Transit of Venus, June 8th, 2004

Essen, equatorial view

Tips and Instructions for the Transit Day

This project's aim is to determine the distance to the Sun by comparing the positions of Venus on pictures which have been simultaneously taken from different sites on Earth. For this reason, it will be necessary to take photos with precisely known orientation at exactly concerted moments at as many sites as possible.

The basic ideas and an evaluation example are again described in additional pages.

In principle, two simultaneously taken photos are enough to determine the distance to the Sun. However, our experiences made during the transit of Mercury last year have shown us that the errors which are due to bad focussing, the process of digitizing, the measured positions etc. make it impossible to get a satisfying result on the basis of two only photos. Therefore, as many pictures as possible should be combined in order to minimize random errors by statistical methods.

Schedule

The participants are asked,

to photograph Sun and Venus during the transit all quarters of an hour, i.e.

exactly at 5.30, 5.45, 6.00, ..., 11.00, 11.15 UT.

In the case that clouds make photographing impossible at the fixed moments, as many pictures as possible should be made with similar time difference. By line fit it then will be possible to interpolate the positions being desired later.

Tips for Photographing

Little sensitive films (e.g. 50 ASA or 64 ASA) seem to be most suitably.

If you let develop the films in a foto shop don't let them cut into parts or frame the slides! The borders of the pictures will be hardly to detect.

The correct time of exposure must be found by series of test pictures, in advance. (By using a sun filter and an optic with f/10, we found 1/1000 s to be the best.)

The focal length should be chosen so that the pictures show an as large part of the Sun as possible. Even better, the pictures should be large enough to show two pictures of the whole Sun. (With a picture size of 24mm*36mm focal lengths of up to ca. 1000 mm are suitably.)

For precise determination of the picture's orientation, we strongly propose to make twice exposed pictures with a camera which is fixed on a tripod. We, additionally, propose a time difference of exact 120 s. In that case, both pictures of the Sun will hardly intersect.

For each picture, the moment of exposure and the corresponding time difference must be exactly recorded.

Digitizing of analog pictures should be done by using a scanner for color slides or by a professional photo shop.

The best photos should be uploaded into our forum. Thereby, the file name must give the most important information:

City(Name)hhmmssUT.jpg

Additional tips are given in the page Twice Exposed Pictures.

Position measurements

There exist several tools for measuring the positions of Venus with respect to the Sun's face. In this project, we offer the program Bildauswertung.exe. It is contained in the zipped archive. With that program, the sizes and positions of Sun and Venus on digitized pictures (BMP or JPG format) can be measured by fitting to them circles of variable size and position.

Pixel radius and centre of the Sun are determined by fitting a circle to it.
By fitting a further cicle to the second picture of the Sun the amount and the direction of the Sun's displacement (and, thereby, the direction from East to West!) are determined. If the corresponding time difference and the celestial position of the Sun are known the scale of the picture and the angular radius of the Sun ρ_S can be derived from these values (see Example 1).
By fitting of two additional circles to both discs of Venus the rectangular positions x'=x/ρ_S and y'=y/ρ_S of them are measured with respect to the Sun's centre. The x-axis shows to West, the y-axis to North.
The results are to put in an Excel sheet. which additionally calculates the line fits. An example sheet may serve as explication.
The completely filled (with the name of the observer, the city and the geographical coordinates) tabulars are to upload into our forum with the following file name:

City(Observers's name).xls

Additional measurements during the transit

As precise measurements as possible of the contact times (see Contact time method)
Measurement of the exact moment at which the Sun takes its highest position above the horizont (local noon) and of the value of that elavation (by means of a gnomon, for instance, see our projects Determining the own Geographical Coordinates and Determining the Radius of the Earth).
Perhaps additional determination of the Sun's angular radius (for instance, by measuring the size of pin hole pictures of the Sun or by measuring the time which the Sun needs to move by its own diameter, see the project Measuring the Angular Radius of the Sun)

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Udo Backhaus
last modification: March 28^th, 2008