We measure time based on motions in space. The Earth rotates on its axis once a day. The Moon orbits the Earth about once a month. The Earth orbits the Sun once a year. That leaves the week as the only aspect of our calendar not directly tied to the Earth, Moon, or Sun. The week, as it turns out, is based on the other planets of our solar system–at least, those easily visible to the naked eye.
Early astronomers were able to distinguish planets from stars because planets seem to move against the starry background. The stars are always rising, moving across the sky, and setting due to Earth’s rotation. They seem to form the same patterns all the time; we never see them move relative to each other. (In fact the stars do have proper motion, but we don’t notice it over a time frame as short as a human life or even over several generations). Anything shifting noticeably over several days was a ‘wandering star’, or planet. Early astronomers identified seven ‘wanderers': the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn, and the Greeks placed them in just that order.
This order, of course, is wrong; it makes the basic error of putting the Sun in orbit around the Earth when in fact the Earth orbits the Sun. Fixing this error by replacing the Sun with the Earth, however, makes the order from Mercury to Saturn correct. That’s because the order is based on something directly observable–the planets seem to move among the background stars at different rates. Ancient observers saw the Moon reappear near the same set of stars once a month. Saturn, on the other hand, takes 29.5 years to reappear in the same part of the sky.
The different speeds are even more apparent when two or more planets are near one another in the sky (an alignment called conjunction). Any planet in conjunction with Saturn catches up to Saturn and then passes it. It’s never the other way around. Any planet (other than Saturn) in conjunction with Jupiter catches and passes Jupiter, never the other way around. For early astronomers, slowness was associated with distance. By carefully observing the planets’ motions and planetary conjunctions, early observers could place them in order.
Ancient Roman writer Dio Cassius was among the first to explain how the order of the planets from slowest to fastest (and thus from outside in) generated the week. The system involves the 24-hour day and an astrological belief that each hour was ‘ruled’ by a planet following the order above, such that Saturn’s hour was followed by Jupiter’s, then Mars’, then the Sun’s, and so on. Further, whichever planet governed the first hour of each day governed that whole day. On Saturn’s day, then, the hours were as follows:
1) Saturn 2) Jupiter 3) Mars 4) Sun 5) Venus 6) Mercury 7) Moon 8. Saturn 9) Jupiter 10) Mars 11) Sun 12) Venus 13) Mercury 14) Moon 15) Saturn 16) Jupiter 17) Mars 18) Sun 19) Venus 20) Mercury 21) Moon 22) Saturn 23) Jupiter 24) Mars 25) Sun
Since there are 24 hours in a day, the 25th hour of Saturn’s day is the first hour of the next day. Therefore, Saturn-day is followed by Sun-day. Redo the list of hours, this time starting with the Sun, such that hours 1, 8, 15, and 22 are the Sun’s. Hour 25 becomes the Moon’s hour, which means the Sun-day is followed by Moon-day. Repeat the list with the Moon in first position, and eventually the following order of days emerges:
1) Saturn-day 2) Sun-day 3) Moon-day 4) Mars-day 5) Mercury-day 6) Jupiter day 7) Venus-day
If Venus governs the first hour, Saturn governs the 25th, and the cycle begins again. A full table of the hours and days is here (this list also has the name of the days in 30 different languages).
You probably recognize Saturday, Sunday, and Monday in this list. To get the other English day names from this list, we have to translate by replacing the planet names, which are names of Roman deities, with roughly equivalent Germanic deities. Languages derived directly from Latin have preserved the Roman gods’ (thus the planets’) names more faithfully. For example, you can recognize Latin luna (the Moon) in French lundi, Spanish lunes, and Italian lunedì.
Similarly, Mars-day is martes in Spanish, mardi in french, and martedì in Italian. Germanic tribes, however, replaced the Roman war god Mars with their own warlike god Tiw (or Tyr for the Norse). Thus, Mars’ day became Tiw’s day or Tuesday.
‘Mercury-day’ is recognizable in French mercredi, Spanish miércoles, and Italian mercoledì. The Germanic pantheon had no messenger god that corresponded well to the Roman Mercury, so they equated him with Woden (Norse Odin). Both Woden and Mercury were gods who escorted the recently deceased to the underworld. Also, Woden became the fastest god when he rode his eight-legged horse Sleipnir.
Jupiter’s original name in Latin was Jovis (‘Jove’ to English writers); the name Jupiter is a contraction of Jovis pater (‘father Jove’). ‘Jove-day’ is recognizable in French jeudi, Spanish jueves, and Italian giovedì. Although Jupiter, like the Greek Zeus, was the king of all the gods, his actual domain was the weather. In particular, he was the god who caused storms and struck people with lightning. Thus Germanic tribes assigned his day to Thor, their god of thunder. Thor’s day is Thursday.
‘Venus-day’ is still recognizeable in French vendredi, Spanish viernes, and Italian venerdì. Germanic tribes replaced Venus’s name with that of Frigg, the wife of Woden who was associated with married women and whom they called upon to help in giving birth. Frigg-day is Friday.
As the Germanic tribes had no one in their pantheon who even roughly corresponded to Saturn, Saturn’s name remains in Saturday. Ironically, the Latin-based languages have lost ‘Saturn-day’ as the day’s name. Spanish sábado and Italian sabato derive from the word ‘sabbath’ (as does French samedi, through a more complex etymology). This is due to the influence of the Catholic Church, which was loath to name the days of the week after pagan gods, and sought to replace the planetary names.
The Church designated Sunday ‘Lord’s Day’ (dies dominicus), called Saturday the sabbath (sabbatum), and numbered the weekdays from 2 to 6. Except in Portugal, however, the numbered weekdays never replaced the planetary days in popular usage. Everyday people in southern Europe did adopt the Church’s terms for the weekend days. Northern Europe, largely outside the influence of the Catholic Church, was less affected by this; we retain ‘Saturday’ and ‘Sunday’ in English as a result.
In November and December 2008, you can make for yourself some of the observations that helped astronomers of antiquity imagine the solar system. The two brightest points of light in the southwest tonight are Venus and Jupiter. They outshine all stars we ever see at night and are visible even in twilight. But don’t wait too late; you’ll need to look in the hours right after sundown before the two planets set. Venus, lower to the horizon, is the brighter of the two. Its closeness to us and the clouds that cover the whole surface and reflect most sunlight back into space cause Venus to outshine the much larger Jupiter.
Watch as Venus gets closer and closer to Jupiter each night this month. This is exactly how ancient astronomers could tell that Venus and Jupiter were not stars. On November 30 and December 1, watch as Venus passes 2 degrees ‘under’ Jupiter. (The crescent Moon also passes by on these nights). Imagine ancient Greek astronomers concluding that Venus is closer because it is faster. Keep watching each night in December as Venus pulls away from Jupiter, getting higher in the dusk sky while Jupiter sinks into the Sun’s glare by early January. Early astronomers would have seen this as the Sun catching up to Jupiter while Venus pulls away; observations like this account for the Sun’s position in the ancient order of ‘planets’. Of course, we now know better–the Sun’s apparent motion is really ours. Earth is going around the far side of the Sun from Jupiter’s position, putting Jupiter behind the Sun as the New Year opens.
Venus remains an evening star until March 2009. Compare Venus to the stars around it, and you’ll see it slow down and then move ‘backwards’ towards the Sun’s position each night in March. That’s because Venus will have come around to our side of the Sun, and will be passing us up on its faster orbit.
Should you make any of these observations on a Thursday or Friday, you can reflect on why those days have those names.