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Star Observations & Personal Equation
Image: Album - Photographs
Source: Museum Victoria
Observing the precise positions of stars required a huge amount of technology, calculation and concentration on the part of the observer. The astronomer would watch the star as it crossed the field of view of the transit telescope, then press a button as the star crossed each of several fine lines in the eyepiece, and press a button when the star met a crossing horizontal and vertical line. Each time the button was pressed a mark was made on a rotating sheet of paper on a precisely controlled chronograph.
Then the astronomer had to shift position and read several microscopes directed at the finely ruled scales on the transit circle, to determine both declination and right ascension (the celestial equivalent of latitude and longitude). Including the taking of the temperature and air pressure, there were as many as 22 different observations to be made for a single star. (See Maunder 1900, pp. 156-63 for a description of the procedure at Greenwich Observatory.)
The observation itself was only the start. Each observation had then to be adjusted carefully to allow for many factors that would influence the measurement: the bending of the star's light rays by the earth's atmosphere, the known errors of the instrument, and other technical factors such as precession, nutation and aberration. These laborious calculations had to be made using pencil and paper and logarithmic tables. A transit observation of 3 or 4 minutes could take hours of computation.
Even the observer had to be taken into account as a source of error. In 1796 the British Astronomer-Royal had dismissed his assistant because he had a 'vicious way of observing the times of the Transits too late', by as much as a second. Astronomers realised that each one had a slightly different way of recording the time; some anticipated the star crossing the line a moment too early, others were slow. Frequent testing allowed the calculation of each observer's peculiar but fairly consistent habits, or 'personal equation', and this could be allowed for in the final calculation of the star's position.
Melbourne Observatory's astronomers would undertake observations of the same stars, to compare their personal equations and adjust their observations to a common standard. When in 1883 Australian observatories undertook to more accurately determine the longitude of each observatory, Pietro Baracchi, a young scientist who was soon to join the Melbourne Observatory staff, was sent to each Australian observatory to compare his personal equation with those of the primary observers in each colony. Typically the differences were no more than a quarter to a third of a second, but it was important to adjust for these if accurate determinations of longitude were to be made.
Maunder, E. Walter (1900). The Royal Observatory Greenwich: A Glance at its History and Work, London: The Religious Tract Society.
Schaffer, Simon (1988). 'Astronomers mark time: discipline and the personal equation,' Science in Context, 2, pp.115-145.
Items per page: 10 50 (showing 321 - 330) 373 items
Standard Troy pound, Parliamentary Copy of the British Standard Pound, no. 29, 1844. Used as a primary standard for the Colony of Victoria. The standard pound was placed in the custody ...From: Melbourne, Australia Images: 2
Photograph of comet, probably taken at Melbourne Observatory. Photograph in a photograph album used at Melbourne Observatory, late 19th century. The photographs show the main Melbourne ...
The 8 inch South Equatorial Telescope, Melbourne Observatory. The telescope was built by Troughton & Simms, London and installed in 1874. Photograph in a photograph album used at Melbo ...
Photograph of the Moon at first quarter, taken by Joseph Turner with the Great Melbourne Telescope at Melbourne Observatory around 1874. Photograph in a photograph album used at Melbou ...From: South Yarra, Australia Images: 2
Photograph of the Moon, taken by Joseph Turner with the Great Melbourne Telescope at Melbourne Observatory around 1874. Photograph in a photograph album used at Melbourne Observatory, ...From: South Yarra, Australia Images: 2
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Terrestrial radiation thermometer, made by Negretti & Zambra, London. Range - 10 to +130 degrees Farhenheit. Used at Melbourne Observatory.From: London, United Kingdom Images: 0
Main observatory building, Melbourne Observatory, from northwest. Photo taken between 1883 (after construction of East Transit Room, on left, and 1892, when Computing Room was construct ...
Chronograph for recording exposures on the Astrograph Telescope, Melbourne Observatory. The Astrograph, by Grubb of Dublin, was installed at Melbourne Observatory in 1891. Photograph i ...
Robinson's anemometer, recording equipment. Possibly at Melbourne Observatory. Photograph in a photograph album used at Melbourne Observatory, late 19th century. The photographs show t ...
Altazimuth instrument, probably one used at Melbourne Observatory. Photograph in a photograph album used at Melbourne Observatory, late 19th century. The photographs show the main Melb ...