  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.
