black drop effect

Has the black drop phenomenon for transits of Venus been explained to
the satisfaction of the majority of the scientific commumity?

In article ,
(alistair) wrote:

Has the black drop phenomenon for transits of Venus been explained to
the satisfaction of the majority of the scientific commumity?

Probably not, most workers are not particularly interested in this.

The underlying reason for accurate Venus transits is to determine
the scale of the Solar System. Since we can do this more accurately
with radar nobody cares about the details of transits anymore.

Dark skies,

tom

--
We have discovered a therapy ( NOT a cure )
for the common cold. Play tuba for an hour.

alistair (or somebody else of the same name) wrote in message
thusly:

Has the black drop phenomenon for transits of Venus been explained to
the satisfaction of the majority of the scientific commumity?

Sorry, could you tell us what this effect is supposed to be please?
--
Paul Townsend
I put it down there, and when I went back to it, there it was GONE!

Interchange the alphabetic elements to reply

In article , pvstownsend@zyx-
abc.fsnet.co.uk says...
alistair (or somebody else of the same name) wrote in message
thusly:

Has the black drop phenomenon for transits of Venus been explained to
the satisfaction of the majority of the scientific commumity?

Sorry, could you tell us what this effect is supposed to be please?


I was wondering that too, but a quick google gives satisfying results.

http://www.exploratorium.edu/venus/question3b.html

Nice picture of it there. Also, although it's mainly talking about
Mercury, this one gives what sounds to me like a *good* explanation:

http://www.aas.org/publications/baas.../aas203/26.htm

"...black-drop effect comes from a convolution of the instrument's
point-spread function and the solar limb darkening..."

It's a bit mathematical for man-in-street, but I'd say a good bottom
line is that it shows limitations of, and is a function of, the fact
that we're using instruments with limitations to observe the event,
rather than the divine perfect perception that we too often take for
granted when we look at something.

Most interesting. Whatever, I'm looking forward to it.

Peter Harding wrote in message ...
In article , pvstownsend@zyx-
abc.fsnet.co.uk says...
alistair (or somebody else of the same name) wrote in message
thusly:

Has the black drop phenomenon for transits of Venus been explained to
the satisfaction of the majority of the scientific commumity?

Sorry, could you tell us what this effect is supposed to be please?


I was wondering that too, but a quick google gives satisfying results.

http://www.exploratorium.edu/venus/question3b.html

Nice picture of it there. Also, although it's mainly talking about
Mercury, this one gives what sounds to me like a *good* explanation:

Maybe transits should be looked at MUCH closer. Atmospheric refraction
might do it, but might not just a "teensy" part of the effect be due
to the gravitational deflection of light from the sun's edge taking it
away from our "line of sight"?

Jim G

In article ,
says...
Maybe transits should be looked at MUCH closer. Atmospheric refraction
might do it, but might not just a "teensy" part of the effect be due
to the gravitational deflection of light from the sun's edge taking it
away from our "line of sight"?

??

Wouldn't this apply to looking at something on the other side of the
sun? Venus is on our side of it during a transit. Or are you suggestion
that the relatively minor gravitational influence of Venus is
responsible?

I think if you want to look at transits MUCH more closely, a good first
step would be to show that the mechanism given in

http://www.aas.org/publications/baas.../aas203/26.htm

doesn't fully account for the effect. Then you'll have a reason to look
for e.g. gravitational effects.

"Peter Harding" wrote in message
...
In article ,
says...
Maybe transits should be looked at MUCH closer. Atmospheric refraction
might do it, but might not just a "teensy" part of the effect be due
to the gravitational deflection of light from the sun's edge taking it
away from our "line of sight"?

??

Wouldn't this apply to looking at something on the other side of the
sun? Venus is on our side of it during a transit. Or are you suggestion
that the relatively minor gravitational influence of Venus is
responsible?

I think if you want to look at transits MUCH more closely, a good first
step would be to show that the mechanism given in

http://www.aas.org/publications/baas.../aas203/26.htm

doesn't fully account for the effect. Then you'll have a reason to look
for e.g. gravitational effects.

The point-spread function also has to include terrestrial atmospheric seeing
effects. But the problems encountered in the 18th century were with small
telescopes (early achromatic refractors of 2-4 inches), so the effect was
largely instrumental (Airy disks of a couple of arcsec). In the 19th
century, observers used bigger telescopes where seeing was more of an issue.

The original supposition that the effect was caused by the atmosphere of
Venus has been pretty well discounted now--its "edge" is far too sharp
(observed by Mariner and other probes).

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

To the Group:

"Mike Dworetsky" wrote in message
...


"Peter Harding" wrote in message
...
In article ,
says...
Maybe transits should be looked at MUCH closer. Atmospheric
refraction
might do it, but might not just a "teensy" part of the effect be due
to the gravitational deflection of light from the sun's edge taking
it
away from our "line of sight"?

??

Wouldn't this apply to looking at something on the other side of the
sun? Venus is on our side of it during a transit. Or are you suggestion
that the relatively minor gravitational influence of Venus is
responsible?

I think if you want to look at transits MUCH more closely, a good first
step would be to show that the mechanism given in

http://www.aas.org/publications/baas.../aas203/26.htm

doesn't fully account for the effect. Then you'll have a reason to look
for e.g. gravitational effects.

The point-spread function also has to include terrestrial atmospheric
seeing
effects. But the problems encountered in the 18th century were with
small
telescopes (early achromatic refractors of 2-4 inches), so the effect was
largely instrumental (Airy disks of a couple of arcsec). In the 19th
century, observers used bigger telescopes where seeing was more of an
issue.

The original supposition that the effect was caused by the atmosphere of
Venus has been pretty well discounted now--its "edge" is far too sharp
(observed by Mariner and other probes).

Couldn't the effect be simply edge diffraction? I realize the Sun's light
isn't a point souce...

David A. Smith

In article Xz6mc.69592$Jy3.57171@fed1read03, "N:dlzc D:aol T:com
\(dlzc\)" N: dlzc1 D:cox says...
Couldn't the effect be simply edge diffraction?

Does it look like it?

http://www.exploratorium.edu/venus/question3b.html

What would "simply edge diffraction" look like? What makes you suggest
this?

Some people who actualy do maths have accounted for it as described. No,
it's not simply edge diffraction.

Dear Peter Harding:

"Peter Harding" wrote in message
...
In article Xz6mc.69592$Jy3.57171@fed1read03, "N:dlzc D:aol T:com
\(dlzc\)" N: dlzc1 D:cox says...
Couldn't the effect be simply edge diffraction?

Does it look like it?

http://www.exploratorium.edu/venus/question3b.html

Yes, it does. Single slit diffraction, with a = planet radius, and theta a
function of planet radius and the distance from the Earth observer to
(basically) the planet's center.

What would "simply edge diffraction" look like? What makes you suggest
this?

Just curious. Of course, multiple wavelengths at the source would make
hash of this idea pretty quick. I'd wonder if the "black-drop" had any
spectral anomalies at the periphery.

Some people who actualy do maths have accounted for it as described. No,
it's not simply edge diffraction.

OK. Thanks.

David A. Smith