Cosmos and sky

Physical cosmology (often simply described as 'cosmology') is the scientific study of the universe, from the beginning of its physical existence. It includes speculative concepts such as a multiverse, when these are being discussed. In physical cosmology, the term cosmos is often used in a technical way, referring to a particular spacetime continuum within a (postulated) multiverse. The particular cosmos in which humans live, the observable universe, is generally capitalized as the Cosmos.

In physical cosmology, the uncapitalized term cosmic signifies a subject with a relationship to the universe, such as 'cosmic time' (time since the Big Bang), 'cosmic rays' (high energy particles or radiation detected from space), and 'cosmic microwave background' (microwave radiation detectable from all directions in space).

According to Charles Peter Mason in Sir William Smith Dictionary of Greek and Roman Biography and Mythology (1870, see book screenshot for full quote), Pythagoreans described the universe.

Cosmology is a branch of metaphysics that deals with the nature of the universe, a theory or doctrine describing the natural order of the universe. The basic definition of Cosmology is the science of the origin and development of the universe. In modern astronomy, the Big Bang theory is the dominant postulation.

Philosophy of cosmology is an expanding discipline, directed to the conceptual foundations of cosmology and the philosophical contemplation of the universe as a totality. It draws on the fundamental theories of physics – thermodynamics, statistical mechanics, quantum mechanics, quantum field theory, and special and general relativity – and on several branches of philosophy – philosophy of physics, philosophy of science, metaphysics, philosophy of mathematics, and epistemology.

Prior to the Copernican Revolution, the Ptolemaic system, also known as the geocentric model, was widely accepted. This put the Earth at the center of the universe, with the sun and other planets revolving around the earth in an epicyclic orbit. Aristotle's geocentric model was also broadly acknowledged, along with his claim that the planets rotated but did not orbit. The reasoning behind this was due to the belief that all objects outside of the lunar sphere were celestial bodies, and therefore could not change, as they were made of quintessence.

There were notable critiques of this model prior to Copernicus. In the Islamic world, Ibn al-Haytham doubted Ptolemy's notion of the planetary orbits, and Muhammad al-Battani recalculated the parameters. However, both still agreed with the geocentric model.

One of the first known astronomers that supported the Heliocentric theory was Aristarchus of Samos. After observing a lunar eclipse, he came to the conclusion that the sun was farther away from earth than the moon and that the sun was much larger than Earth. He also claimed the sun was a star. While Aristarchus was later an influence on Copernicus and his groundbreaking work, prior to the 17th century Aristarchus' findings were obstructed by the more established theories of Ptolemy and Aristotle.

The cosmos (/ˈkɒzmɒs/, US also /-moʊs, -məs/) is another name for the universe. Using the word cosmos implies viewing the universe as a complex and orderly system or entity.

The cosmos, and understandings of the reasons for its existence and significance, are studied in cosmology – a broad discipline covering scientific, religious or philosophical aspects of the cosmos and its nature. Religious and philosophical approaches may include the cosmos among spiritual entities or other matters deemed to exist outside the physical universe.