<HTML>Martin Stower wrote:
>
> Dave Moore wrote:
>
> > Okay, folks, here's a view looking south from Giza in 10430
> > BC, the low point of Zeta Orionis (Alnitak) in the
> > precessional cycle.
>
> How precisely can the date of this low point be determined?
>
> Bauval also talks about meridian transit of Orion's Belt
> (from the diagrams, Alnitak) on the vernal equinox in 10500
> BC <i>at sunrise</i>. Presumably this would compromise
> visibility . . . but (also) how good is the coincidence?
Let's deal with the dates first:
At 10430 BC, Alnitak was 9.53 degrees above the southern horizon at its highest. At 10500 BC, Alnitak was 9.57 degrees above the southern horizon at its highest.
In other words, there's precious little difference (certainly too small to be seen with the naked eye). 10500 BC is an acceptable figure, even though it is 70 years out.
At both dates, there would be no problem during the night seeing Alnitak, as the atmospheric extinsion at 9.5 degrees above the horizon is about 1.6 magnitudes, so Alnitak (mag. 1.9) would have appeared as mag. 3.5, i.e. slightly fainter than Megrez (mag. 3.3) - the faintest star in the Plough.
Saiph, on the other hand, is a completely different kettle of fish. It would <B>not</B> have been visible at this time, as I've pointed out.
However, as regards to sunrise - this is an easy answer. <B>No stars are visible at the moment of sunrise</B>, whither they are crossing the meridian or not. The brightness of the daylight sky is equivalent to mag. -3.5 (IIRC), so only Venus (mag. -4.4) and the Moon (mag. -12.6 when full) (and the occasional nova and supernova) are visible in daylight.
Best Regards,
Dave</HTML>
