The subject of this study is the pre-main sequence (PMS) evolution of stars less massive than ~1.2 solar mass, using lithium as the main tool for confronting model predictions with observations. In addition, it is proposed to use Li as a test of the possible substellar nature of brown dwarf candidates. The main results of this work are: 1.) The

By integrating the differential equation system of the single-body meteor theory which describes meteor flight in the atmosphere, we show that the population index r depends on the radiant zenithal distance zR. When zR is large, r diminishes, requiring correction in order to obtain reliable number density profiles. A new method is proposed to

A new semi-empirical method to derive the absolute flux calibration in the near infrared (J,K,L and L' bands) is proposed. The method consists of the application of the InfraRed Flux Method (IRFM) to a selected sample of stars with accurate direst measurements of their angular diameters. The absolute fluxes in each band are obtained minimizing the

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It is a typical lament of the astronomers of today that there has been a dearth of bright comets in the 20th century and, particularly in its latter half. Many go on to point out the large number of very bright comets that were observed in the 18th and 19th century to reinforce the unfavourable comparison with modern times. Analysis of bright naked