If one makes certain of the time of sunrise and sunset through observations of the sky and applying astronomical theories, they could act accordingly. If trustworthy astronomers notify us with certainty about the beginning and the end of the month of Ramadan in regions where the weather is often cloudy and the moon is not visible, it is sufficient. Performing prayers in leather clothing purchased from the bazaar of Muslims is permissible even though one doubts whether or not the leather belongs to an animal which has been Islamically slaughtered. However, if one is certain that the leather has been imported from non-Muslim countries and the seller is an irresponsible person who has not investigated about the leather then it is not permissible to perform prayer with it.