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Schröter Effect 2017


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About this observation
David Basey
Time of observation
07/11/2017 - 09:56
358mm Newtonian
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Since early May I have been able to make visual observations of the planet Venus on 31 separate days. Part of the each observation is to measure the apparent phase of the planet and compare this to the theoretical value. As usual, for much of the apparition, the observed phase is less than that dictated purely by geometric concerns. This is the well known Schröter Effect.

With the 2017 elongation approaching its close I thought it might be of interest to display the results. Observations were normally made both in yellow light with a Wratten 15 filter and blue with a Wratten 47.

Notice how:-

  1. The deficit is greater in blue than yellow light.
  2. The lag seems to increase with increasing phase up to about 0.8 and then decrease close to zero as it approaches 0.95. 


pmulli's picture

Hi David nice piece of work, what is the latest explanation of this phase anomaly.  I read somewhere that it is a contrast effect between the Terminator and the dark side of the planet'


David Basey's picture

Hi Peter,

I think in many ways you pays your money and you takes your choice as to the cause.

Clearly it could have a physiological basis, personally I’m not convinced by a contrast explanation in that I would expect this to give a phase larger than predicted rather than smaller but I could be wrong. 

The alternative of course is that the deficit has its origin in the planets thick atmosphere. Clearly the planet’s terminator is fainter than the limb and it may be that it fades below detection earlier than pure geometry would dictate. However if this were the case you perhaps would expect the greatest anomaly to be at half phase, which does not seem to be the case.

Scattering of some sort is likely involved. In a recent e-mail, Richard McKim, Director of the Mercury & Venus Section, points out that forward scattering does take place at small phases and indeed at these times the apparent phase can actually be larger than predicted. As the planet moves round in its orbit the direction of the scattering changes with respect to the Earth and this may play a part.


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