Solar diameter with 2012 Venus transit: history and opportunities

TL;DR

This paper discusses methods to accurately measure the solar diameter using Venus transits, addressing observational challenges and highlighting the potential of the 2012 transit for high-precision measurements with professional and amateur efforts.

Contribution

It introduces a novel approach to correct for black drop and seeing effects, enabling unprecedented accuracy in solar diameter measurements during Venus transits.

Findings

Potential accuracy better than 0.01 arcsec with high-quality images

Effective correction techniques for black drop and atmospheric refraction

Feasibility of amateur observations for scientific data collection

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 Observatories of China for observing that transit.Finally a miminal observational schedule is suggested.