Dr J A Hamilton, later Rector of Mullabrack, was closely involved in the conception of the Observatory at Armagh and may well have made the initial suggestion to his Archbishop. It was natural that he should be chosen as its first director.
Though generously supported by its founder, the Archbishop, his early death in 1794 resulted in the loss of a number of instruments originally ordered for the Observatory. As a result Hamilton’s observations were not as comprehensive as they might have been. Nevertheless he initiated the early series of meterological recordings and observations of stars which paved the way for the future scientific work of the observatory. From the beginning, he expressed a desire to work closely with the astronomer at the other public observatory in Ireland, Dunsink Observatory in Dublin – a cooperation that has continued until the present day.
The need for accurate positions of stars was one of the principal reasons for the establishment of the observatory at Armagh. They were required firstly for navigation and secondly to provide a framework for the measurement of the positions of the planets.
The second reason was scientifically more important as it was only by careful, frequent and accurate observations of the planets, that Newton’s law of gravitation, one of the most fundamental laws of physics, could be verified. Indeed, during the 18th century, French astronomers believed they had observed discrepancies in the motions of the planets, which were not accounted for by Newton’s laws. English astronomers disputed these findings and were anxious to vindicate the reputation of the great Newton. Whilst in fact the English astronomers on this occasion were proved right – it was the small discrepancies that were discovered in the orbit of the planet, Mercury, in the 19th century, that eventually brought down Newton’s theory of gravitation, and lead to Einstein’s theory of general relativity.
The positions of the stars and how they were measured
With the need for accurate positional observations established, how were they to be obtained? Firstly, we must understand the coordinate systems for stars. In a directly analogous way to latitude and longitude on the earth, astronomers used celestial latitude, (termed declination) and celestial longitude, (termed right ascension). If one imagines the earth’s coordinate system, (latitude and longitude), projected onto the celestial sphere, (the sky), one can grasp the basic similarity. Just as we can measure the latitude of a point on the earth’s surface by it’s distance from the equator, (or 90 degrees minus its distance from the pole), similarly we can determine the declination by measuring the distance of a star from the celestial equator, (or 90 degrees minus the distance from the celestial pole). Also, just as we measure longitude on the earth from an arbritrary zero line, the Greenwich Meridian, we measure right ascension or celestial longitude from an arbrtrarily defined point in the sky called the first point of Aries.
To measure these coordinates for a particular star the simplest and most commonly used instruments were the meridian and transit circles. Both of these instruments consist of a simple refracting telescope which swivels around a horizontal axis which lies exactly east-west. The telescope has a small field of view and is only capable of observing stars in a very narrow strip of the sky. This strip, adjacent to the meridian, passes through: the south and north horizon points, the celestial pole, and the zenith directly overhead. From the time at which the stars cross the meridian astronomers can deduce their right ascension or celestial longitude.
The Troughton Equatorial Telescope
The first major instrument purchased for Armagh Observatory, the telescope made by Troughton of London, is a masterpiece of English instrument-making of the 18th century, only one other instrument of its type exists in the world today. It was purchased by Archbishop Robinson on the recommendation of the Astronomer Royal of England who was very impressed with its novel design.
As explained earlier, the simplest method of measuring the coordinates of stars and planets was to record the time at the instant they crossed the meridian using a transit circle. However the major difficulty with this technique is that observers had only one opportunity per night to make such a measurement – when a star crossed the meridian. Troughton and some of the other instrument makers of his time realized that, if instead of using a horizontal east-west axis, they could mount an instrument on an axis that pointed to the celestial pole, it would be possible to measure the coordinates of a star at any time of the night when that star was visible. This would be particularly valuable if one were trying to obtain a sequence of planetary observations in a poor climate when clouds obscure the sky for much of the night. The astronomers were somewhat dubious that the instrument makers of the day could accomplish the high standard of stability and accuracy, in this more complicated design, than in the simple and well tried meridian and transit circles. Manufacturers, and particularly Troughton, felt they could. To improve stability Troughton used massive stone pillars for support and for rigidity used conical brass tubes to support the central telescope ring.
In the final outcome, although many useful observations were made by this telescope by J A Hamilton, the astronomers were proved right, in that the telescope could not match the accuracy of the simple transit and meridian circle. It was an expensive mistake.
