We have a few more days of 2020 to go according to the Julian calendar, but in the solar calendar, 2020 is done!
See here for the difference between astronomical and meteorological winter: https://www.metoffice.gov.uk/weather/learn-about/weather/seasons/winter/when-does-winter-start
The Irish for solstice is ‘an grianstad’, which literally means ‘the sun stop’. This is because the sunrise appears to ‘stop’ its annual motion through the sky and remain the same point for several days around the solstice.
The reason that solstices happen is because the earth rotates on its axis at an angle of approximately 23.4º as it orbits the sun. This is called the earth’s axial tilt, or obliquity. https://www.timeanddate.com/astronomy/axial-tilt-obliquity.html
Because of this axial tilt, when the earth orbits around the sun, some parts of the earth are tilted towards the sun, and some are tilted away from it. This is what gives earth its seasons. The earth’s axial tilt is currently 23.4º, but this varies between 22.1º and 24.5º.
But there’s more to earth’s cycles than axial tilt! Axial tilt is part of a series of larger orbital cycles known as Milankovitch cycles. https://climate.nasa.gov/news/2948/milankovitch-orbital-cycles-and-their-role-in-earths-climate/
Solstices have been an important feature for civilisations, as structures such as #Newgrange attest. The solstice was so important to ancient civilisations that they oriented the design of significant tombs to ensure the magical event of the solstice was part of their rituals.
As the @opwireland broadcast from Newgrange this morning noted, we waited today for an event that was planned 5,000 years ago. While the sun didn’t shine into the chamber today, there are hopes that tomorrow will be a brighter day for the live broadcast. http://www.discoverboynevalley.ie/livestream-winter-solstice-newgrange-2020
