The major argument against
>their use for time keeping appears to be that most such are covered in
>inscriptions many of which describe why they were erected. No text yet
>discovered suggests their use as a time measuring device even though there
>was room to do so. The shadow clock therefore appears to predate the use
>of a vertical obelisk gnomon for formal solar timekeeping purposes.
The major argument, in my eyes, against the use of obelisks as gnomon in Egypt is that most of them were erected as pair of twins.
Karlheinz Schaldach
