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Astron. Astrophys. 325, 1174-1178 (1997)

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2. The measurements

Table 1 gives a list of measurements of the solar diameter available at the Paris Observatory library both in archives and documents from 1660 to 1995. Columns 1 to 4 contain respectively a reference number (1 to 30), name of author, date and method. Columns 5 to 7 give the number (N) of individual measurements, the calculated value of the semi-diameter at 1 AU (R1) and the estimated error ([FORMULA]) for one individual measurement in each set. The contents of columns 8 to 10 will be discussed in Sects. 3 and 4 below. In the case of the 17th and 18th centuries (reference numbers 1 to 13), I have studied the measurements by comparing with modern theories and ephemerides. For each individual measurement made by each observer, I determined the value of R1 using the VSOP82 theory of Bretagnon (1982). In particular, this comparison has led to an explanation of the periodical term noted by Débarbat (1982); this proves to be a fictitious term related to the fact that the secular change in the eccentricity of the Earth's orbit produced by planetary action had not been taken into account. The error for each individual measurement ([FORMULA]) was estimated, mainly on the basis of remarks by the observers, related to instrumental difficulties: micrometer, clock, reticule. The data dispersion ([FORMULA]) could only provide a reasonable order of magnitude for the experimental uncertainties. In addition, similar observations have been made recently (Toulmonde 1995), either (i) using a well-preserved historical instrument (dated 1677) or (ii) with the aid of a small modern telescope which was assumed to be similar to that used by early observers. Both series of measurements enable the past observations to be better assessed, considering the timing for the solar limb (eye and ear method) and the friction of the micrometer screw. Astrometric instruments around 1700 had a very small objective (2 to 3 cm in diameter) and with a focal length from 2 to 3 m. For micrometric measurements, the full disk of the solar image was seen in the field of view; the objective had only a single lens, which was often employed with a diaphragm to diminish the geometric and chromatic aberrations. In my detailed analysis of these observations, I showed that the measurements made with such instruments gave results for the solar diameter some 10" (0,5 per cent) larger than the standardised values calculated using modern ephemerides. For the data from the 19th and 20th centuries, analyses by various authors have been divided into two groups. Items numbered 14 to 23 in Table 1 are based on the measurements from the beginning of the 19th century up to the middle of the 20th century; those numbered 24 to 30 relate to the most recent measurements. It should be noted that in many instances, the period covered by any specific series of measurements covered several decades, with observations made by different personnel, using a variety of instruments at the same observatory.


[TABLE]

Table 1. Observations in chronological order and R1 values. In column 2, names in parenthesis are those of authors who have analysed the observations and not the observers themselves. Methods are given in col. 4: PP = projected transit time, MI = micrometer, DP = observed transit time, HE = heliometer, ME = transit time at meridian circle, AS = solar astrolabe.


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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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