## Project Preview: Measuring the Sun's Parallax

You will find below `pre-publication' information about an important project which we have prepared as one of the groups participating in Astronomy On-Line. If all goes well, the project will be placed under the Collaborative Projects on Monday, November 4, 1996, following some further, minor changes, etc. We hope that all interested groups will make themselves known to us as soon as possible.

We will try to determine the size of the solar system by measuring the distance of a nearby minor planet and deducing the sun's distance from it.

The radius of the Earth's orbit around the Sun is one of the most fundamental constants in astronomy: the Astronomical Unit (AU). Its value is the basis not only for determing the dimensions and structures of space but for measuring the astrophysical properties of planets and stars. Because of the great length of the AU, all related effects are very small. For this reason, the measurement of the Astronomical Unit is very difficult and it was one of the main problems of astronomy for hundreds of years. Up to now, there exists no possibility to determine by means of measurements carried out in schools with the equipment there.

Because the expected parallax effect is in the range of some arcseconds (for an asteroid with a distance of aproximately 1 AU), it will be necessary to take pictures with CCD cameras and telescopes with focal lengths as long as possible.

Therefore we are looking for groups interested in parallax measurements by means of CCD imaging. If we will find groups (distributed all over the Earth, if possible), we hope to be able to determine the asteroid's parallax and by that to measure the distance of the Sun. We will offer the pascal source code of a simple program which calculates the Sun's parallax by combining two different positions of the asteroid, taken simultanously at different observatories or at different times of the same night at the same observatory, respectively.

We hope that the result of this cooperation will be a quite good value of the Astronomical Unit, expressed as a multiple of the Earth's radius and therefore in kilometres. Typical pictures and evaluations will be published as soon as possible after the measurements during the hot phase of Astronomy On-Line.

A preliminary version of the project description can be downloaded via http://www.uni-koblenz.de/~backhaus/aol/project.html.

Interested groups are kindly requested to take test pictures of the asteroids (84) Klio and (990) Yerkes, respectively (ephemeris data of these and some other asteroids can be reached via our project description) and to contact us via e-mail at backhaus@uni-koblenz.de.

Udo Backhaus
AstroAG Koblenz

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Last update: Oct 29, 1996