Resumen:

Reports from the 2nd and 3rd Century AD attribute the first measurement of the angular size of the Sun to Thales of Miletus, in the 6th Century BC. Cleomedes, also in the 2nd Century AD, described a method to perform the measurement, based on timing the duration of the sunrise. Several modern authors have suggested Thales used Cleomedes' method, but others are skeptical of the connection. Here I present an objection that has not been discussed in the literature, namely, that the proportionality between the size of the Sun and the duration of sunrise is not constant, but changes with latitude and time of the year, due to what I call the ``asymmetric sunrise effect''. I show that this effect is large enough to have prevented Thales from obtaining the roughly accurate recorded value.

Resumen:

Knowledge of fundamental parameters such as age, metallicity, distance, among others, of stellar clusters is relevant when it comes to understanding the structure and evolution of galaxies. This is due to that clusters are excellent tracers of the different subsystems that make up galaxies (bulb, disk, bar, halo). One of the most powerful tools to study in depth the astrophysical characteristics of star clusters is the color-magnitude diagram. In the specific case of stellar clusters, their stars members are distributed in said diagram along sequences that depend on the evolutionary state of the cluster (age), its metallicity and its distance. In order to carry out a precise astrophysical analysis, it is essential to characterize and reliably identify the stars that are typical of clusters. In this work we present a method of decontamination of color-magnitude diagrams of star clusters, developed in Python, which allows to identify the own stars of the cluster with a statistical assignment of membership for each one of them.