Measuring the Solar Radius From Space During the 2012 Venus Transit

Abstract

We report in this work the determination of the solar radius from observations by the Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) instruments on board the Solar Dynamics Observatory during the 2012 June Venus transit of the Sun. Two different methods were utilized to determine the solar radius using images of Sun taken by the HMI instrument. The first technique fit the measured trajectory of Venus in front of the Sun for seven wavelengths across the Fe i absorption line at 6173 Å. The solar radius determined from this method varies with the measurement wavelength, reflecting the variation in the height of line formation. The second method measured the area of the Sun obscured by Venus to determine the transit duration from which the solar radius was derived. This analysis focused on measurements taken in the continuum wing of the line, and applied a correction for the instrumental point spread function (PSF) of the HMI images. Measurements taken in the continuum wing of the 6173 Å line, resulted in a derived solar radius at 1 AU of 959. 57 ± 0. 02 (695,946 ± 15 km). The AIA instrument observed the Venus transit at ultraviolet wavelengths. Using the solar disk obscuration technique, similar to that applied to the HMI images, analysis of the AIA data resulted in values of R = 963. 04 ± 0. 03 at 1600 Å and R = 961. 76 ± 0. 03 at 1700 Å.

Observations of the Annular Eclipse on 2012 May 21 by the General Public in Japan

“…However, as Fig. 4 (from Emilio et al 2012) shows, the measured radius of the Sun shows scatter by about ±500 km, which indicates that the radius of the Sun is not yet determined precisely.…”

On-Orbit Performance of the Helioseismic and Magnetic Imager Instrument onboard the Solar Dynamics Observatory

“…Using HMI data collected during the 2012 Venus transit, Emilio et al. ( 2015 ) derived a 1 AU solar radius in the continuum wing of the line of arcseconds, equivalent to . Similarly, Couvidat et al.…”

“…The height of formation near the 6173 Å Fe i spectral line changes with wavelength by a few hundred kilometers (Fleck, Couvidat, and Straus, 2011 ; Emilio et al. , 2015 ). Because the standard HMI observing sequence samples the solar Fe i line at six wavelengths separated by about Å, the apparent size of the Sun varies with wavelength by as much as half a pixel.…”

O antigo adapta-se ao moderno: verificação do valor da Unidade Astronômica a partir do trânsito de Vênus reproduzido com o software Stellarium

“…Não devemos esquecer também do seu valor científico: recentemente os trânsitos de Mercúrio de 2003 e 2006 foram utilizados para a medição do raio do Sol com excelente exatidão (a incerteza foi de apenas 65km) [11] e posteriormente este resultado foi ainda melhorado pelo mesmo grupo com a utilização do trânsito de Vênus de 2012 (neste segundo estudo, a incerteza foi de 15km) [12]. Na próxima Seção faremos uma adaptação do método de Halley e Deslile que pode ser facilmente reproduzida em sala de aula seja no ensino médio ou superior, em cursos de introduçãoà Astronomia.…”