[Avodah] Gezeros in the BHMK

[Simon Montagu]

Zev Sero wrote:
> Bimchilas kevodo, that makes no sense.  The half-hour was to see the
> sun (or its shadow) move noticeably from its noon position.  The sun
> moves at exactly the same rate regardless of the day's length.  If on
> 21-Jun the shadow reaches a certain point on the BHMK wall at exactly
> 30 minutes past noon, then on 21-Dec it will also reach that point at
> exactly 30 minutes past noon, and not a moment earlier.
> 

I'm not sure that the seifa follows from the reisha. Isn't the "equation 
of time" also a factor here?