Dynamic spectra of the pre-flare-subtracted H-alpha spectra of the Sun-as-a-star prominence eruptions. (a) The prominence eruption on 2017 June 19 (event “S1”). (b) The prominence eruption on 2021 May 5 (event “S2”). The data of the dynamic spectra were taken from Otsu et al. (2022), and the spectral fitting is newly done in this paper. In both events, the emissions from flare ribbons are behind the limb and not visible from the Earth, so the emission almost comes from the erupted prominence. Some of the white-colored region is the unavailable wavelength at that time. The cyan points are the central wavelength of the emission component. For the event S1, the velocity evolution is fitted by a linear function, and the deceleration is derived as 0.1089 ± 0.0002 km s⁻².