Study and test of a solar astrolabe for Tamanrasset observatory

Abstract

Among the important quantities in astronomy and astrophysics there is the Sun diameter. From this quantity, the ephemerid are established, the dates of contacts for the solar eclipse are calculated, and the standard solar models are elaborates. In parallel with the astrophysical studies of the Sun, astrometry observations may lead to improved values for this particular reference. The measurement of the solar diameter is not as simple as may be thought. During the last centuries and years a big discussion and disputation about its constancy and variation took place. Following the suggestions of Newcomb (1835-1909) during the "Conférence Internationale des Etoiles Fondamentales" held in Paris en 1896, a set of astronomical constants was adopted [1]. Astronomers were asked to use it in order to ensure an easy comparison of measurements made in different observatories. The adopted solar parallax was equal to 8".26. for the semi-diameter of the Sun, the value 959".63 was given, based on a study made by Answers (1838-1915) in Berlin [1]. In 1891, answers gave two values of Sun diameter in his conclusion. The first one is the above-adopted value, which is obtained from observations made with transit instruments, is 962".73. The difference of 3".10 in diameter (1".55 in the radius) between them was considered as due to a so-called irradiation. From that time, the ephemerides (Nautical Almanac, Connaissance des Temps, American Ephemeris), use the second value under the form of Sun semi-diameter 16'1".18 (961".18 ) for the eclipses and the value 15'59".63 (959".63), with the corresponding variations due to the changes in the distance between the Earth and the Sun, for the ephemerides [1]. Many authors, mainly since the beginning of the 19th century, have studied the problem of Sun diameter variations using a long series of Sun diameter date. Among them, we can mention Sofia et al, who found a decrease of 0".010 per year over the period 1925-1979 [2] and a decrease of 0".019 per years between 1700 and 1990 [3]. In contrast, other authors have found irregularities, for example Leone (1973)[43]and Dunham (1980)[44] gave values of Sun diameter variations between 1" and fractions of it. At Alern observatory in France, Laclare.F has been found relatively short-term variation by using Fourier analysis [1]. Such variations and discrepancies in the solar diameter are not a new phenomenon; they can be seen from modern analysis of observations in the past. An example is be found in a preliminary work by Smith and Massine [4] from which the following values for the horizontal diameter of the Sun were given [1].