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The opportunity of the 2016 transit of Mercury for measuring the solar diameter and recommendations for the observation

The transit of Mercury occurred two times in this century: 2003, May 7 and 2006, November 8. In 2016 there is another opportunity to observe this phenomenon and measure the solar diameter with the method of comparing the ephemerides with the observations. This method has been presented by I. I. Shapiro in 1980, the data of the observed transits (since 1631) have been re-analyzed by Sveshnikov (2002) and an improvement on the observed data, to avoid the confusion given by the black-drop effect, proposed by C. Sigismondi and collaborators since 2005 exploits the measurement of the chord drawn by the solar limb with the disk of the transiting planet and its extrapolation to zero through the analytic chord fitting the observations before the black drop, in the ingress/egress phases. A network of European observers (IOTA/ES) and observatories (coronograph of Bialkow, PL 56 cm; IRSOL, Locarno CH - 45 cm Gregorian telescope) is active for the 2016 transit. Recommendations to improve the observation of the ingress/egress phases and examples of the data analysis on the 1999 Mercury transit observed by TRACE are also done: the planet should be centered in the field of view of the telescope to avoid optical distortions, and the observations should be prolonged well after the black drop phase, estimated in about 40 seconds and more after the internal contact t2 in the ingress and before the internal contact t3 in egress.