Solar diameter with 2012 Venus transit

TL;DR

This paper discusses methods to accurately measure the solar diameter using Venus transits, addressing observational challenges like the black drop phenomenon and atmospheric effects, with potential for high-precision results from the 2012 transit.

Contribution

It introduces a technique to overcome observational distortions and improve solar diameter measurements during Venus transits, enhancing accuracy with ephemeris calibration.

Findings

Black drop phenomenon can be mitigated with circle fitting techniques.

Atmospheric refraction corrections improve measurement accuracy.

Chinese observatories are well-positioned for high-precision 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.