[time-nuts] zero crossing of venus

[Chris Albertson]

People have tried to time these transits for a LONG time.  No one has had
very good success because of the physics.  First venus has an atmosphere
the refracts sunlight.  There is also an optical illusion when two circles
are close we think we see them overlap.  It was once important to measure
this so they could determine the size of the solar system but no days we
can bounce radars of the planets and directly measure distance and velocity

Yes venus is a disk, not a point.  That is why they define 1st and 2nd
contact as when each edge of tens crosses the edge of the sun.   There are
four events to time.

A good way to measure this today is with a video camera.  The camera takes
30 frames per send and later you can figure out in which frame each
"contact" was made.  And then you look at the time code to figure out when

On Tue, Jun 5, 2012 at 8:56 PM, Andrew Rodland <andrew at cleverdomain.org>wrote:

> Jim Lux <jimlux at ...> writes:
>
> >
> > On 6/5/12 5:20 PM, Tom Van Baak wrote:
> > > Attached are two snapshots of a NASA live feed -- an interesting
> reminder
> about the difficulty measuring
> > timing signals with great precision.
> > >
> > > When you look closely, the leading edge of the sun is rather
> ill-defined,
> not unlike many 1PPS pulses. I
> > suppose with enough photos, modeling, and image processing one could
> pinpoint
> when the transit (zero
> > crossing) really occurs to great precision. Does anyone know more
> details how
> this is done? Is the
> > state-of-the-art at the millisecond level? microsecond? nanosecond?
> > >
> > > Thanks,
> > > /tvb
> > >
> >
> > or do something like compare the "centroid" of venus to "centroid +
> > radius" of sun (or segment thereof.. )
> >
> > it's pretty easy to get 0.1 pixel centroid precision, from what the star
> > tracker, tiny moon finder folks tell me.
>
> Drifting off topic, but the (apparent) radius of Venus is actually
> significant
> here, being about 1 minute of arc — about 1/30th the apparent radius of
> the Sun.
> In the case of the current transit of Venus, there are 18 minutes between
> the
> time when the leading edge of Venus touches the Sun's disc from the
> outside, and
> the time when the trailing edge of Venus touches the Sun's disc from the
> inside.
>
> So if you want to talk precision, don't treat Venus as a point. It's too
> nearby
> for that.
>
> Andrew
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.




-- 

Chris Albertson
Redondo Beach, California