The geometrical information of the Sun

Wang Xiaofan, wxf@nao.cas.cn, National Astronomical Observatories, Chinese Academy of Sciences
Sigismondi Costantino, sigismondi.costantino@gmail.com, ICRA and Sapienza University of Rome

Abstract
It is still a controversial and interesting problem whether or not the solar diameter (radius) is a constant on a secular basis. The historical data set on solar radiation and solar radius is very useful to the study (reconstruction) of the solar influence on Earth's climate. The measurement of solar diameter based on full-disc images on board of satellites is compared with the ground based observations. However, the observation on satellites could not provide a long duration monitor of solar diameter with the same instrument in the same stable condition, due to the short satellite lifetimes. Hence, we compared the solar diameter measurement results from ground observations to the space observations and tried to use the space observations to "cross-calibrate" different ground observations results. In this way, a reliable network for long term solar diameter monitor could be built. The solar oblateness is a more challenging measurement than solar diameter, since the signal amplitude is on the level of milli-arcsecond, suitable almost only to satellite observations. It is not affected by the satellite orbital variation like the diameter observation is. But the results are strongly dependent on the state of instrument and on the calculation process.