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Yeah, Newgrange is a great gig that usually turns out to be a damp squib.
Newgrange doesn't face directly East, but a bit South of that. Newgrange has a rising passage tomb, with a larger burial chamber at the top. On the various days around the Solstice, the rising sun shines increasingly up the passage. Then on the Solstice itself, the rising sun is low enough to shine completely up and illuminate the passage, illuminate the upper burial chamber, and confirm the exact solstice date. That's the theory, and the theory works.
The trouble is, it's always cloudy in the Ireland, and we're like a loo for the various passing weather formations. They all pass around us, but when a few clouds want to 'lift a leg' somewhere, those ones pass over Ireland and lift their legs here, while the rest politely look the other way. So you only see sunshine on a winter solstice 00.01% of the time. In fact you never see a cloud free day here. Personally, I'm an hour's drive from Newgrange, but I never bothered getting up in the night, driving for an hour, and traipsing up only to freeze my backside & discover it was clouded over.
The winter solstice is actually confusing, because the next day, the sun goes down 2 or 3 minutes earlier, although it rises 5 minutes earlier. So the Romans, without such monumentally precise assistance, weren't sure about the solstice until the 25th or so. Hence the Great Roman Festival of "Natalis Sol Invictii"(= Saturnalia = "Birth of the Invincible Sun") wasn't celebrated until the 25th. So the stuff we associate with Christmas was going on long before (and during) the time Christ was alive. It simply was rebranded when Rome was paganising what little was left of "Christianity."
The Office of Public Works holds a 'lottery' where people are chosen to be up there on the interesting days. They probably pick carefully and junk the rest.
The winter solstice is actually confusing, because the next day, the sun goes down 2 or 3 minutes earlier, although it rises 5 minutes earlier. So the Romans, without such monumentally precise assistance, weren't sure about the solstice until the 25th or so. Hence the Great Roman Festival of "Natalis Sol Invictii"(= Saturnalia = "Birth of the Invincible Sun") wasn't celebrated until the 25th.
You find the solstices with a long pole: note the day that its shadow is shortest and then next year look for when the shadow falls in the same place. Now the equinoxes are a different question — even Ptolemy couldn't manage an accurate determination of an equinox.
Saturnatia ≠ Dies Natis Solis. The former is an old Roman festival (which started on 17th), the latter an import from Syria in the late empire.
The date of 25th is to do with the Julian / Gregorian calendar change over. The solstice was originally on the 25th. When the Gregorian calendar was introduced, they didn't restore it to what Julius Caesar had used, but used the date for the spring equinox specified at the First Council of Nicea. But that was in the 4th century, so the calendar had already slipped by 3 days.
I'd have thought the equinox was easy enough. That's the day the sunrise is exactly eastwards, and sunset is exactly westwards, but maybe Ptolemy didn't know that. It's also 90 days after the winter solstice which we had a handle on, thanks to Newgrange.
And how do you determine the compass points without a compass? How do you know that the solstices and equinoxes quadrisect the orbit until you know there they are? The ancient pioneers of astronomy did an incredible job. Anyone with an interest in such things should read James Evans's The History and Practice of Ancient Astronomy — and you'll even learn how to build your own astrolabe and equatorium!
I've celebrated Heliogenna at noon, offering frankincense, golden chrysanthemums, and a (badly-sung) hymn. And now I've eaten Heliogenna Cake for tea (aka lemon swiss roll). Let's all pray that next year may see some light at the end of our current tunnel.
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