The Schröter effect was first coined by Sir Patrick Moore and was named after Johann Schröter who first noted it when he was observing Venus in the 1790's. In essence, the effect is an anomaly that is observed when determining the phase of Venus. When the planet is approaching its Greatest Elongation East (GEE) in the evening sky dichotomy is observed to take place a few days earlier than predicted whilst when the planet is approaching Greatest Elongation West (GEW) in the morning sky dichotomy is observed to take place a few days later than expected. I think it is now generally accepted that this is a genuine optical effect caused by the thick atmosphere of Venus and is not due to observer error.
I don't think it is too hard to understand in a hand-waving way why this occurs. Most predictions about the phase of a planet or moon assume that the body is a sphere with a well-defined surface. This is largely to help make predictions about phase relatively easy to compute (see my previous blog entry). To aid with this description I include my first diagram from my earlier post:-
Venus is shown here in the gibbous phase and let us imagine that it is in the evening sky. For this to be the case Venus would have appeared from behind the Sun after a superior conjunction and then steadily increased its angular distance from the Sun as it approaches its GEE. Its phase initially would be almost full and then it would be less and less gibbous until dichotomy (half phase) is reached around GEE. In this picture we assume that Venus has a hard spherical surface and that light from the Sun can just reach the edge of the planet at the terminator (see, for example, point T).
Now imagine that Venus has an upper atmosphere that is like a thick haze. Light can penetrate this haze to a certain degree and be reflected from it, but the well-defined surface is now obscured by the haze and lower atmosphere cloud. Light destined for a point on the terminator T enters the atmosphere at a point ahead of T towards O. As it penetrates the atmosphere some of the light is scattered by the haze and so the amount of light that would have reached T is much reduced. The effect is to soften the terminator and pull its apparent edge towards O.
At the 'poles' of the planet at P and P' something else happens. Light enters the atmosphere ahead of point P and P' as it does at T. The same scattering and light reduction occurs here too due to the haze. Light that would have terminated at P or P' on a hard surface continues diffuse into the haze beyond P and P' into the dark. However, unlike at T the effect is amplified because we are looking at the atmosphere edge on - we are seeing this forward scattering over a wider depth. The net effect is that we don't see the terminator move inwards at P and P' as we did at T.
I hope you can see from my description that the overall effect is that the terminator is pushed by this scattering process to be a little ahead (sunwards) of its predicted position. This means that as we approach EGE the observed phase is always less than its predicted phase and it is this that causes dichotomy to be observed a few days early.
Now if Venus is in the morning sky then just after inferior conjunction it is a very thin crescent. On the following days its angular separation from the Sun increases until GEW is reached. As this happens the crescent phase of Venus gradually thickens until dichotomy occurs. Again because Venus has a thick atmosphere the terminator of Venus will be a little bit sunwards of where it is predicted to be. This means that the observed phase of Venus will be less than the predicted value. This will cause dichotomy to be seen a few days later than expected.
For a full description of this model of how the atmosphere affects the phase of Venus you can read the paper in the BAA journal by Anthony Mallama published in 1996. I think the idea that the observed phase is altered by scattering is compelling and it is supported by the fact that the effect is more pronounced in blue and ultraviolet light rather than yellow (see, for example, this nice observation by David Basey in 2017). It is well-known that blue and ultraviolet is more scattered than yellow light. Also, the idea that light scattering in the upper atmosphere can diffuse into the dark portion of the visible disc is supported by the observation of the extension of the cusps of the terminator into areas which are predicted to be dark when Venus is a very thin crescent (see, for example, this image by John Sussenbach in 2017).
For further discussions of the Schröter effect have a look at this article by William Sheehan in 2017.
All text and images © Duncan Hale-Sutton 2025
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