[time-nuts] zero crossing of venus

[Chris Albertson]

On Wed, Jun 6, 2012 at 6:09 AM, Jim Lux <jimlux at earthlink.net> wrote:

>  Does anyone know more details how this is done? Is the state-of-the-art
>> at the millisecond level? microsecond? nanosecond?
>>
>
I'm thinking "few milliseconds is the best we can do, ever.  Especially
from the ground.  By 1874 thy had decent clocks and even photography and
good telescopes.  The problem is that the exact time is not well defined
visually.   The infamous "black drop" effect prevents accurate timing.
 This is where Venus and the Sun appear to be moving closer at a slow rate
and then as they get closer, venus "jumps".


does anyone have a reference to the math and process used to measure
distance from earth to sun using transit of venus?

I think it is as simple as a "similar triangles" geometry problem.   There
was a recent article on this in Sky and Telescope.

Chris Albertson
Redondo Beach, California