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Brian Albin
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Have You Seen the Schröeter Dichotomy? new
      #6040138 - 08/22/13 01:21 PM

We have long been able to accurately predict when Mercury or Venus will reach the half & half phase (Dichotomy) when the terminator between light side and dark side becomes a straight line. But in the year 1793, Astronomer Johan Schröeter observed that on Venus, Dichotomy is observed several days from the calculated date. The same thing happens with Mercury.
Neat huh?
At the time of publication of the book in which I read of this, this puzzle of how it is that observation does not agree with calculation, had not been solved.


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brianb11213
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Re: Have You Seen the Schröeter Dichotomy? new [Re: Brian Albin]
      #6040192 - 08/22/13 01:48 PM

The phase is smaller than predicted because the lit portion immediately adjacent to the terminator is only faintly illuminated and tends to merge into the sky background.

When Venus is a very thin crescent, the brilliantly lit portion of the disc is mostly hidden by the planet and the area just past the terminator is lit faintly by twilight ... so the phase may be a little bigger than predicted (in fact, a whole twilit ring when the planet is within a couple of degrees of the sun at inferior conjunction).

There really isn't much mystery about this.


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azure1961p
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Re: Have You Seen the Schröeter Dichotomy? new [Re: brianb11213]
      #6040226 - 08/22/13 02:05 PM

Agreed and well told Brian. In fact one could say a number of these visual mysteries has been laid to rest with the advent of CCD imaging. Nice as it frees up observers to focus on the what's real - mysterious or not .

Pete

Edited by azure1961p (08/22/13 02:07 PM)


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krakatoa1883
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Re: Have You Seen the Schröeter Dichotomy? new [Re: brianb11213]
      #6042007 - 08/23/13 02:49 PM

Quote:

The phase is smaller than predicted because the lit portion immediately adjacent to the terminator is only faintly illuminated and tends to merge into the sky background.



The phase anomaly discovered by Schroeter is a real phenomenon much more complex than your explanation. A fine summary of the subject can be found on this article


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krakatoa1883
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Re: Have You Seen the Schröeter Dichotomy? new [Re: azure1961p]
      #6042019 - 08/23/13 02:58 PM

Quote:

In fact one could say a number of these visual mysteries has been laid to rest with the advent of CCD imaging.



No, Pete, the phase anomaly can be observed and measured also on CCD images and the results obtained with both techniques are in very good agreement. For more details please have a look to this report we have published some years ago. Sorry it is in Italian, however there is an abstract in English and you can go directly to figure 19 which is self explanatory.

Regards

Edited by krakatoa1883 (08/23/13 04:02 PM)


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azure1961p
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Re: Have You Seen the Schröeter Dichotomy? new [Re: krakatoa1883]
      #6042297 - 08/23/13 05:33 PM

Thank you Raf. I have bookmarked the link for reading later.

Pete


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Brian Albin
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Re: Have You Seen the Schröeter Dichotomy? new [Re: azure1961p]
      #6044290 - 08/24/13 09:52 PM

Thank you krakatoa1883, for the report by McCue & Nichol.
I see how their aerosol explanation makes good sense for Venus.

I read that this Dichotomy discrepancy also happens with Mercury, which of course does not have an atmosphere. Perhaps related is the report that Mercury shows ashen lighting as is seen on Venus. I thought the ash light on Venus was probably a spreading of light through the upper atmosphere by reflection from particle to particle. But again Mercury's lack of an atmosphere forbids a single solution being applied to both worlds.

All in all, I find this Schröeter effect an interesting puzzle.


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Sarkikos
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Re: Have You Seen the Schröeter Dichotomy? [Re: Brian Albin]
      #6046651 - 08/26/13 09:57 AM

I read the title of this post and thought it was going to be about an episode of "The Big Bang Theory."


Mike


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Chopin
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Re: Have You Seen the Schröeter Dichotomy? new [Re: Sarkikos]
      #6051349 - 08/28/13 07:27 PM

Quote:

I read the title of this post and thought it was going to be about an episode of "The Big Bang Theory."


Mike






I was thinking the same thing.


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Rick Woods
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Re: Have You Seen the Schröeter Dichotomy? new [Re: Brian Albin]
      #6051574 - 08/28/13 10:03 PM

Quote:

The same thing happens with Mercury. Neat huh?




I don't think this is accurate. My sources say it's an effect of the dense atmosphere of Venus, whereas the atmosphere of Mercury is far to thin (almost a vacuum) to exhibit this effect.


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Brian Albin
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Re: Have You Seen the Schröeter Dichotomy? new [Re: Rick Woods]
      #6059743 - 09/02/13 05:48 PM

My thought was, the fellas who say it is caused by Venus' atmosphere must be wrong because this same Dichotomy discrepancy is observed with Mercury, so it must not be the atmosphere.

But perhaps the same observed effect happens on two different worlds from separate and different causes. To me that seems unlikely, but unlikely is only statistics, not facts; so different causes may be the answer.


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Rick Woods
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Re: Have You Seen the Schröeter Dichotomy? new [Re: Brian Albin]
      #6060004 - 09/02/13 09:11 PM

As far as I know, there is no dichotomy discrepancy at all involving Mercury. If you could point me at some documentation to the contrary, I'd love to see it.

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John Boudreau
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Re: Have You Seen the Schröeter Dichotomy? new [Re: Rick Woods]
      #6060288 - 09/02/13 11:50 PM

Quote:

As far as I know, there is no dichotomy discrepancy at all involving Mercury. If you could point me at some documentation to the contrary, I'd love to see it.




I'm not familiar with any seriously reported dichotomy discrepancy with Mercury either, but I'd also be interested in seeing any such reports.

Since the planet is quite dark and has large scale features with varying albedo there have certainly been observations of an irregular terminator, but I think the observers going back at least to the 18th century had some understanding that there were true albedo differences that caused this, although up until just a few decades ago there were some that believed that thin clouds were also playing a part at times. In addition to light and dark large scale features along the equatorial and mid-latitude regions, much of the northern area of Mercury is relatively bright while the southern region is noticeably darker--- this effects the 'horn' extension of Mercury near dichotomy and in crescent phases as the northern can be quite pronounced while the southern is relatively blunt.


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krakatoa1883
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Re: Have You Seen the Schröeter Dichotomy? new [Re: Brian Albin]
      #6060580 - 09/03/13 07:50 AM

Hello Brian

the occurrence of a phase anomaly for Mercury is very uncertain as there are no as many estimates of observed dichotomy vs predicted as we have for Venus. Moreover while Venus looks of quite uniform brightness in white light Mercury does not, therefore the phase estimates are influenced by albedo variations. A detailed study of the errors involved in estimating the phases of Venus and Mercury was published by Kirby in the volume 80 of the JBAA. He demonstrated that while careful estimates of Venus dichotomy can be obtained through a telescope as small as 6 inches (although in practice I would say 4 inches may suffice) , Mercury requires a 12 inch telescope for obtaining reliable estimates of dichotomy (may be this is a conservative figure, however I personally wouldn't consider anything less than 9 or 10 inch for this work) . At present there are no enough data on the occurrence of Mercury dichotomy gathered with apertures of this size for concluding that Schroeter effect does involve Mercury phases.


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brianb11213
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Re: Have You Seen the Schröeter Dichotomy? new [Re: Rick Woods]
      #6061301 - 09/03/13 04:28 PM

Quote:

As far as I know, there is no dichotomy discrepancy at all involving Mercury. If you could point me at some documentation to the contrary, I'd love to see it.



T W Webb, "Celestial Objects for Common Telescopes", vol. 1., p.57-58 (Dover edition)

"The Phases. These will be easily seen, and are only remarkable because the breadth of the enlightened part has been sometimes found less than it should be from calculation. Schroter noticed this: Beer and Madler confirm it..."


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Rick Woods
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Re: Have You Seen the Schröeter Dichotomy? new [Re: brianb11213]
      #6061907 - 09/03/13 11:26 PM

That's a new one on me.
I wonder if it's the relativistic effect he's referring to? The thing that made LeVerrier beat his head against Vulcan for so long?


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krakatoa1883
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Re: Have You Seen the Schröeter Dichotomy? new [Re: brianb11213]
      #6062125 - 09/04/13 04:33 AM

These old reports are not always reliable. The reference to Beer and Madler, in particular, is based on only one observation made on September 29, 1832 with a small refractor. Although small discrepancies on dichotomy occurrence are sometimes still reported, recent observations failed to confirm Schroeter's effect for Mercury and are not conclusive (see ALPO reports for example).

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Brian Albin
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Re: Have You Seen the Schröeter Dichotomy? new [Re: krakatoa1883]
      #6062134 - 09/04/13 05:06 AM

I may have believed too quickly what I read.
I got that from Fred Price' Planet Observer's Handbook. His Mercury chapter, the paragraph "The Phase Anomaly" says: "It was Schröter who first noticed the discrepancy when he was observing Venus in the year 1793. It has been evident with Venus since that time, and Mercury sometimes shows it."

And the Mercury paragraph: "History of Observation" says: "In August 1793, Schröter observed that the terminator of Mercury was slightly concave at a time when it should have been straight according to calculations. Beer & Madler made many observations in 1836 that confirmed Schröter's and showed that Dichotomy occurred about 6-8 days later or earlier than the calculated time, depending on the direction in which Mercury was moving."

The text goes on to say that William Herschel doubted Schröter's findings.

If it were only Schröter and Madler who saw this, perhaps that is not enough observers for a consensus.

Perhaps it is a fault of viewing with too small a telescope. I think Madler used 3 3/4 inches. Wikipedia says Mr Schröter used a 6 1/2 inch mirror.


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