The solar eclipse of 2017 August 21 was observed with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) on the Large Binocular Telescope (LBT), which is located at Mt. Graham International Observatory (MGIO), Arizona, USA. At this location, a partial eclipse was observed with maximum obscuration of about 61.6%. The 11-millimeter-aperture, binocular Solar Disk-Integrated (SDI) telescope, located on the kitchen balcony of the LBT building, feeds sunlight to PEPSI, which has recorded a total of 116 Sun-as-a-star spectra in the wavelength range of 5300 – 6300 Å, with a spectral resolution ${\mathcal{R}} \approx 250{,}000$ and signal-to-noise ratio of about 733:1. The temporal evolution of the Fraunhofer Na i D doublet at $\lambda $ 5890/5896 Å is analyzed using contrast profiles that illustrate subtle changes in the spectral line, not obvious in the intensity profiles. Line bisectors are used to study the height-dependent signature of convective motions. Sun-as-a-star spectra illustrate the radial atmospheric stratification and are affected by limb darkening, solar differential rotation, convective motions, and magnetic activity. During a partial solar eclipse, the contribution of these features is modified by the passage of the Moon, resulting in a transit spectral signature. These observations are compared with synthetic Na i D spectra generated by the Spectropolarimetic NLTE Analytically Powered Inversion (SNAPI) code, based on state-of-the-art Bifrost atmospheric parameters, applied to a geometrically accurate model of the solar eclipse. The model is in qualitative agreement with the observations. However, the discrepancies indicate that models need to be improved, where high-resolution eclipse spectroscopy can serve as a benchmark.
