Editing Leucippus (section)

=== Cosmology ===
Leucippus said that the void extends infinitely, expanding across the entire universe. He also said that there is an infinite number of atoms, spread across the void. The Earth and the [[cosmos]]—including the Sun, the Moon, the stars, and anything else visible in the night sky—exist together in the void.{{Sfn|Furley|1987|p=136}}

Leucippus said that the cosmos was created when a large group of atoms came together and swirled as a vortex. They shifted around each other until they were sorted "like to like". The larger atoms gathered in the center while the smaller ones were pushed to the edge. The smaller atoms became the celestial bodies of the cosmos. The larger atoms in the center came together as a membrane from which the Earth was formed.{{Sfn|Furley|1987|pp=140–141}}{{Sfn|Kirk|Raven|1957|pp=411–412}} Ancient writers disagreed about what Leucippus meant when he described the membrane: [[Aetius (philosopher)|Aetius]] said that the smaller atoms were part of the membrane, encasing the larger atoms, but [[Diogenes Laertius]] said that the larger atoms formed a membrane themselves and the smaller ones were excluded.{{Sfn|Kirk|Raven|1957|pp=411–412}} Leucippus also believed that there were distant cosmoses in other parts of the void; this makes him the first known philosopher to propose the existence of other worlds besides Earth, though some ancient doxographers have attributed these ideas to the earlier [[Ionian philosophers]].{{Sfn|Kirk|Raven|1957|p=412}}

Like other pre-Socratic philosophers, Leucippus believed that the Earth was in the center of the cosmos.{{Sfn|McKirahan|2011|p=327}} He said that the other celestial bodies orbited around the Earth, with the Moon being the closest to the Earth and the Sun being the farthest.{{Sfn|Graham|2008|p=338}} He described the stars as orbiting the fastest.{{Sfn|Graham|2008|p=339}} While initially "moist and muddy", the stars dried and then ignited.{{Sfn|Furley|1987|pp=140–141, 145}}

Leucippus adopted the idea of the Ionian philosophers that the Earth is flat.{{Sfn|Kirk|Raven|1957|p=412}} According to Aetius, Leucippus thought of the Earth as "drum-shaped", with a flat surface and some degree of depth.{{Sfn|Graham|2008|p=339}} He said that the flat Earth is tilted on its horizontal axis so that the south is lower than the north, explaining that the northern region is colder than the southern region, and the cold compacted air of the north can better support the Earth's weight than the warm rarefied air of the south.{{Sfn|Graham|2008|pp=339–340}} Aetius also tells of Leucippus's explanation for thunder: that it is caused by fire being compressed in clouds and then bursting out.{{Sfn|Graham|2008|pp=340–341}}

Many early philosophers were confused by the fact that earthly objects fell downward while celestial objects moved in a curved trajectory. This prompted many of them to believe in a non-earthly substance that composes the celestial bodies. With his model of the cosmos, Leucippus was able to justify why these entities move differently even though they are made of the same substance.{{Sfn|Furley|1987|pp=146–147}} Leucippus gave no explanation for how motion began, for which he was criticized by Aristotle.{{Sfn|Furley|1987|p=149}}{{Sfn|Kirk|Raven|1957|p=417}} It is unclear whether Leucippus considered vorticies to arise by chance or as a deterministic outcome.{{Sfn|Gregory|2013|pp=459–460}}