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George, you wrote:

> A discrepancy of 8% is hard to accept. Something is wrong, as Marcel
> suspects. And such a big discrepancy should be easy to check for.

Yes, the discrepancy is big. I thought I would have a reliable scale
from my sunset observations. This scale is now put in question. It
turns out to be not so easy, that's why I start to ask around, hoping
that someone may have done a similar experience and may have even
found the reason for it.

> I suspect the angular calibration of his camera Perhaps he should check that
> more simply, in terms of the angle subtended by an object a few metres away,
> rather than by relying on distant topography. Near enough to that he can
> check dimensions with a tape, yet large enough so that the camera focus
> doesn't need to change from infinity, and also so that uncertainty in the
> position of the effective optical centre of the lens is unimportant (within
> much better than 8%).

One reason could be distortion, but I wouldn't attribute it to barrel
or pincushion distortion for the following reason: These optical
distortions are important when referring to all of the image. The 6x
zoomed pictures of the sun or the geographical feature (an island with
a well defined shape) use only a very small part of the full picture
size and they are usually near the centre. With the resolution used
for those images the full size of the pictures is 958 x 1278pixels
compared to the size of the island about 60 x 210pixels or the
diameter of the sun about 105pixels. Within this small range these
distortions are likely to be negligible.

It doesn't seem to be a distortion of the computer screen when
measuring the size of the geographical feature with GoogleMap. In
order to verify this I made the measurement also with GoogleEarth
which has a tool for measuring distances; the result was the same.

> Alternatively, even though Marcel considers that the Sun image is not
> overexposed, perhaps it is, and he is looking at a bright halo of light
> around it.

The scale derived from the sun is a statistical result from over
hundred measurements, having a standard deviation of a little bit less
than +/-1.5% or pixels. Indeed, I do also have overexposed photos, but
they don't enter this statistics. The colour of the sun's
circumference is a good indicator.

Any suggestions for possible reasons are welcome! In the meantime I
try to get a better estimation for this difference.

Marcel

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