[time-nuts] zero crossing of venus

[Andrew Rodland]

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