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Carsten Sønderskov Jørgensen: Christian Horrebow's sun observations (in Danish, resume in English)

English resumé

To understand how the Sun works one way is to examine the sunspot records from the last 400 year. Since sunspots are caused by magnetic activity, they are a sign of higher general activity on the Sun, which is important for space weather and the Earth’s climate. In order to use the old sunspot records in the best way, a better understanding of the methods used are needed. It has therefor been the aim of this project to examine the observations made by Christian Horrebow at Rundetårn, Copenhagen during the years 1761-76. His observational protocols have been analyzed to determine which observations were made and which methods were used. In addition articles published by him in two Danish journals have also been analyzed.

It has been found, that the sunspots were first observed using Machina Parallactican and Qvadrante. This was done during the years 1761-1766, with either the two instruments at the same time (so two observers were needed) or the quadrant was used to find both coordinates. This later method must have been a time consuming process and the first a complicated one since two observers had to work together. In 1767 they started using Machina Æquatorea to find both sets of coordinates in a much simpler process. In this instrument there were one vertical line and three horizontal, with one of the later being adjustable. During this first year the instrument was corrected several times, which is likely why Christian Horrebow only published his sunspot data from 1768-1776. In these articles he also explained symbols, which are corrections to some observations and from those it is clear, that he saw himself able to observe with an accuracy down to ¼ second of arc. In his article about the time during the Venuspassage in 1761, he shows how to find the correct time of this observation with the use of the time control from previous days. A similar method could be used, if one needs the correct time for any observations in the protocols. Besides these observations there are also an extensive one of a solar eclipse on 4 June 1769 and several comets are followed throughout the protocols.

For the time controls they used the method of measuring corresponding altitudes for the Sun with a quadrant and from this determine the time of noon. In addition they observed the Sun at noon with Rota Meridiana to determine how badly their clocks had been running since last days noon. It has been found, that they used sidereal time for all observations with starting point placed at the vernal equinox, which is why their calculated noon increases about four minutes each day in the protocols.

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RePoSS: Research Publications on Science Studies is an electronic preprint series issued by the Center for Science Studies, University of Aarhus. It has been given the ISSN 1903-413X.

RePoSS publishes preprints, manuscripts of talks, student works (e.g. Masters theses) etc. that contribute to the field of Science Studies as understood by the research group.