Measuring the Distance to the Sun

1. Obviously, we must drill very careful focussing. It is important for measuring the angular radius of the Sun, but crucial for a sufficient exact determination of the planet's positions.

2. The appropriate exposure time is equally important.

3. The exact knowledge of the picture's orientation is crucially important. Only on professionally taken single pictures it may be known with sufficient exactness. Up to now, we are not quite sure if it is possible to keep the orientation sufficiently constant between two single pictures.

4. Even the exactly known (to one second, if possible) moment at which the picture is taken is important. If the size of the picture is large enough for two nonintersecting pictures of the Sun the time difference should be choosen large enough ($>$ 2 min). In this case, both positions can be well determined.

5. There is a chance for satisfying values of the solar parallax only with a sufficient basis length. Even the professional pictures of the GONG network yield a good measure only for the largest distance (Learmonth/Teide).

6. It has become evident that it will not be possible to get good results by comparing single pictures.

Of importance are as extensive series as possible which are made up of well focussed and correctly exposured double photos with exactly known moments of exposure! The distance between the sites of observations should be as large as possible.

The concrete photographing procedure for the transit day will be described later!