Why are the days of the week ordered the way they are?
Taking into account that each day of the week is dedicated to a wandering star easily visible, why is the order Moon – Mars – Mercury – Jupiter – Venus – Saturn – Sun if from the Sun the order should be: Sun – Mercury – Venus – Moon – Mars – Jupiter – Saturn and from the Earth: Moon – Sun – Mercury – Venus – Mars – Jupiter – Saturn?
Question sent by QUIM BOSCH (Barcelona). ENRIC MARCO answers:
According to the geocentric model, with the Earth as the centre of the Universe, since Ancient times the planets (or wandering stars) have been ordered according to the time it takes for them to travel around the Earth. The Moon goes around the Earth in less than a month, While for Saturn it takes over thirty years. The theory was that the longer it took for it to orbit the Earth, the further it would be. Thus, from the outside to the inside, a model of Universe was build up placing Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon over seven crystalline concentric spheres in decreasing order of their synodic periods, i.e., as seen from the Earth.
In Babylon they named the wandering stars after their main gods and it seems that they also used these names for the seven days of the week. Later on, the Greeks took this naming and used the names of their equivalent gods, and after that they were also transferred to Latin forms. Although Christianity, once turned into the official religion of the Roman Empire, tried to erase the reference to the pagan gods, it didn’t succeed. (Much later, some Romanic languages, like Portuguese, and Slavic languages, like Russian, changed their nomenclature in order to just enumerate the days of the week, for instance, segunda feira is “Monday” in Portuguese.)
We owe the current order to Dio Cassius, a Christian historian of the 3rd century. According to Cassius, astrologists ascribed the 24 hours of each day of the week to the seven wandering stars in a cyclical sequence. The first hour of the first day of the week was ascribed to Saturn and the following to Jupiter, Mars, the Sun, Venus, Mercury and the Moon respectively. Thus, the eighth hour of the first day was ascribed again to Saturn, and also the fifteenth and twenty second. Following this cycle for every hour and every day in the week, the first hours of the following days would be ascribed to the Sun, the Moon, Mars, Mercury, Jupiter and Venus, respectively.
Therefore, each day of the week received the name of the planet to which its first hour had been ascribed. This way, the sequence of the days was: Saturn, Sun, Moon, Mars, Mercury, Jupiter and Venus, which corresponds to our days Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday and Friday. And remember that for the Jews the week starts on Saturday, Shabbat, name that comes from the planet Saturn in Hebrew, Shabbetai, as can be seen in the Babylonian Talmud.
N.B.: This article refers to the names of the days of the week in Spanish and Catalan and other Latin languages, which slightly differ from the forms used in English.
Enric Marco Soler. Department of Astronomy and Astrophysics at the University of Valencia.