by neufer » Sun Jun 20, 2021 9:25 pm
Precession schmecession
- The Earth's obliquity (i.e., its axial tilt with respect to the orbital plane) is the real critical factor here. The Earth's obliquity was about 0.7° greater (24.14° vs. 23.44°) when Stonehenge was first built circa 3000 BCE. This would be, more or less, equivalent to Stonehenge effectively being about 0.7° further north (51.88°N vs. 51.18°N) in a sunrise/sunset algorithm:
As such, the Summer solstice sunrise/sunsets occurred about 0.8° (i.e., 1.6 solar diameters) further north in azimuth than they do today.
https://en.wikipedia.org/wiki/Milankovitch_cycles#Axial_tilt_(obliquity) wrote:
<<The angle of the Earth's axial tilt with respect to the orbital plane (the obliquity of the ecliptic) varies between 22.1° and 24.5°, over a cycle of about 41,000 years. The current tilt is 23.44°, roughly halfway between its extreme values. The tilt last reached its maximum in 8,700 BCE. It is now in the decreasing phase of its cycle, and will reach its minimum around the year 11,800 CE.[7] Increased tilt increases the amplitude of the seasonal cycle in insolation, providing more solar radiation in each hemisphere's summer and less in winter. However, these effects are not uniform everywhere on the Earth's surface. Increased tilt increases the total annual solar radiation at higher latitudes, and decreases the total closer to the equator.
The current trend of decreasing tilt, by itself, will promote milder seasons (warmer winters and colder summers), as well as an overall cooling trend. Because most of the planet's snow and ice lies at high latitude, decreasing tilt may encourage the termination of an interglacial period and the onset of a glacial period for two reasons: 1) there is less overall summer insolation, and, 2) there is less insolation at higher latitudes (which melts less of the previous winter's snow and ice)>>
[quote="APOD Robot" post_id=314347 time=1624162090 user_id=128559]
[url=https://apod.nasa.gov/apod/ap210620.html] [img]https://apod.nasa.gov/apod/calendar/S_210620.jpg[/img] [size=150]Sunrise Solstice over Stonehenge[/size][/url]
[b] Explanation: [/b] Even given the [url=https://visibleearth.nasa.gov/images/53974/earths-orbital-precession]precession of the Earth's rotational axis[/url] over the millennia, the [url=http://www.nasa.gov/sun]Sun[/url] continues to [url=https://www.english-heritage.org.uk/visit/places/stonehenge/things-to-do/solstice/]rise[/url] over [url=http://www.english-heritage.org.uk/visit/places/Stonehenge]Stonehenge[/url] in an [url=https://en.wikipedia.org/wiki/Archaeoastronomy_and_Stonehenge]astronomically significant way[/url]. [/quote]
[c][size=200][color=#FF0000][b][u]Precession schmecession[/u][/b][/color][/size] :!: [/c]
[list][size=150][b][u][color=#0000FF]The Earth's obliquity[/color][/u][/b][/size] (i.e., its axial tilt with respect to the orbital plane) is the [b][u]real critical factor[/u][/b] here. The Earth's obliquity was about 0.7° greater (24.14° vs. 23.44°) when Stonehenge was first built circa 3000 BCE. This would be, more or less, equivalent to Stonehenge effectively being about 0.7° further north (51.88°N vs. 51.18°N) in a sunrise/sunset algorithm:
[list]https://keisan.casio.com/exec/system/1224686065[/list]
[b][color=#0000FF]As such, the Summer solstice sunrise/sunsets occurred about [u]0.8° (i.e., 1.6 solar diameters) further north[/u] in azimuth than they do today.[/color][/b][/list]
[quote=https://en.wikipedia.org/wiki/Milankovitch_cycles#Axial_tilt_(obliquity)]
[float=right][img3=22.1–24.5° range of Earth's obliquity]https://upload.wikimedia.org/wikipedia/commons/a/ae/Earth_obliquity_range.svg[/img3][/float]
<<The angle of the Earth's axial tilt with respect to the orbital plane (the obliquity of the ecliptic) varies between 22.1° and 24.5°, over a cycle of about 41,000 years. The current tilt is 23.44°, roughly halfway between its extreme values. The tilt last reached its maximum in 8,700 BCE. It is now in the decreasing phase of its cycle, and will reach its minimum around the year 11,800 CE.[7] Increased tilt increases the amplitude of the seasonal cycle in insolation, providing more solar radiation in each hemisphere's summer and less in winter. However, these effects are not uniform everywhere on the Earth's surface. Increased tilt increases the total annual solar radiation at higher latitudes, and decreases the total closer to the equator.
The current trend of decreasing tilt, by itself, will promote milder seasons (warmer winters and colder summers), as well as an overall cooling trend. Because most of the planet's snow and ice lies at high latitude, decreasing tilt may encourage the termination of an interglacial period and the onset of a glacial period for two reasons: 1) there is less overall summer insolation, and, 2) there is less insolation at higher latitudes (which melts less of the previous winter's snow and ice)>>[/quote]