Measuring the Solar System - the Story of the Transits of Venus

This is the catalogue for an exhibition at Armagh Observatory to coincide with the transit of Venus on 8 June, 2004.

Introduction

In the 16th and 17th centuries our knowledge of the motions of the planets rapidly improved. This was due to the work of several outstanding astronomers and two brilliant mathematicians.

The astronomers were: Copernicus, who proposed that the Earth moves around the Sun, Tycho Brahe, who made meticulous naked eye observations of the planets, and Galileo, who first utilised a telescope for astronomy.

The mathematicians were: Kepler, who showed that the planets moved in elliptical orbits, and Sir Isaac Newton, who was able to explain the relative positions and motions of the planets from a simple application of his Universal Law of Gravitation.

Newton's theory was so comprehensive that it was felt by many that the science of astronomy was now complete. Only one vital piece of information was missing, the distance from the Earth to the Sun, for, though Newton's theory was able to predict the relative distances of all the planets from the Sun, it could not predict the actual distance to any one of them. But, once the distance to one planet was known, all others would follow. It was as if the distance to Dublin from Armagh was known to be twice the distance to Belfast, but neither distance was actually known in miles.

Section I	Items illustrating the background to the problem.
Section II	Jeremiah Horrocks, Edmund Halley and the importance of the Transits of Venus
Section III	The Astronomical Unit
Section IV	The 19th century transits of Venus
Section V	The Legacy of the Transits of Venus

John Butler
Armagh Observatory
May 2004