An observational project of amateur astronomers to determine the scale of the solar system with the aid of the Venus transit in 2004. Groups investigate the historical calculations and observations, set up new procedures, prepare the observation and carry out the evaluation.

Actuality, Introduction

On June 8^th 2004, in major parts of Europe, Africa and Asia – in particular in the complete German speaking area - a Venus transit takes place which is observable during it‘s entire duration. This is a rare astronomical event. The last transit of Venus took place on Dec. 6^th 1882, the one following the next on June 6, 2012 (which will only be partly visible in the German speaking part of Europe). Assuming good weather, Venus will be seen as a small, black disk moving slowly across the sun. For various reasons these transits could only be observed and evaluated by professional astronomers in the past. The astronomer E. Halley (1656 to 1742) had the idea to determine the distance from the earth to the sun (the astronomical unit, AU) from the transit times (together with a bit of geometry) with an error of only a few percent. In particular the transit of Venus of June 3^rd 1769 was observed and then evaluated: for instance from Varda (Sweden) and Tahiti.

The mathematical and astronomical instruments available today enable the present day amateur astronomers for the first time to observe and evaluate such a transit. From this emerged the idea for the project „Venus 2004", initiated by the Astronomical Society of Zurich, Switzerland.

1.- We want to learn about the history of the past Venus transits and the corresponding kinematic conditions and to re-enact for the first time on occasion of the coming transit of Venus of June 8^th, 2004 with the means available to the amateur what has been only possible with professional means in the past.

The coming Venus transit we want to

• prepare well
• observe by different methods
• process the raw data and standardise in order to enable the data exchange

We want to interest other amateur astronomers in this project to observe this same transit at the same time from the southern hemisphere.

2.- We want to evaluate these observations and data by different methods, taking into account the given means, in order to determine the parallax of the sun and from that the astronomical unit with the best possible accuracy.

3.- We want to do all this for our own pleasure and within the scope of the teamwork among friends. We appreciate the help of professional astronomers.

We have become aware of the fact that the success of the project depends essentially upon the motivation and cooperation of a sufficient number of amateur astronomers, but also upon the meteorological conditions on June 8^th, 2004 and, above all, whether we will be able to evaluate the large number of observational data in a statistically correct way.

1. History

The history group has first reported on the large scale explorations in 1761 and 1769, in particular on the travels of Le Gentil. For various reasons these endevours were not successful: inaccuracies in determining the geographical position; the black drop effect; inadequate kinematic models; lack of statistical methods.

E. Halley proposed his idea already early in the 18^th century. It was L. Euler (1707 to 1783) who finally worked out the mathematics in detail including statistical treatment. He arrived at an estimate of the parallax of 8.80" in 1770. J.F. Encke (1791 to 1865) of Berlin, Germany, evaluated the observations again using the least squares method but arrived at a surprisingly inaccurate result.

2. Kinematics

Several people in this group (H. Blatter, F. Messmer, R. Hauswirth) followed up Halley’s ideas (cf. internet address below). The result is that amateurs today can determine the AU in good quality by measuring the contact times (in particular 2^nd and 3^rd contact) at observation points that are as far apart as possible (e.g. Europe and South Africa). The accuracy will be determined essentially by the accuracy of the times of contact.

It is for that reason that we are seeking cooperation from astro amateurs of South Africa. Switzerland and South Africa are extremely well suited for observation of the event.

Another approach is to measure the radial distances of Venus from the sun during transit at various times and compare them with the geocentric ephemeris. This enables one solitary observer to determine the parallax (cf. R. Brodbeck at the internet address below).

3. Instruments / Observation

Various problems facing an observer have been discussed, in particular the Black Drop Effect. This seems to have seriously impaired earlier observations of the contact times.

The primary goal the group has set itself is to establish guidelines for the observation. The result, influenced by the experience of some members in (semiprofessional) groups for observing minor planets is the „Observation Record / Beobachtungsprotokoll" which is part of this report. A manual as a guide for carrying out and recording the observations is in preparation.

This Observation Record should be tried out on the occasion of the Mercury transit on May 7^th, 2003.

4. Evaluation / Results

It has been decided to do the evaluation of the data centrally, that is, all observations have to be submitted to Hugo Jost who is working on the procedures to evaluate the data.

5. Contacts

Contacts have been established with Germany and South Africa and we intend to contact La Réunion as well, where an observatory is located with which Swiss astronomers have had contact in the past.

It has been pointed out above that it is crucial to this project to have observations from far apart places on our planet. It is the hope of the Swiss astro amateurs engaged in this project that we can establish cooperation with a number of amateur astronomers in South Africa to carry through the method Blatter-Messmer.

6. AstroInfo

This is a portal in the internet dedicated to the astro amateurs. The project is represented there under www.astronomie.info/projectvenus . - This report is in part a translation of what is to be found in German under the given address and it will also be made available there. – Several articles as well as the Observation Record can be downloaded from