We have 23 RC dates from Portal Tombs ,all are termini ante quos and clearly cannot date the build .The best example of a date associated with build is Sperris , where the deposit was overlain by the doorstone .The results show that the first Portal tombs were built around 400-3800 BC and no later than 3600 BC .The dates from the deposits at two PT’s not far from Trethevy , Sperris and Zennor were 3633-3557 BC and 3342 -3024 BC . It is not unreasonable to assume that Trethevy was built around the same period i.e 3024 -3633 BC .It does not need an expert anything to calculate the the azimuth and altitude of the sun at any given date but it is worth pointing out to those that are doing observations in the 21st C that the azimuth and altitude at a certain time of day on a particular date this century would differ from that around 3024 BC . Depending on the month the azimuth could differ by 7 degrees which is the case at summer solstice or a difference of 6 degrees altitude in November .If these changes are not incorporated into calculations regarding light and shadow phenomena found at monuments like portal tombs then what is seen today is not the same as what would be seen at the same time and date when they were built .
It is not a major problem for these particular measurements but when recording data like longtitude , it is worthwhile getting it right , making mistakes when you go to bother of noting decimal point(s) does not inspire confidence . The longtitude for Trethevy is 4.55 not 4.2 as mentioned elsewhere .
 Reply | with quote | Posted by tiompan
3rd May 2011ce
08:20 |
Portal tomb observation problems . (tiompan, May 03, 2011, 08:20)
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