Tag Archives: parallax

Old-school Cometary Theory: Above or Below the Moon?

5 Jan


How Do We Know if Something is “Above the Moon”?

Where do comets come from? Other than the sins of human beings rising every hour until the stench and horror turns into a comet (a reference to Andreas Celichius, The Theological Reminder of the New Comet, 1578).

So, Oort Cloud? Human sin? It could go either way.

Imagine that you’ve never seen a model of the Solar System. You have to construct it from what you see from the ground. You’re looking out over the horizon and you want to create a model of the universe.

You might come up some main areas from what you see; the stuff on Earth, terrestrial stuff, also sometimes referred to as ‘meteorological’. This isn’t just stuff on the ground. It’s anything that comes from our planet; clouds, rain, lightning, tornadoes, even meteors which only light up and become obvious when they enter our atmosphere, could be considered “meteorological”.

But you know there’s stuff above all the rain and the clouds. The Moon, for example, is often behind clouds, but conspicuously never in front of the clouds. The Sun is sometimes behind the moon, but never in front of the Moon.

You have the Sun and Moon areas, the things that move remarkably quickly in relation to you and dominate the sky and what you see. Then you have planets, and they move, but not like the Sun and Moon. They move slowly. Mercury moves the fastest, and if you’re living in pre-telescopic times, Saturn moves the slowest. But it does move, and its movement is obvious over a few weeks. Then there’s the stars. They don’t appear to move in relation to each other at all. Orion will forever be chasing a bull, followed by his dog, and squishing a bunny. So it’s logical from this perspective that there are only 2 or 3 regions of space: Earth, planet stuff, and the celestial sphere.

Why Won’t Comets Just Go Behind the Moon Already?

How do you prove something is somewhere beyond the moon? Well, an easy way would be if the Moon would just occult it. If the moon passed in front of Halley’s Comet, that would be a dead giveaway the comet was farther away than the moon. Alternatively, if the comet passed in front of the moon, then… it would have to be closer to us than the moon. Because that’s just how vision works.

But even before people knew that the slower planets were all farther away, we knew that Mercury and Venus were closer to us than the other planets (although which planet was closest to us, Mercury or Venus, was not figured out for some time). The placement of planets in relation to Earth, that is, the relative distances of planets, can be ascertained when planets transit, eclipse, or occult other planets. Just like when we see the Moon eclipsing the Sun, we say “hey, I guess the Moon is closer to us than the Sun”, when we see Venus pass in front of Mars, we say “hey, I guess Venus is closer to us than Mars.”

Comparison of Models

Note that regardless of whether it’s the Tychonic, Geocentric, or Heliocentric model, Mercury and Venus are always listed as the closest planets to Earth. Additionally Mars, Jupiter, and Saturn are correctly placed.

Transits and occultations were important for understanding the relative distances to planets, that is, which is closer, Venus or Mars? Mars or Saturn? For example, Kepler observed that Mars occulted Jupiter in 1591, and Maestlin observed Venus occult Mars in 1590.

Chinese astronomical records indicate that Mars occulted Saturn in 1027, Venus occulted Jupiter in 757, and a bunch of planets were occulted by the Moon a bunch of times. There’s a never ending record of the Moon passing in front of a bunch of things which we don’t have to get into, suffice it to say the Moon’s the closest to us and humans have known that for forever.

So now, building our Solar System by relative distance: If Venus occults Mars (in 1590) and Jupiter (in 757) and Mars occults Jupiter (in 1591) and Saturn (in 1027) then LSAT logic dictates that:

  1. Venus must be closer to us than Mars, Jupiter, and Saturn.
  2. Mars must be farther away than Venus but closer than Jupiter and Saturn.
  3. And either Jupiter or Saturn has to be the farthest away – ideally we’d get Jupiter to occult Saturn at some point. It happens, but rarely. I haven’t found a written record of it happening (if you find one, please send it my way with a source), but pretty much the oldest models of the Solar System we know of place Saturn farther out than Jupiter, so people knew. I don’t know if occultations were passed down orally or what, but they knew. I was unable to find a solar system model, even a really old one, even a really obscure one, with Jupiter farther out than Saturn. And I looked. Real hard. Took my whole Saturday.
  4. Since the moon has occulted a bunch of stuff at one point or another, humans knew the basic setup of the solar system well before the telescope, certainly before modern probes. It only takes a few eclipses and occultations to figure out which planets are the farthest from us, which are the closest, and that the moon is closer to us than everything else. Since no planet was (or ever will be) eclipsed by a star, it was also obvious very early on that the stars were much farther away than anything in the Solar System. And virtually every old-school model gets this general setup correct.

While I couldn’t find a specific historical example of Jupiter occulting Saturn, here are some examples of old-school models that placed Saturn past Jupiter:

Again, that old date of the Venus/Jupiter occultation, AD 757, that was way before the invention of the telescope. That was before heliocentrism really took off. But people still knew the relative setup of the planets because they were paying attention.

This is not, by any stretch a complete list of occultations that were written down all over the world. But isn’t it convenient that way before fancier methods came along astronomers were able to see which planets passed in front of each other and ascertain their relative distances in the Solar System?

Well… No. It’s not convenient, or luck, or coincidence. The Solar System is actually on a plane.

The problem is, comets aren’t generally on that plane. If they were, we could just wait until the Moon passes in front of them and it becomes super obvious that they live “above the Moon” but no. They don’t do that. The fact that comets aren’t on the ecliptic plane is important in the discussion on where comets are coming from.

Who Thought Comets Were Coming From Where?

Now, there were people that believed that comets came from Earth. Like wind, or lightning, comets blew in like a hurricane. Aristotle was one of them, true to his nature, he believed wrongly that comets were coming from the Earth itself. Now, he lived a long time ago so he didn’t have the Internet, but other Greeks before him, notably the Pythagoreans (according to Aristotle), believed that comets were planets, or perhaps just one planet, and lived where planets lived: Above the moon.

Other early sources of believing comets were in fact celestial was Apollonius of Myndus, a contemporary of Aristotle, and according to Apollonius (by way of Seneca), the even older Babylonians/Chaldeans thought that comets came from beyond the moon.  But according to Epigenes, the Babylonians thought they were atmospheric phenomena and came from Earth:

Going back to Aristotle, he believed that comets came from Earth; that they rose up from dry windy exhalations. He believed, basically, that they were atmospheric phenomenon – like weather. To prove this theory, Aristotle noted that comets are foreshadowed by wind and drought. That the wind and the drought proved that comets were atmospheric and came from Earth.

  •  “We may regard as a proof that their [comets] constitution is fiery the fact that their appearance in any number is a sign of coming wind and drought.”
  • “So when comets appear frequently and in considerable numbers, the years are, as we say, notoriously dry and windy.”

Aristotle, Meteorologica, H.D.P. Lee version (trans. 1951)

Now if you’re wondering, ‘wait, is there a correlation between comets and drought?’ No. No there’s not. This is one step away from Monty Python logic.

By the way, this isn’t even sort of the oddest “proof” that comets are actually from Earth. In John Edwards’ 1684 Cometomantia, John argues that yes, sure, astronomers say that comets are above the moon, but then why do they occur more frequently in Autumn? And also, smell bad? Answer me that learned astronomers!

Let’s Get Listy

If I go through every single person we have on record saying where comets come from, we’ll be here all day. So first up, an incomplete list of people who believed comets came from the Earth, Earth’s atmosphere, or “below the moon”:

  • Xenophanes of Colophon (570-475 BCE)
  • Aeschylus (525-456 BCE)
  • Heraclides of Pontus (390- 310 BCE)
  • Aristotle (384-322 BCE)
  • Metrodorus of Chios (300s BCE)
  • Theophrastus (371-287 BCE)
  • Strato of Lampsacus (335-269 BCE)
  • Epigenes (100s BCE)
  • Panaetius of Rhodes (185-110 BCE)
  • Posidonius (135-51 BCE)
  • Boethus (75-10 BCE)
  • Albert Magnus (1193-1280)
  • Georg von Peuerbach (1423-1461)
  • Nicolaus Copernicus (1473-1543)
  • Johannes Vogelin (1480s-1549)
  • Peter Apian (1495-1552)
  • Bartholomaeus Scultetus (1540-1614)
  • Scipione Chiaramonti (1565-1652)

People who believed comets were actually celestial objects living above the moon in true outer space:  

  • Babylonians (according to Apollonius) (circa 1700s BCE)
  • Pythagoreans (according to Aristotle) (500s BCE)
  • Anaxagoras (510-428 BCE)
  • Hippocrates of Chios (470-410 BCE)
  • Democritus (460-370 BCE)
  • Diogenes of Apollonia (400s BCE)
  • Zeno of Citium (334-262 BCE)
  • Apollonius of Myndos (300s BCE)
  • Artemidorus of Parium (it’s unclear)
  • Lucius Annaeus Seneca (4 BCE-65 CE)
  • Michael Maestlin (1550-1631)
  • Johannes Kepler (1571-1630)
  • Orazio Grassi (1583-1654)

And what about those people that just couldn’t choose a solid above or below the moon region? 

  • In the 1500s Jean Pena suggested that some comets exist beyond the moon. This was a pretty common comeback in the late 1500s.
  • Bernardino Telesio wrote on the comet of 1577. He wrote a book, On Comets and the Milky Way in which Telesio said that though comets originated from below the moon, some of them, such as the Great Comet of 1577, can rise high enough to become a part of the celestial region, beyond the moon.
  • Thomas Digges treated comets as celestial objects but then seems to switch in 1576 to treating comets as terrestrial objects in A Perfit Description of the Caelestiall Orbes. This was just sad, as the comet of 1577 basically proved comets were not from Earth (see page 33 of Donald Yeoman’s Comets).

And then there’s Galileo. What did Galileo think of comets? In 1619 Orazio Grassi wrote on the three comets of 1618 noting that since they essentially moved very slowly – slower than the moon – comets must be far away, and indeed exist beyond the moon.

This set Galileo off in a way that begs the question: Did Galileo think comets originated from Earth or from outer space?

The answer is, we don’t know for sure. We know he liked to argue with people. We know that with the help of a man named Mario Guiducci, Galileo took Grassi to task, arguing that comets could show no parallax because they might not even be real. They could actually be, like rainbows, an illusion.

Galileo went on to poke others who had made statements about comets, including Tycho Brahe who thought comets lived in the region between the Sun and Venus.

Kepler came to Brahe’s defense, and implied that Galileo was just jealous of Tycho Brahe, actually. But in his most famous work, Galileo stated that he did not care whether comets originated from above or below the moon.

The Comet of 1577

The Great Comet of 1577 was incredibly important in firmly moving comets from “could be meteorological” to “definitely coming from outer space”. Because the comet was pretty obviously farther away than the Moon.

So how do you figure out distance without a space probe and a really big measuring stick? With trigonometry. Of course. Which is largely the tool that old-school astronomers used to make measurements of the heavens.

Now we could talk about parallax but it really belongs in a video about the Cosmic Distance Ladder and that would take like, 70 hours. Suffice it to say, it’s not a coincidence that things closer to us look like they’re moving faster. People knew this. They knew clouds were closer to us than the Moon and that clouds moved faster than the Moon. From our vantage point on Earth (you know, the only vantage point) they knew the Moon moved faster in the night sky than planets and that it was closer to us than planets. No one ever said, ‘you know, the stars seem to not be moving in relation to each other at all, but I’ll bet they’re closer to us than the Moon’. That person doesn’t exist. I mean, maybe they existed – I just haven’t found them.

It’s not like the theory of comets originating from above the moon happened over night and was accepted because of one comet, but if one comet could be given the most credit, called the most influential, it is definitely the Great Comet of 1577. Similarly, the acceptance of comets as objects that originate well and truly from outer space cannot be credited to any single astronomer, but if one astronomer could be given the most credit, called the most influential on this topic, it’s Tycho Brahe.

Tycho showed that the comet had moved less in the night sky than the moon in relation to the background stars, that is, the comet showed less parallax. This required the use of a fine tuned instrument, but let’s first talk about what parallax is.

See people couldn’t say ‘oh this object has moved 4 miles today’ because they didn’t know how far away things were. All people could do is divide up the sky and say, ‘here’s my sphere, and it has 360 degrees, and each degree we’ll divide that further into 60 segments, we’ll say every degree has 60 minutes in it, and every minute has 60 seconds in it.

So all you can say at this point is, here’s how much this thing in the sky appears to have moved. That’s why you go back and you read these old books and it’s always like, ‘the comet moved 8 degrees and 3 minutes last night’ and never ‘8 km and 3 meters’. Because they’re really just saying what percentage of the sky the comet has moved across.

The problem with this is what if the object moves less than a degree? You need a quadrant big enough put the arc-minute notches onto. Actually if you were to get a circle and put all the degrees and all the minutes and all the seconds on it, you’d have 1,296,000 marks on your circle. In order to make those super accurate parallax measurements, I’d need to be able to put a whole lot of notches onto my circle/semicircle/quadrant/sextant.

So what’s a person to do?

Get a bigger quadrant. Which begs the question, how does one get a giant quadrant? Well, it helps to be propped up by the king. And that’s exactly what happened to Tycho Brahe. King Frederick the II of Denmark gave Tycho Brahe an island, a castle, and a whole lot of money to do his experiments with, making Tycho Brahe probably the highest paid astronomer of all time. And his instruments were the best, and the biggest. Only one other astronomer that I know of had a larger quadrant than Tycho Brahe; Ulug Beg, and he wasn’t just an astronomer for the king, he was the king.

Ah it’s good to be king!

Wait hang on, what’s parallax?

So let’s talk briefly about ‘parallax’. You know how when you’re driving down a road, the fence close to you flies by but the mountains in the distance don’t really appear to move all that much? Sure the mountains are bigger, but also, things closer to you appear to move more rapidly. [do the finger thing] My finger didn’t change size, my head didn’t move more, but it sure moves a lot less when it’s farther away.

The takeaway here is, when traveling past things that are far away, they appear to move slower than things that are close to us. A jet traveling hundreds of miles per hour in the distance appears to be moving slower than a jet traveling hundreds of miles an hour right over your head.

Now the Comet of 1577 was super bright. And also seen pretty much worldwide.

  • Japan
  • Czech Republic
  • Sweden
  • Belgium
  • Palestine
  • Peru
  • India
  • Turkey

Which is important, because what Tycho did (theoretically) was get another astronomer, 500 miles away to observe both the comet and the moon on the same night at the same time as he was observing them.


Ok, so let’s pretend the Earth is a sphere – go with me on a journey – and you’re looking at the moon, which is relatively close to us. There’s a guy over here, we’ll say he’s in Sweden, and a guy over here, we’ll say he’s in Prague, and they’re looking at the Moon and we’ll say it’s midnight. And guy over here’s looking at the moon going, yeah, it’s uh… kinda near the Pleiades and guy over here’s like it’s actually a little farther away from the Pleiades, and then they see the comet and they’re like, ‘yep, it’s in the same spot’. Well, that’s how you know, the comet is above the moon, beyond the moon, farther from Earth, than the moon.


This is a wild exaggeration and should only be used for illustrative purposes. However you can look at the moon from one location in Stellarium, and then choose another location roughly 500 miles away and see that yes, it does move in relation to the background stars.

Turns out, the moon was in a different location in the sky, because of course it was, but the comet wasn’t.


Again, an exaggeration. In reality, the comet would have appeared to move almost imperceptibly in relation to the background stars.

[Do the finger blink comparator again] Which means this was the moon [close] and this was the comet [far].

Your head is the Earth in this example.

Ok so, Tycho theoretically wrote all of this down , but I swear to god , it’s not translated.

Everything I have found is in Latin. Latin. Do I look like a person who learns Latin in their off time? Everything I’ve seen is a secondary source because nobody translated Tycho Brahe’s actual work on the 1577 comet?! This is Tycho Brahe! One of the most famous astronomers of all time – not Simon Stevin! – and I gotta read about him by some American? Why aren’t the Danish doing something about this? Who is the president of history?

Anyway, the Great Comet of 1577 was seen everywhere. It was seen in Japan, it was seen Europe, it was seen in India. The comet was big, bright, and far away.

For a list of texts and treatises on the Great Comet of 1577 check out C.D. Hellman’s 1944 book: The Comet of 1577. There were many.

So theoretically at this point, Tycho, and to be fair many other astronomers, realize that this comet had to be “above” the moon. It could not be atmospheric phenomena. Comets could not be created like tornadoes or wind or hurricanes – in short, they were not in fact coming from Earth.

People knew that the higher up something was, the more people could see it around the world, because it wouldn’t be below the horizon as quickly as something that was much closer, in this case the moon. In fact, about 70 years after the comet of 1577, a textbook writer in Paris, Pierre du Moulin, wrote that the fact that the comet was seen by so many in so many countries demonstrated its great height.

“Aristotle holds that comets are fiery exhalations: but the astronomers of this time have observed that a comet was above the moon. If that comet was a fiery exhalation, it would have always kept its tail behind it, in the manner of a torch, which when carried always keeps its flame behind it. And the fact that it was seen by so many in so many countries demonstrates its great height.”

And I think it’s important to note that this is not just one guy altering our view of astronomy, this is mostly just the most famous guy with the biggest annual income of any astronomer ever, altering our view of astronomy. Even before the comet of 1577 Girolamo Cardano (known primarily for his theories of probability and gambling) had stated that the comet of 1532 was above the Moon, precisely because it appeared to move slower than the Moon.

And it wasn’t an overnight change. Some astronomers still went down this road where they reasoned, ‘OK some comets live in outer space, but some still come from Earth’. It wasn’t an immediate discovery that kicked comets above the moon and well into outer space, but it’s not an overstatement to say that the Great Comet of 1577 was the nail in the coffin of an unfortunate spate of Aristotelian theories about comets being from Earth. After 1577 you pretty much had to explain why you still though comets were coming from Earth.

If I had to distill the not-so-linear-progression of old-school cometary thought, it would be:

  1. People knew the setup and relative distances of the Solar System because the Moon, Sun, and planets are on a plane and do eclipse each other from time to time.
  2. Comets are not on that plane and therefore people needed a different way of ascertaining distance.
  3. But people had noticed and knew that the farther away objects were, the slower they appeared to move in relation to the background stars.
  4. Armed with some pretty good logic, they then used a combination of parallax and large, finely tuned instruments to make comparatively accurate measurements of distances to “great” comets.

Thus ultimately placing comets beyond the moon, in at least the realm of planets, coming somewhere from true outer space.

I know I said last time to join me next time to talk about what are comets really? But I grossly underestimated how big of a historical argument it was that comets were above or below the moon, and how long it took for people to move past that.

So I guess instead, I’d file this one under Old-school Cometary Theory – you know, before people knew what comets even were. Join me next time to talk about New School Cometary Theory! And when I say New School I really mean, what are comets even, really?