The Great Planet Race

During July and August 2010, you can watch a great planet race as Venus and Mars close in on and then pass Saturn. Observing this will also give you a sense of how the ancients, even thousands of years ago, could distinguish the planets from the stars and from each other.

Thousands of years ago, the earliest astronomers noticed that they could form patterns with the stars.  They also noticed that these patterns remained the same throughout their lives and even across generations.

In contrast to these “fixed stars,” there were seven “wandering stars.”  Consistent observation revealed that five points of light in the night sky shifted position noticeably from night to night.  After a year, even the slowest of these was clearly “out of place.”   The other two “wanderers” were the sun and the moon.  From the Greek word for “wanderer,” today we call these moving objects planets.  (The sun and moon were thus “planets” until we understood the solar system better.)

Cellarius ptolemaic system

With more careful observation, we can clearly distinguish the planets from one another using only the naked eye.  One of the planets far outshines all the others, and in fact outshines everything in the sky except the sun and moon.  Ancients named this one after the goddess of love and beauty–Aphrodite for the Greeks and Venus for the Romans.  Another planet has a distinctly reddish tint compared to all of the others, whose light is closer to pure white.  This one has therefore borne the names of gods of war, such as the Greek Ares and Roman Mars.

“Jupiter of Smyrna” currently residing
in the Louvre in Paris, France.

An important way to distinguish among the planets was to observe them when several were close together and to note which ones moved faster compared to others.  The fastest moving planet, the one that always passed up the others (unless it appears to stop and head the other way), received the name of the swiftest god.  For the Greeks, this was the messenger god Hermes, for the Romans it was Mercury.  On the other hand, there were two planets so slow that the motion was barely noticeable from night to night, but could be detected over months or years.  These two were considered rulers of heaven since they were the farthest away.  After all, ancients noticed that faraway objects, such as ships sailing at the limit of their vision, seemed to be going slower than similar objects close by.    The very slowest planet is also the dimmest; any other planet at its brightest outshines it.  The second slowest, on the other hand, outshines all stars at night and all planets except Venus.  Thus, the Greeks identified the slowest planet with the former, deposed ruler of heaven (Kronos/Saturn).  The planet which is brilliant despite its great slowness and distance was the true ruler of heaven (Zeus/Jupiter).

Now, you can go outside and make these same types of observations. In July 2010, face west at dusk to find three planets.  Venus is by far the brightest, outshining all the planets and stars.  Mars and Saturn are to the left and  slightly higher in the sky.  Although they aren’t nearly as bright as Venus, Mars and Saturn easily outshine all stars in their immediate vicinity and are therefore just as noticeable.  Mars is between Venus and Saturn and slightly below a line joining those two.

During the rest of July, Mars will close the gap on Saturn, until by July 31 it appears less then 2 degrees under the ringed planet.  Meanwhile, Venus will have closed to less than 8 degrees to the right of the pair.  Keep watching in August as Mars pulls ahead of Saturn while Venus begins to form a compact triangle with them both.  Venus is less than 3 degrees below Saturn on August 8 as it continues to gain on Mars.  Finally, Venus catches up to Mars and is less than three degrees below it on August 19.  By the end of August, the three planets have reversed their order; Venus has become the leftmost of the three, while Saturn will be on the right.

As you keep observing, you will also notice that Venus begins to slow down a little once it has “won” the race.  Also, it begins to move off of the imaginary line joining Mars and Saturn.  This is due to the geometry of our solar system.  Mars and Saturn are traveling slower than earth on their outer orbits.  Thus, Earth is leaving them behind and will pass on the far side of the Sun from them.  That’s why we see Saturn (in September) and Mars (in December) drop into the Sun glare.

Venus, on the other hand, is orbiting inside Earth’s orbit and is therefore going faster than Earth.  We’re seeing Venus come out from behind the Sun, and then around to our side of the Sun.  That’s why Venus seems to slow down–it’s starting to move towards the Earth.  Once Venus is on our side of the Sun, we see it move backwards (or retrograde) against the background stars.  In October, when Venus “laps” the Earth, we can’t see it at all.

Viewers with perfectly clear horizons who observe right at dusk may also glimpse Mercury.  The elusive Messenger is to the lower right of the other three planets and roughly in line with them.  Mercury won’t catch up to the other three however.  By August, Mercury will have come around to our side of the Sun, so we’ll see it head back towards the Sun’s glare before it aligns with Saturn, Mars, or Venus.

The King of Planets, Jupiter, sits this one out.  he makes a grand entry into the eastern sky at dusk this September, is up all night long on September 21, and dominates the evening sky throughout the fall.

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.

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.