What the Heck is a Tues, Wed, Thurs, or Fri?

Earth Mars and Moon to scale
Creative Commons License photo credit: Bluedharma

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.

Apollo Belvedere
Creative Commons License photo credit: Alun Salt

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.

UN_Zeus
Creative Commons License photo credit: Pro-Zak

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.

Happy New Year!

The New Moon of Monday, September 29, is an important one to many people of the world. In the Hebrew Calendar, it marks Rosh Hashanah (literally, ‘head of the year’) which is the beginning of year 5769. On Tuesday night, September 30, Muslims across the world will see the first slender crescent of this lunar cycle. That will mark the end of Ramadan and the beginning of the next month, Shawwal. 1 Shawwal is ‘Eid ul Fitr, one of the greatest holidays in the Islamic calendar. This week, then, is a good time to think about the Moon, why it’s here, how it orbits, and how we have used it to measure time.

Moon and stars
Creative Commons License photo credit: joiseyshowaa

Unlike our months, Hebrew and Islamic months begin with the New Moon. Because twelve lunar months add to only 354 days, less than the 365.25 day solar year, an extra month is occasionally needed to keep the months roughly aligned with the seasons. In a 19 year cycle, years 3, 6, 8, 11, 14, 17, and 19 have the extra month. The year that is ending, 5678, is number 11 in its cycle and was a leap year.

Interestingly, the Jewish year has two ‘beginnings’. Tishrei (the month which begins now) is the first month of the civil calendar, and the month where 5678 becomes 5679. However, it is actually the seventh month of the religious calendar, which begins at Nisan (the month of Passover).

The Islamic calendar functions slightly differently. Its months begin with the first visible crescent low in the west at dusk, which is not with the actual New Moon. Keep in mind that at New Moon, the Moon is in line with the Earth and Sun, and the entire near side of the Moon has nighttime (and is therefore dark). The New Moon is visible, therefore, only if it blocks the Sun during an eclipse.

Since this New Moon occurs early Monday morning, the 29th, we expect it to be visible by Tuesday evening, the 30th. Observant Muslims, then, will continue to fast in daylight hours Monday and Tuesday. Upon seeing the Moon Tuesday night, they will know that Ramadan has become Shawwal, and they may break their fast on Wednesday.

Due to early controversy as to which years would have it, Muhammad outlawed the 13th month that kept Islamic months tied to the seasons. As a result, Ramadan (and each other month in that calendar) begins 11 days earlier each year according to our Gregorian calendar.

The Moon is Earth’s only natural satellite, orbiting our planet once every 27.3 days. However, a cycle of moon phases (say, from New Moon to the next New Moon), takes 29.54 days. This is because the Earth itself is moving during each 27.3 day Moon orbit. Since it is much easier to observe the Moon’s changing phase cycle than to observe the Sun directly, the 29.54 day phase cycle was the basis of many ancient calendars. Words for ‘moon’ and ‘month’ are related in English and are identical in many other languages. There is some evidence that our word ‘moon’ is ultimately related to an Indo-European word for ‘measure.’ Given how long we’ve measured time by the Moon, it is easy to take its presence for granted.

Released to Public: Jupiter Montage (NASA)
Creative Commons License photo credit: pingnews.com

However, our Moon is quite remarkable in several ways. Moons in our solar system are generally much smaller than the planet they orbit. Jupiter and Saturn, for example, are about 25 times bigger across than their biggest moons. Earth, though, is only 3.67 times the diameter of our Moon. Also, moons usually orbit in the same plane as their planet’s equator. Our Moon, though, orbits within about 5 degrees of Earth’s orbital plane, called the ecliptic, which is not the plane of the equator since Earth is tilted 23.5 degrees on its axis.

This leads most astronomers to believe that the Moon did not form with the Earth, but is the result of a collision with with an object roughly the size of Mars. According to this theory, the impactor (sometimes called ‘Theia’) struck a glancing blow on the Earth and was completely destroyed, and the Moon formed from the debris of Theia’s and Earth’s mantles.

This impact is what left Earth with a Moon much larger than what a planet Earth’s size would normally have, and left that Moon near Earth’s orbital plane (where the impact occured). Our relatively big moon has crucial effects not only on our tides, but also on the stability of Earth’s tilt.

Earth’s orbital tilt of about 23 and a half degrees as it goes around the Sun causes the seasons. The axis precesses, describing an aparent circle roughly every 26,000 years, but the amount of tilt (obliquity) stays nearly the same. Because the Moon acts a counterweight, the obliquity varies only between 22.1 degrees and 24.5 degrees over about 41,00 years (we are now at 23.44 degrees and decreasing). Even this orderly variation, called the Milankovitch cycle, is enough to influence our Ice Ages. Imagine the impact on Earth’s climate if there were no Moon, and the obliquity varied chaotically. This is exactly what happens at Mars, where the tiny moons Phobos and Deimos are not massive enough to influence Mars’ tilt.

Public Domain: Apollo 8 Looks at the Moon (NARA/NASA)
Creative Commons License photo credit: pingnews.com

One thing our Moon does have in common with most others is that it orbits the Earth and rotates on its own axis at the same rate. This is called ‘synchronous rotation’ and it occurs because the Moon is not exactly uniform in composition. From the time the Moon formed, the slightly heavier side was attracted to the Earth. Over time, this effect de-spun the Moon until it attained synchronous rotation. The Moon’s gravitational attraction also de-spins the Earth, although much more slowly as the Moon is less massive. As it does so, the Moon moves slightly farther from the Earth (just over 3 cm per year). The Moon is now 1.5 meters farther away that it was when Apollo astronauts went there. Don’t worry, though, by the time the Moon is far enough away to escape, the Sun will have become a red giant and swallowed both Earth and Moon anyway.

What is the shape of the Moon’s trajectory around the Sun? Perhaps not what you’d expect.

So, I encourage every one to watch for the reappearance of the Moon in the evening sky this week, even if you aren’t celebrating a New Year or an ‘Eid. The Earth’s companion gives all of us something to appreciate.