Solar diameter with 2012 Venus Transit

Abstract

The role of Venus and Mercury transits is crucial to know the past history of the solar diameter.Through the W parameter, the logarithmic derivative of the radius with respect to the luminosity, the past values of the solar luminosity can be recovered. The black drop phenomenon affects the evaluation of the instants of internal and external contacts between the planetary disk and the solar limb. With these observed instants compared with the ephemerides the value of the solar diameter is recovered. The black drop and seeing effects are overcome with two fitting circles, to Venus and to the Sun, drawn in the undistorted part of the image. The corrections of ephemerides due to the atmospheric refraction will also be taken into account. The forthcoming transit of Venus will allow an accuracy on the diameter of the Sun better than 0.01 arcsec, with good images of the ingress and of the egress taken each second. Chinese solar observatories are in the optimal conditions to obtain valuable data for the measurement of the solar diameter with the Venus transit of 5/6 June 2012 with an unprecedented accuracy, and with absolute calibration given by the ephemerides. Fruitful observations can be obtained also by amateur astronomers, by following the instructions in this paper. All ground-based observations designed to achieve this goal are warmly welcome to be analyzed by the author, presently visitinig the Huairou Solar Station of National Observatory of China for observing that transit. Finally a miminal observational schedule is suggested.

Transits of Venus 2004 and 2012 and solar diameter from ground: Method, results and perspectives for Mercury transit of 2016

“…The proportion between the duration of the transit and the diameter was used, according to the first order solution of the equation of the length of the measured chord. Nevertheless the velocity of Venus across the solar disk was not constant during the transit, and it was observed in Rome with the 120/1000 Konus refractor and a K12 eyepiece with a haircross during that transit by Sigismondi (2004). So the proportion ΔT /T = (T obs − T calc)/T calc = ΔR/R valid to the first order, has to be corrected because of the different angular velocities at the ingress and at the egress.…”

“…Besides the long debate about the sphericity of the Sun and the verifications of the theory of General Relativity and alternative theories, already presented in this meeting to this same parallel section (Sigismondi, et al 2015), there is a mere thermodynamical fact related to the compression and expansion of the outer shell of the Sun, eventually measurable as shrinking or enlargement of the solar radius. This argument has implications from classical astrophysics to climatic (and economic) history of the World, as the little ice age (1645-1715 Maunder minimum) and the global warming phases (ancient climatic optimums and the current).…”

“…Its ability to observe and the perfectioned optics of the telescopes, particularly clean, allowed this significant achievement. About the other transits, always in couples spaced each 108 years, 1769 (again on the astronomical unit and the black drop problem), 1874 and 1882 (after the advent of the photography) In a paper of Sigismondi (2012) there are some more details on these transits, while in this one we want to focus on diameters and irradiation effects. The last two transits in the digital era are the first to be exploited to measure the solar diamter overcoming the black drop effect, due to the interplay between limb darkening function of the Sun and the diffraction pattern of the telescope.…”

“…The visual observations of Sigismondi (2004) from Rome helped us to understand this phenomenon and the difference of velocity between ingress and egress, due to the vectorial composition of Venus and Earth's orbital velocities and Earth's rotation velocity, different from early morning (directed toward the Sun) to noon (maximum tangential value). The radial velocity of Venus at egress is 0.0508"/s.…”