A lunar eclipse is actually the Earth transiting the Sun when viewed from the moon. Thus we can obtained the Earth’s transmission spectrum by observing lunar eclipses. During my PhD study, I observed three lunar eclipses with different instruments and configurations. These observations yield three interesting results:
(1) The high resolution transmission spectrum of the Earth’s atmosphere
Using the 2.16m telescope (Xinglong, China), we obtained the high resolution transmission spectrum of the Earth’s atmosphere. A line-by-line transmission spectral model was built using the HITRAN database to fit the observed spectrum in order to quantitatively analyze the atmospheric species. This research provides important hints for future exo-Earth characterization.
(2) The centre-to-limb variation of solar lines during lunar eclipses
We observed another lunar eclipse with CFHT (Canada France Hawaii Telescope) and obtained a sequence of Earth’s transmission spectra. There are obvious Na and Ca features imprinted in the obtained transmission spectrum and we proved that these features are due to the line profile variations along the solar disk centre-to-limb instead of actual absorption in the Earth’s atmosphere. The effect is called the centre-to- limb variation (CLV). We demonstrated that this CLV effect has lead to erroneous claims of the Na and Ca detection in previous lunar eclipse studies.
Publication: Yan et al. 2015b, A&A, 574, A94.
(3) The chromatic Rossiter-McLaughlin (RM) effect as a method to detect transmission spectrum
During the lunar eclipse, the Earth transits different parts of the rotating Sun as seen from the Moon. Thus the observed radial velocity (RV) is expected to change during transit. We have used the HARPS/ESO instrument to measure the precise RV of the partially illuminated Moon during a lunar eclipse. The RV curve is then obtained at a high signal-to-noise ratio. By measuring the amplitudes of the RV curve at different wavelengths, we are able to obtain the transmission spectrum of the Earth’s atmosphere. The Rayleigh scattering and the ozone are clearly detected. This demonstrates that this chromatic RM effect is a powerful method of detecting transmission spectrum. The method has a great advantage on large groud-based telescopes because it attenuates the telluric atmosphere noise by measuring precise RV of the stellar lines.
Publications: Yan et al. 2015c, ApJL, 806, L23