# How Did the Ancients Know These Five Were Planets? **URL:** \ **Category:** Factual Questions\ **Created:** [October 7, 2004, 1:17am UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956 "2004-10-07T01:17:25Z")\ **Posts on this page:** 13\ **Page:** 2
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [October 7, 2004, 1:46pm UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/21 "2004-10-07T13:46:58Z")
> [@](#): > > Hijack alert! > > Isaac Asimov once postulated that the number 7 was/is sacred in so many cultures because that was the number of planets\* known to the ancients. I find the idea interesting, but cannot vouch for its accuracy. Is there anyone here who can? > > - The list of planets being different in antiquity because of the exclusion of the Earth and inclusion of the Sun and Moon Exactly why seven is seen as a mystical number isn’t extremely obvious. Odd numbers definitely seem to have mystic significance (Three does – Trinities of Gods, thee notices for danger, etc. Five does as well – pentagrams, etc. Seven certainly does.) I suspect the fact that you couldn’t divide these num,bers by two made them odd and suspect back at the bare beginnings of math. My own p[ersonal suspicion is that seven gets a lot of significance because there are seven days in the week. By why should we have a week of seven days? Because the period of the moon is 29 days and change, and 28 is close to that, and is easily divisible by two and four. Divide by two and you getr fourteen day periods (a fortnight). Divide by four and you get seven – the week. (I suppose you could round up instead and get three ten day “weeks” in a month, but I never heard of anyone trying that experiment.) Four also fits in well with four easily distinguished phases of the moon – full, new, and the two quarters. All other divisions aren’t so easily defined (you get “gibbous” for a few days, but full only for one). The seven days of the week fit in well with the seven classical “planets”, and their names reflect this – Sun-Day, Moon-Day, Saturn-Day. For some reason they went with Northern gods for the other days, but I believe they have the same astronomical significance (Wodens-Day has Woden = Odin = Jupiter, king of the gods, etc.) ---
**Author:** ![Neurotik](https://avatars.discourse-cdn.com/v4/letter/n/91b2a8/32.png) [@Neurotik](https://boards.straightdope.com/u/Neurotik)\ **Post date:** [October 7, 2004, 1:55pm UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/22 "2004-10-07T13:55:44Z")
I was assumed it was because 3, 5 & 7 would probably have been the first prime numbers ancient people would have known about. ---
**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\ **Post date:** [October 7, 2004, 9:15pm UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/23 "2004-10-07T21:15:40Z")
> [@Colophon](#): > > > [@ultrafilter](#): > > > > It’s important to note that the ancients probably had no idea that planets were unlike stars. > > This is an important point. Just because the ancient Greeks called them “planets” doesn’t mean they knew that they were giant round lumps of rock or gas. So, although they knew the planets were different from the stars in that they moved around, they still had no idea what they actually were. And just to nail the point, the Greeks called them “_asteres_ planetes”, literally “wandering _stars_.” ---
**Author:** ![Odinoneeye](https://avatars.discourse-cdn.com/v4/letter/o/97f17d/32.png) [@Odinoneeye](https://boards.straightdope.com/u/Odinoneeye)\ **Post date:** [October 7, 2004, 11:04pm UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/24 "2004-10-07T23:04:43Z")
> [@dtilque](#): > > One thing to note is that record keeping was essential to knowing how many planets there were. Illiterate cultures knew there were wanderers, but didn’t know, for example, that the morning star was the same object as the evening star (unless they learned this from a neighboring literate society). Only by recording Venus’ positions over a long period of time can it be established that they were the same object. > > The same would be true of the other planets. They could follow an outer planet from night to night, but eventually it would not rise during the night and they would lose track of it. I once read that the morning star aspect of Mercury was called Apollo before they figured out it was the same planet. Anyone else hear of this? ---
**Author:** ![t-keela](https://avatars.discourse-cdn.com/v4/letter/t/a3d4f5/32.png) [@t-keela](https://boards.straightdope.com/u/t-keela)\ **Post date:** [October 8, 2004, 1:08am UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/25 "2004-10-08T01:08:34Z")
> [@Odinoneeye](#): > > I once read that the morning star aspect of Mercury was called Apollo before they figured out it was the same planet. Anyone else hear of this? You may be correct as I’m recalling this from memory but IIRC Apollo was originally the Greek God who drove a chariot pulling the sun across the heavens. He was adopted by the Romans later. Mercury on the other hand was the God of Commerce and considered a thief. His speed enabled him to “get away”. IIRC Hermes was his Greek counterpart and he was the brother of Apollo. Not the same God. then again…I’m all ears if you can show me different. 🙂 ---
**Author:** ![stockton](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@stockton](https://boards.straightdope.com/u/stockton)\ **Post date:** [October 8, 2004, 1:54am UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/26 "2004-10-08T01:54:43Z")
> [@t-keela](#): > > …IIRC Apollo was originally the Greek God who drove a chariot pulling the sun across the heavens. He was adopted by the Romans later. > Mercury on the other hand was the God of Commerce and considered a thief. His speed enabled him to “get away”. IIRC Hermes was his Greek counterpart and he was the brother of Apollo. Not the same God. You are correct, **t-k**. Hermes (offspring of Maia and Zeus) stole a herd of cattle from Apollo on the day he (H) was born. Apollo was majorly pissed so Hermes gave Apollo a lyre and taught him how to play it (when he was 2 days old). Apollo was very pleased and allowed Hermes to keep the cattle. Hermes=Mercury. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [October 8, 2004, 2:13am UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/27 "2004-10-08T02:13:35Z")
The driver of the sun chariot was Helios. Apollo appears to have assumed his attributes in later myths, and the two became conflated: > **[Encyclopedia Mythica](https://pantheon.org)** > > Encyclopedia Mythica is the premier encyclopedia on mythology, folklore, and religion. Instant mythology since 1995. > **[Global Industry Market Sizing - NationMaster](https://www.nationmaster.com/)** > > Looking for country statistics? NationMaster gives access to market sizing and trends across 300 industry verticals and a global coverage. > [@](#): > > Helios is the young Greek god of the sun. He is the son of Hyperion and Theia. By the Oceanid Perse, he became the father of Aeëtes, Circe, and Pasiphae. His other two daughters are Phaethusa (“radiant”) and Lampetia (“shining”). He had a son, named Phaeton, whom he once allowed to guide his chariot across the sky. The unskilled youth could not control the horses and fell towards his death. Apollo was the son of Leto and Zeus. ---
**Author:** ![Larry\_Mudd](https://avatars.discourse-cdn.com/v4/letter/l/f14d63/32.png) [@Larry\_Mudd](https://boards.straightdope.com/u/Larry_Mudd)\ **Post date:** [October 8, 2004, 3:37am UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/28 "2004-10-08T03:37:11Z")
> [@hammerbach](#): > > Isaac Asimov once postulated that the number 7 was/is sacred in so many cultures because that was the number of planets\* known to the ancients. I find the idea interesting, but cannot vouch for its accuracy. Is there anyone here who can? Just to hijack a little further– [Lorne Greene] There are those who believe… [/Lorne Greene] …that there may have been some occult knowledge of the Nine Planets. There’s a Hermetic text (generally called _The [Discourse on the Eight and Ninth](http://www.gnosis.org/naghamm/discorse.html)_,) found at Nag Hammadi, which contains a certain amount of astrological/astronomical metaphor, and refers to a hidden “eight and ninth” beyond the revealed seven. Of course, even in translation it’s far from clear that they’re referring to planets, and my Coptic is, uh, a little rusty, but it’s kind of interesting. all the same: > [@](#): > > For I see! I see indescribable depths. How shall I tell you, my son? […] from the […] the places. How shall I describe the universe? I am Mind, and I see another Mind, the one that moves the soul. > > […] > > What he had finished praising, he shouted, “Father [Hermes] Trismegistus! What shall I say? We have received this light. And I myself see this same vision in you. And I see the eighth, and the souls that are in it, and the angels singing a hymn to the ninth and its powers. And I see him who has the power of them all, creating those \ in the spirit.” > > “It is advantageous from now on, that we keep silence in a reverent posture. Do not speak about the vision from now on.” > > “My son, write this book for the temple at Diospolis in hieroglyphic characters, entitling it ‘The Eighth Reveals the Ninth.’” > > “I will do it, my \, as you command now.” > > “My \, write the language of the book on steles of turquoise. My son, it is proper to write this book on steles of turquoise, in hieroglyphic characters. For Mind himself has become overseer of these. Therefore, I command that this teaching be carved on stone, and that you place it in my sanctuary. Eight guardians guard it with […] of the Sun. The males on the right are frog-faced, and the females on the left are cat-faced. And put a square milk-stone at the base of the turquoise tablets, and write the name on the azure stone tablet in hieroglyphic characters. My son, you will do this when I am in Virgo, and the sun is in the first half of the day, and fifteen degrees have passed by me.” > > “My father, everything that you say I will do eagerly.” _shrug_ ---
**Author:** ![Sampiro](https://avatars.discourse-cdn.com/v4/letter/s/2acd7d/32.png) [@Sampiro](https://boards.straightdope.com/u/Sampiro)\ **Post date:** [October 8, 2004, 8:27am UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/29 "2004-10-08T08:27:57Z")
Actually there was some ancient postulations that the planets were worlds. Lucretius among others (a poet-philosopher rather than a scientist) posited that they might even be earthlike and occupied. (He also suggested that humans descended from lower animals, but again, more for aesthetic than for scientific reason.) Some post-Platonic educated Greeks also speculated that if seen up close each planet may be one of the five Platonic solids. ---
**Author:** ![vetbridge](https://avatars.discourse-cdn.com/v4/letter/v/b5e925/32.png) [@vetbridge](https://boards.straightdope.com/u/vetbridge)\ **Post date:** [October 8, 2004, 5:24pm UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/30 "2004-10-08T17:24:58Z")
If you face east, locate the north star, then bend waaaay over backwards you can sometimes see uranus. ---
**Author:** ![Kimstu](https://avatars.discourse-cdn.com/v4/letter/k/ecd19e/32.png) [@Kimstu](https://boards.straightdope.com/u/Kimstu)\ **Post date:** [October 8, 2004, 8:53pm UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/31 "2004-10-08T20:53:00Z")
**ultrafilter:** _It’s important to note that the ancients probably had no idea that planets were unlike stars._ Not quite true. They certainly had no way of knowing what planets were actually made of via spectroscopy or anything like that, but the classical cosmological models certainly considered planets (including the sun and moon) to be different from stars. The stars were considered to be much farther away and to be located on a single outermost sphere (stellar parallax and proper motion weren’t known at the time). The planets were bigger to the naked eye (sun and moon certainly, but even the five star-planets look somewhat different from stars because they’re not far enough away to be point sources of light). I believe Aristotle considered, though, that all celestial objects (moon on outwards) were made of a different substance than the four elements of Earth, Air, Fire, and Water that composed the “sublunar” world in variously proportioned mixtures. The more perfect celestial objects were a fifth element or “quintessence”. **LM:** _There’s a Hermetic text (generally called The Discourse on the Eight and Ninth,) found at Nag Hammadi, which contains a certain amount of astrological/astronomical metaphor, and refers to a hidden “eight and ninth” beyond the revealed seven._ Could be (and probably is) just meaning to imply celestial **orbs** or spherical shells that were part of the standard geocentric cosmology, not additional planets. I think the eighth orb was generally that of the fixed stars and the ninth that of the Unmoved Mover or some such Neoplatonic concept. **Odion:** _I once read that the morning star aspect of Mercury was called Apollo before they figured out it was the same planet._ I never heard that, but it’s true that some classical sources refer to earlier authorities as having thought that the evening and morning appearances of the same inferior planet (usually Venus, not Mercury) were actually different objects. This was kind of a dig by the classical Greeks at their more backwards ancestors who hadn’t figured out periodic planetary motions yet. **CM:** _Exactly why seven is seen as a mystical number isn’t extremely obvious. Odd numbers definitely seem to have mystic significance […] By why should we have a week of seven days? Because the period of the moon is 29 days and change, and 28 is close to that, and is easily divisible by two and four. Divide by two and you getr fourteen day periods (a fortnight). Divide by four and you get seven – the week. (I suppose you could round up instead and get three ten day “weeks” in a month, but I never heard of anyone trying that experiment.)_ They certainly [did:](http://personal.ecu.edu/mccartyr/constantine.html) > [@](#): > > Actually there were several kinds of “weeks” used. The early Romans observed an eight-day market week; other tribal and primitive people had market weeks of four, five or six days; the Egyptians and Greeks divided their months into ten-day periods. The linked site is almost certainly incorrect, though, in stating that the seven-day week came from the “Assyrians”. Akkadian-using Mesopotamians, Assyrians and Babylonians alike, reckoned a 29- or 30-day lunar month but identified all its days individually, without smaller “week”-like units. As far as I’ve ever been able to find out, the seven-day week is a Hebrew invention, and may indeed have come about exactly as **Cal** hypothesizes. **Cal** ’s also right that the mystic nature of seven is probably more closely related to this weird-odd-indivisible number phenomenon than to any notion of the planets. Note also that seven is the first “does not divide” number in Mesopotamian base-sixty mathematics, dating back at least into the third millennium BCE; division was performed by multiplying by the number’s base-60 reciprocal (3 = 60 x 0;20, 4 = 60 x 0;15, and so on). Mesopotamian division tables (still preserved on a lot of cuneiform clay tablets) looked something like this: 1 60 2 30 3 20 4 15 5 12 6 10 7 does not divide 8 7 30 9 6 20 10 6 11 does not divide And so on. The numbers that didn’t have finite base-60 reciprocals—that “didn’t divide” 60—were definitely perceived as kinda weird and special mathematically. How much that influenced their mystic aura, however, I don’t know. ---
**Author:** ![foolsguinea](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/foolsguinea/32/14751_2.png) [@foolsguinea](https://boards.straightdope.com/u/foolsguinea)\ **Post date:** [October 11, 2004, 8:08pm UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/32 "2004-10-11T20:08:44Z")
> [@CalMeacham](#): > > The seven days of the week fit in well with the seven classical “planets”, and their names reflect this – Sun-Day, Moon-Day, Saturn-Day. For some reason they went with Northern gods for the other days, but I believe they have the same astronomical significance (Wodens-Day has Woden = Odin = Jupiter, king of the gods, etc.) Actually, you can see the actual day/planet organization in the modern Spanish names of the weekdays, which mostly come from the Roman names. I don’t know if the ancient Norse equated gods with planets, but I suspect that our English names come from syncretic attempts to translate the gods’ names. Monday - lunes - the moon, **Luna** Tuesday - martes - Tyr (a warrior god) as translation of **Mars** Wednesday - miércoles - Woden needed recognition, so replaced **Mercury** Thursday - jueves - Thor as translation of **Jove** /Jupiter (both are thunder gods) Friday - veirnes - Frigg or Freya takes the place of **Venus** Saturday - sábado (“sabbath” due to Christianity) - **Saturn** Sunday - domingo (“lord’s day” due to Christianity) - the sun, **Sol** ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [October 11, 2004, 9:00pm UTC](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956/33 "2004-10-11T21:00:49Z")
> [@Kimstu](#): > > 9 6 20 9 6 40 🙂 [Previous page](https://boards.straightdope.com/t/how-did-the-ancients-know-these-five-were-planets/267956.md?page=1)