Essen, horizontal view

Internet Project Observing, Photographing and Evaluating the Transit of Venus, June 8th, 2004

Essen, equatorial view

Photographing the Sun

In order to be able to determine the position angle of Venus with respect to the direction to east we propose to photograph the Sun twice on each picture with fixed camera (see our project Transit of Mercury). Precondition of this procedure is, of course, that the size of the picture is large enough for more than one Sun. The smaller the Sun's face is compared with the size of the picture the longer can the time intervall be chosen and the more exactly can the orientation of the picture be determined.

We use daylight slides (36mm*24mm) together with an optic of 800mm focal length. In this case, the both exposures can be seperated by up to 6 minutes (see below).

As our experiences with Mercury show there are several crucial points to be regarded:

• Use a little sensitive film (50 ASA, for instance).

• The focus must be as exact as possible because a bad focus causes uncertainties in determing the Sun's radius and errors in measuring the position of the planet. But this position has to be measured with greatest precision! But: The visual impression of a good focus when looking through the viewfinder is no guarantee for a well focussed picture!
  The edge of the Sun, possible sunspots and, perhaps, Venus itself will offer the best opportunity of focussing. Using a magnifier for the viewfinder may facilitate the focussing but because of its own focus it must be tested in advance: The pictures below have been taken using a new (and not properly adjusted) magnifying glass!

• The exposure time must be suitably chosen so that the picture is neither to bright nor to dark. Some hints may be useful:
  • If the camera can exposure automatically it should be tested if shortening the time by one or two steps yields better results (see the examples below).
    90 seconds time difference

    Automatic exposure	Automatic exposure -1 step	Automatic exposure -2 steps

  • Most often the brightness of the picture is measured in the center of the picture. Therefore, by automatic exposure, the Sun will be taken with different brightness depending on its position on the picture (see the left picture below).
  • Perhaps it will be better to choose a fixed exposure time. With our optics, 1/1000 second has proved to be suitable. Of course, it depends on the Sun's elavation above the horizont and on the state of the sky!

• The time difference between the both pictures should be adjusted to a fixed value and maintained carefully:
  • If you will have found the suitable time difference you will be shure to catch as much of the "both" Suns as possible - and that is a precondition of precise measuring their positions.
  • Choose the time difference as large as possible, if possible so that the both pictures don't overlap. In this way you can minimize the probality of Venus to be positioned in the overlapping region of both pictures.
  • If you know the time difference you also know the exposure moments of both pictures and the position of Venus on both pictures can be evaluated.
  • If you know the time difference and the declination of the Sun you will be able to determine the angular radius of the Sun.

    Example: The pictures above and below have been taken on March 8^th, 2004 (δ_S=-4.7°). Because of the rotation of the heaven (or of the Earth, if you prefer this viewpoint) in each minute the Sun will move by 15'*cos(δ_S)=14.95' and, therefore, 89.70' in 6 minutes. In this way, we found ρ_S=16.2' (Guide 8.0 gives us ρ_S=16.11'.)

• Multiple exposuring must be exercised!
  • Does your camera offer the possibility of double exposuring the same picture? That means: Are you able to wind up the camera without transporting the film?
  • Are you able to keep the position of the camera unaltered during winding it up? A very stable tripod is necessary for this reason.
    120 seconds time difference

    Fixed exposure 1/250s	Automatic exposure

    In the pictures above we found the direction of displacement to be constant within +-0.5°, approximately.

  • Does the counter count a new picture when you wind up without transporting the film?
  • Between the double exposured pictures, is the film transported by the same amount as usual (with our camera it is transported less!). For savety, you should perhaps always insert one additional frame.

Position Measurements

On a picture with two solar discs which have been taken with fixed camera, the line connecting the both discs represents the direction from (celestial) east to west. The position angle θ' of Venus is measured counterclockwise with respect to the direction to west. The position of Venus can be specified in the following two manners:

1. by the position angle θ' and the relative distance r'=r/r_S or
2. by the rectangular coordinates x'=r'cosθ' and y'=r'sinθ'.

Mercury, photographed from Namibia, May 7^th, 2003 at 9.45 UT
Position measurement with Bildauswertung.exe

We offer the little program Bildauswertung.exe which allows to fit circles to the solar discs and to the discs of Venus. Resulting, it calculates both positions of Venus. Following the above outlined procedure it additionally calculates the angular radius of the Sun if its declination angle and the time difference between the both pictures are known.

In the case of a single exposured picture the program determines the position angle with respect to the direction to the right edge of the picture unless the orientation of the picture is known.

If several pictures have been taken from the same site time and position are saved in a text file in a format which can easily be imported to an Excel worksheet. Excel than determines the line fit to the rectangular coordinates:

This fit makes it possible to interpolate or to extrapolate the positions to a common moment for different sites. The distance between two of those positions gives the parallactic shift β of Venus from which the solar parallax π_S can be deduced following the procedure described in our basic paper.

The procedure is described in more details in our Mercury project and in the corresponding evaluation page. We have put detailed material for exercises into our forum.

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Prof. Dr. Udo Backhaus
last modification:  March 28^th, 2008