Observations of the Sun with a Danjon-type visual astrolabe have been recorded since 1975 at the CERGA Observatory. The primary goal of these observations is to determine the orbital elements of the Earth around the Sun. However, they also provide very good quality, homogeneous values of the solar diameter. The working principle of the astrolabe, which relies on the method of equal elevations1, has been described elsewhere2. The diameter value is obtained from the difference between the zenith distances of the solar centres corresponding to the instances of successive limbs crossing the same almucantar. The solar diameters are obtained up to 16 times per day. The heliographic latitude of the observed points varies throughout the day and changes with the seasons. In the present study, we have discarded the data taken at zenith distances >60° because of their lower quality on average. We consider then all the retained measurements on equal footing. The existence of a long period, ∼1,000 days, in the variation of the solar diameter has already been reported3. The low-frequency domain of the power spectrum deserved a more detailed analysis which the more recent data permit. Here, we present this analysis, which reveals several previously undetected periodicities.
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Delache, P., Laclare, F. & Sadsaoud, H. Long period oscillations in solar diameter measurements. Nature 317, 416–418 (1985). https://doi.org/10.1038/317416a0
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