Precessional dating
Hancock’s own original contribution to pseudohistory is his idea of precessional dating. This assumes as axiomatic that many ancient monument complexes were laid out so as to represent constellations of stars, a notion for which there is not a shred of historical evidence.
Assuming this to be so, we take a set of ancient monuments (say, the pyramids) and decide what group of stars their layout most resembles (Orion’s belt).
The resemblance is not exact. This is where “precession” comes in. The arrangement of the constellations as seen from the Earth changes over time, and it is easy to obtain programs for your home computer which can show you what the night sky looked like at various times in the past. So in order to find when the pyramids were built, it is only necessary to find out when the pattern made by Orion’s belt in the sky is exactly the same as the pattern made by the pyramids on the ground.
Unfortunately, the answer is “never”, so you do the next best thing and find when the two patterns were the most similar. Then you conclude that this is when the pyramids were built.
If your conclusion disagrees with all the historical and archaeological evidence, you are to be congratulated --- you have proved all the evidence wrong. Hancock’s opinion of real archeological evidence is summed up by his extraordinary declaration that: “My reservations about radiocarbon will continue to apply to sites that are primarily megalithic and that […] demonstrate alignments older than the radiocarbon dates”. How graciously he agrees not to quarrel with carbon dating --- unless, by some chance, it should disagree with his own methods! So historians consider his method to be bunk, because it disagrees with all known history; and Hancock considers history to be bunk --- for exactly the same reason.
As with numerology, the problem with this method is that Hancock allows himself way too much latitude. For example:
In the case of the pyramids, he allows the pyramids to represent a mirror image of the constellation as it appears in the sky.
To stay with the example of the pyramids, he has a whole skyfull of stars to choose from, and only three buildings to match to any group of stars. It would be strange if he couldn't find a rough resemblance somewhere in the night sky.
He doesn’t even have to find a match to an entire constellation --- he claims that the pyramids represent Orion’s belt, but his scheme does not require him to find other monuments in the Giza burial complex representing Orion’s trousers.
The match he seeks can be as far back in time as he chooses, for his results do not have to agree with history or even with paleontology --- indeed, from his point of view, the greater the discrepancy, the better.
As we have noted, he is merely looking for a best fit, not an exact match. The best fit he can come up with for the pyramids is way below the accuracy with which each individual pyramid is aligned to the four points of the compass. Now if you allow the monumental architects any arbitrary degree of imprecision, then the method would be worthless even if the underlying axiom (that buildings represent stars) was perfectly correct.
He allows himself to discover in the East maps of constellations only traditional in Western astrology.
He feels free to pick and choose, from any complex of monuments, which are to represent stars and which are to be ignored. In the case of the pyramids, the reason for his choice is obvious and excusable: they are the most prominent objects in the Giza funary complex. In other cases, he seems to have gone by the principle that he can select buildings that resemble, in their layout, the stars of some constellation, while ignoring the ones that don’t agree with this interpretation.