Huairou Solar Observing Station of the National Astronomical Observatories of Chinese Academy of Sciences has been in operation since 1987. During its operation, successful observations of the solar vector magnetic field have been conducted. On the basis of the achievements at Huairou, we analyze the methods of observing the solar magnetic field, including discussions of the approximation of the transfer theory of the solar magnetic field in the atmosphere, wide field of view polarized observation, and some questions on the inversion of solar magnetic field data. We also present relevant challenges for further research.

The transits of Venus occur in couples every 105/122 years: the observed ones were in 1639; 1761–1769; 1874–1882; and 2004–2012. The next couple will occur in the years 2117 and 2125. We need all four contacts to determine the solar diameter accurately. The black-drop phenomenon blurs internal contacts, so we developed a parabolic analysis of the chords drawn by the disk of Venus on the solar limb. The extrapolation of the zeroes gives the contact timings. We tested this method with some high-quality images obtained in 2004 and 2012, and we applied it to the observations of 2012 in a visual band (Huairou Solar Observing Station, hazy weather) and H-alpha (Shen Zen Astronomical Observatory). To exclude a reduction in the measured diameter by the haze, we made two series of measures at the Clementine Gnomon (Rome) and at the PHYSIS telescope (Rome), under various sky transparencies and with diffraction-limited instruments. The haze and the low altitudes above the horizon reduced accuracy at all first contacts examined, without changing the solar diameter. Our measures obtained in China during the transit of 2012 yielded a photospheric radius R_⊙P = 959.33″ ± 0.06″, based on 76 + 75 diffraction-limited images; this is compatible with the chromospheric radius measured at the base of the spiculae, which is R_⊙C = 959.78″ ± 0.11″, relying on 7 + 5 diffraction-limited series of images.

The transits of Venus occur in couples each 105/122 years: the observed ones are 1639; 1761-1769; 1874-1882, and 2004-2012. The next couple will be in 2117-2125. We would need all four contacts to determine the solar diameter accurately. The black-drop phenomenon blurred always the internal contacts, so we developed a parabolic analysis of the chords drawn by the disk of Venus on the solar limb. The extrapolation to zero gives the contact timings. We tested this method with some high quality images obtained in 2004 and 2012, and we applied it to the observations of 2012 in visual band (Huairou Solar Observing Station, hazy weather) and H-alpha (Shen Zen Astronomical Observatory). To exclude a reduction of the measured diameter by the haze, we made two series of measures at the Clementine Gnomon (Rome) and at PHYSIS telescope (Rome), under various sky transparencies and with diffraction limited instruments. The haze, and the low altitudes above the horizon reduced the accuracy at all first contacts examined, without changing the solar diameter. Our measures obtained in China during the transit of 2012 yielded a photospheric radius R_¤P=959.33”±0.06”, based on 76+75 images diffraction limited; compatible with the chromospheric radius measured at the base of the spiculae was R_¤C=959.78”±0.11”, relying on 7+5 diffraction-limited series of images.