Solar Alignment: Egypt vs. UK Moved?

I was reading an article about how various corridors in the Pyramids and the Temple of Karnak no longer line up with the sun the way that they ‘meant’ to. That, is to say, “The shaft of light would would penetrate down the corridor, at dawn, on the summer solstice, and illuminate the altar.”
Then, “This no longer happens due to the ‘precession of the Earth’s axis.’” That is to say, The ‘Wobble’ in the Earth’s axis has changed enough in the last 4-6,000 years. The poles of the Earth are pointing at different places in the sky than they were 4-6,000 years ago.
Okay, fine.
Except places in the UK like Stonehenge and Newgrange are aligned just fine, and their about the same age as the Pyramids and the Temple of Karnak.

What gives?

Stonehenge has same problem.

The basic reason is that it’s believed that the likes of the Great Pyramid were arranged to align with particular stars, while Stonehenge and Newgrange were aligned with the Sun.
Why there’s the difference is one of those things that I find is easily understood if you visualise it just right, but it can be a bit difficult to explain with just words. Anyway …

Imagine the Earth going round the Sun over the course of a year. That orbit defines a flat plane with the Sun stationary at the centre. The Earth is also spinning about its own axis once a day. Over the course of the year, this axis is roughly fixed and at the moment happens to point towards the very distant star Polaris. So, although the Earth is going around the Sun, this axis always points towards that star.
So imagine somehow building a pyramid at the North Pole. And include a shaft in it that points directly upwards. You’ve now got a shaft that (roughly) points to the Pole Star throughout the entire year. Job done.
Except that over tens of thousands of years the Earth’s axis isn’t quite fixed. Precession twists it away from the direction of the Pole Star. The angle the axis makes with the plane of the Earth’s orbit remains (roughly) fixed, but precession twists it in a cycle. So our elaborate scheme to incorporate an alignment with Polaris in our new pyramid doesn’t work forever.

That’s the simpler case. Now think what an observer at the Sun sees the Earth’s axis do over the course of a year. Because, over the course of that year, it’s fixed relative to the Pole Star rather than the Sun, they see it twist around. Sometimes they see the North Pole towards them, then they see the axis “side-on”, then they see the North Pole leaning away from them, etc. Over the course of the year, they see the axis twist around entirely.
What does precession look like from the Sun? Although the timescale is much, much longer, it just twists the Earth’s axis through the same cycle. Superimpose the two effects and, apart from details about the timing, the overall effect is that the observer at the Sun just sees the Earth’s axis go through this same cycle each year regardless.
Precession thus doesn’t have any fundamental effect on the cycle of relative geometries the Earth and the Sun undergo over the course of a year. From the point of view of someone on the Earth, precession thus doesn’t change the apparant path of the Sun, but does slowly change the positions of the stars.

Since the most obvious and uncontroversial alignments at Stonehenge and Newgrange are to the Sun, these still work. The Sun does still come up over the Heel Stone at the summer solstice sunrise. And always will.
There are more minor effects that do slightly alter these alignments and people do occasionally try to date the monuments using them. Personally, I usually find that these effects are so small that the astronomical datings for such solar alignments are unconvincing. Using precession to date stellar alignments is usually far more convincing.

Incidentally, the main alignments at Karnak are indeed usually suggested to be solar, but do also still work.

Checking Lockyer’s chapter in his old The Dawn of Astronomy, it appears that the more minor effects I mentioned may now screw up the particular alignment he suggested for Karnak back in the 1890s. There’s a slight wobble in the Earth’s axis that changes the angle it makes with the plane of the orbit on timescales similar to precession. Lockyer had someone check at the summer solstice of 1891 and they could only just see the Sun. Assuming that the original alignment was exact, this was a fact that he used to try to date the temple.
But the wobble in question is a fairly small effect and it only causes the alignment no longer to quite work because it was really rather narrowly defined in the first place using multiple doorways. By contrast, for instance, the Heel Stone at Stonehenge is a matter of “the Sun comes up over that stone for about a week around the solstice”. Much less impressive, but also much more robust against the small effects.

It took a couple of reads, but you did an outstanding job. Incidentally, how many books do you own - you continually provide excellent references from a vast range of sources. I’m jealous, frankly. :slight_smile:

Ignorance fought. That was impressive, thanks.

Wikipedia explains astronomical precession:
http://en.wikipedia.org/wiki/Precession_(astronomy)
and a map showing the path of the north celestial pole among the stars due to precession http://en.wikipedia.org/wiki/Image:Precession_N.gif

Another interesting consequence of precession: The ancient Egyptians used the heliacal rising of Sirius (that is, the time of year when Sirius is just barely visible right before dawn, before the Sun comes up and washes it out) as the signal for when the Nile was about to flood (an event of crucial importance to Egyptian agriculture). But the flooding depends on the change of seasons (i.e., alignment of the Earth’s axis relative to the Sun), while the heliacal rising of Sirius depends on the sidereal cycle (the Earth’s position around the Sun, relative to the stars), so the appearance of Sirius and the floods drifted out of synch due to precession. The precession cycle has a period of about 26,000 years, so after about a thousand years, you’d be a couple of weeks off in predicting the floods.

Precession also has an effect on astrology, which most astrologers neglect: The dates of the zodiac were set about two thousand years ago, so the precession since then has thrown all of the signs off by about a month. This is, however, acknowledged in the concept of “astrological age” (such as the infamous Age of Aquarius which is approaching some time soon): That’s the constellation which the Sun is in (sidereal cycle) at the time of Vernal Equinox (cycle of Earth’s axis relative to the Sun), so it’s also subject to precession.

Enough that I daren’t count. And that I’ve mentally suppressed the number an ex-girlfriend came up with when she counted them a decade ago. Though I also don’t think I’m remotely close to any “Top 10 Doper Personal Libraries” by size list.
I may as well note that my copy of Norman Lockyer’s book is a secondhand MIT reprint from the Seventies rather than an 1894 edition. Obscure perhaps, but not particularly rare.