Caption: A cartoon of our understanding of the atomist cosmology (see David Furley (1922--2010), The Greek Cosmologists, 1987, p. 139--151 (hereafter Fu)).

Our understanding is imperfect because the writings of the early atomists are lost, except for text fragments and imperfect summaries.

The atomist cosmology seems to be mostly due to Democritus (c.460--c.370 BCE), but his predecessor Leucippus (first half of 5th century BCE) contributed ideas (Fu-139).

Note the cartoon is NOT definitive since we do NOT have a precise understanding and maybe neither did the atomists.

The later atomists, the Epicureans (as evidenced by Epicurus (341--271 BCE) and Lucretius (c.99--c.55 BCE)) made slight variations from the original atomist cosmology. The Epicureans had very little interest in natural philosophy, except as a background for their ethics---live, drink, and be merry.

Here we just describe the original atomist cosmology keeping in mind that the atomists created a rational myth from first principles and NOT a derivation from first principles.

Features:

1. The atomist cosmology posited an infinite, eternal universe consisting of empty space (the atomist void) and the unbreakable, eternal atomist atoms which had all kinds of shapes which gave them useful properties---but there was no derivation of these properties---they were just imagined.

   The atomists called the universe το παν (to pan = the all) which is distinct from cosmos in their terminology: see below.

2. The atomist atoms had mass and motion, and were always colliding and re-arranging in a completely deterministic fashion.

   Note the atomists had only a qualitative understanding of mass and motion. They were a long way from Newton's 3 laws of motion.

3. The atomist atoms moving in the atomist void had an instability to the formation of vortices.

   The atomists certainly got this idea from the rotation of the celestial sphere plus what they probably identified as miniature versions in fluid motion: eddies, whirlpools, and whirlwinds.

4. The vortices developed a surrounding membrane that was roughly spherical. The atomists may have been thinking along the lines of embryo formation in an early mixing biology and physics (see Fu-143--144).

5. A membrane and its interior constitute a cosmos.

   There are as many cosmoi formed from the vortices.

   We live in our cosmos and can't see out.

   By the by, the word cosmos was reputedly introduced into cosmology by Pythagoras (c.570--c.495 BCE) meaning ordered and beautiful system (see Fu-58--60).

   They are indeed correct who use cosmology and cosmetology as synonyms.

6. Inside the membrane---to revert to the definite article as ancient Greek philosophers would when they usually mean "our"---the atomist atoms arrange themselves to cause up-down direction which becomes gravity somehow.

   At some level in the membrane solid land and ocean form a disk. Above is air and maybe below is air too (see Fu-143).

   The vortex is still swirling the cosmos around. In the cosmos, the axis of rotation (the celestial axis) has become titled to the north and is ∼ 40° above due north as viewed from Greece. The disk of land and ocean may have been thought of as a bit curved to allow for variations in the altitude of the celestial axis, but this is unclear (see Fu-145). The atomist cosmology was essentially a flat-Earth one.

   The fact that it was a flat-Earth cosmology shows that the atomists were obtuse about astronomy and geodesy as understood already in the 5th century BCE when the spherical-Earth theory was already well known with probably with solid evidence.

7. The stars were attached to the membrane and swirled with it. The inner surface of the membrane is, in fact, the celestial sphere interpreted as a real thing---like the celestial sphere of the stars (the outermost celestial sphere of Aristotelian cosmology).

   The membrane like the celestial sphere of the stars was finally ruled out by the Copernican revolution.

   The planets, Sun, and Moon were probably closer and swirled in a more complicated way. Democritus did believe the Moon was closest and shone by reflected light from the Sun The atomists seem to have thought of the astronomical objects hot condensations from the formation of the cosmos (see Fu-145).

   It is clear that the atomists, both the early and later ones, took very little account of even qualitative astronomy perhaps because astronomy evidenced the spherical-Earth theory and a very large celestial sphere/membrane.

   Yours truly thinks that the atomists could have accommodated astronomy if they had tried.

8. The cosmoi are destroyed somehow after finite lifetimes which at least in the case of our comos is much longer than a human lifetime. Maybe they just break up after some point in time, but by one report they break up in collisions between themselves (see Fu-140).

9. It is remarkable that atomists by insight/luck made several hypotheses that turned out to be right or potentially right to some degree:
   1. They posited atoms: atomist atoms, of course.
   2. They had unified physics: celestial and terrestrial physics were the same. This is unlike Aristotelian physics where celestial and terrestrial physics are different: Dual physics (Aristotelian physics or otherwise) would dominate natural philosophy in western Eurasia from circa 400 BCE to 1600 CE and was only finally dismissed by the unified Newtonian physics.
   3. Vortices are key ingredients in star formation and galaxy formation.
   4. The idea of cosmoi being born and dying in a background universe is similar (albeit with a lot less math) to the multiverse of eternal inflation with the cosmoi corresponding to pocket universes. One wonders if human intellect is somehow constrained to imagine only certain possibilities.

10. From the point of view of human enlightenment, it is tragic that the insights of the atomist atoms and ancient Greek astronomy were NOT put together in classical antiquity. But do to that would have taken an imaginative leap that did NOT happen and probably had a low probability of happening before the Scientific Revolution of the 17th century.

11. Democritus videos (i.e., Democritus (c.460--c.370 BCE) videos):
    1. Carl Sagan - Cosmos - Democritus | 3:50: Carl Sagan (1934--1996) schmoozing in Abdera. Too long for the classroom.
    2. Democritus And Atomic Theory | 3:56: A bit dull. Too long for the classroom.