Of the four fundamental interactions, gravitation is the dominant at astronomical length scales. Gravity effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on astronomical length scales. The remaining two interactions, the weak and strong nuclear forces, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales.

This diagram shows Earth location in the universe on increasingly larger scales. The images, labeled along their left edge, increase in size from left to right, then from top to bottom.

The size of the universe is somewhat difficult to define. According to the general theory of relativity, far regions of space may never interact with ours even in the lifetime of the universe due to the finite speed of light and the ongoing expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the universe were to exist forever: space may expand faster than light can traverse it.

Because we cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the universe in its totality is finite or infinite.

Spacetimes are the arenas in which all physical events take place. The basic elements of spacetimes are events. In any given spacetime, an event is defined as a unique position at a unique time. A spacetime is the union of all events in the same way that a line is the union of all of its points, formally organized into a manifold.

Spacetime events are not absolutely defined spatially and temporally but rather are known to be relative to the motion of an observer. Minkowski space approximates the universe without gravity; the pseudo-Riemannian manifolds of general relativity describe spacetime with matter and gravity.

An important parameter determining the future evolution of the universe theory is the density parameter, Omega, defined as the average matter density of the universe divided by a critical value of that density. This selects one of three possible geometries depending on whether is equal to, less than, or greater than 1. These are called, respectively, the flat, open and closed universes.

Observations, including the Cosmic Background Explorer, Wilkinson Microwave Anisotropy Probe, and Planck maps of the CMB, suggest that the universe is infinite in extent with a finite age, as described by the Friedmann–Lemaître–Robertson–Walker (FLRW) models.

Because we cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the universe in its totality is finite or infinite.

The universe is composed almost completely of dark energy, dark matter, and ordinary matter. Other contents are electromagnetic radiation (estimated to constitute from 0.005% to close to 0.01% of the total mass-energy of the universe) and antimatter.

The observable universe is isotropic on scales significantly larger than superclusters, meaning that the statistical properties of the universe are the same in all directions as observed from Earth. The universe is bathed in highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.72548 kelvins.

Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to vacuum energy. Scalar fields having only a slight amount of spatial inhomogeneity would be difficult to distinguish from a cosmological constant.

Ordinary matter commonly exists in four states: solid, liquid, gas, and plasma. However, advances in experimental techniques have revealed other previously theoretical phases, such as Bose–Einstein condensates and fermionic condensates.

A photon is the quantum of light and all other forms of electromagnetic radiation. It is the force carrier for the electromagnetic force, even when static via virtual photons. The effects of this force are easily observable at the microscopic and at the macroscopic level because the photon has zero rest mass; this allows long distance interactions. Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, exhibiting properties of waves and of particles.

With the assumption of the cosmological principle that the universe is homogeneous and isotropic everywhere, a specific solution of the field equations that describes the universe is the metric tensor called the metric,

Some speculative theories have proposed that our universe is but one of a set of disconnected universes, collectively denoted as the multiverse, challenging or enhancing more limited definitions of the universe. Scientific multiverse models are distinct from concepts such as alternate planes of consciousness and simulated reality.

The Indian philosopher Kanada, founder of the Vaisheshika school, developed a notion of atomism and proposed that light and heat were varieties of the same substance.

By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when Earth rotational axis is aligned with its orbital axis. In the Northern Hemisphere, winter solstice currently occurs around 21 December; summer solstice is near 21 June, spring equinox is around 20 March and autumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.

A planet that can sustain life is termed habitable, even if life did not originate there. Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism.

Earth has resources that have been exploited by humans.

Large deposits of fossil fuels are obtained from Earth crust, consisting of coal, petroleum, and natural gas. These bodies form concentrated sources for many metals and other useful elements.

Large areas of Earth surface are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. From 1980 to 2000, these events caused an average of 11,800 human deaths per year. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, wildfires, and other calamities and disasters.

Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion and erosion.

There is a scientific consensus linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather and a global rise in average sea levels.

Cartography, the study and practice of map-making, and geography, the study of the lands, features, inhabitants and phenomena on Earth, have historically been the disciplines devoted to depicting Earth. Surveying, the determination of locations and distances, and to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

• Earth human population reached approximately seven billion on 31 October 2011.
• It is estimated that one-eighth of Earth surface is suitable for humans to live on.
• Three-quarters of Earth surface is covered by oceans, leaving one-quarter as land.
• Half of that land area is desert (14), (82N) The southernmost is the Amundsen–Scott.
• South Pole Station, in Antarctica, almost exactly at the South Pole. (90S)

A country may be an independent sovereign state or part of a larger state, as a non-sovereign or formerly sovereign political division, a physical territory with a government, or a geographic region associated with sets of previously independent or differently associated people with distinct political characteristics. It is not inherently sovereign.

Countries can refer both to sovereign states and to other political entities, such as Vatican City, the largest country in the world is Russia, while the most populous is China, followed by India and the United States of America. The newest country with widespread international recognition as a sovereign state is South Sudan.

The word country comes from Old French which derives from Vulgar Latin (terra) from contra (“against, opposite”). It most likely entered the English language after the Franco-Norman invasion during the 11th century.

A version of “country” can be found in the modern French language as contre, based on the word cuntrée in Old French, that is used similarly to the word “pays” to define non-state regions, but can also be used to describe a political state in some particular cases. The modern Italian contrada is a word with its meaning varying locally, but usually meaning a ward or similar small division of a town, or a village or hamlet in the countryside.

The term “country” can refer to a sovereign state. There is no universal agreement on the number of “countries” in the world since a number of states have disputed sovereignty status. By one application of the declarative theory of statehood and constitutive theory of statehood, there are 206 sovereign states; of which 193 are members of the United Nations, two have observer status at the UN (the Holy See and Palestine), and 11 others are neither a member nor observer at the UN. The latest proclaimed state is South Sudan since 2011.

The degree of autonomy of non-sovereign countries varies widely. Some are possessions of sovereign states, as several states have overseas territories (such as French Polynesia or the British Virgin Islands), with citizenry at times identical and at times distinct from their own. Such territories, with the exception of distinct dependent territories, are usually listed together with sovereign states on lists of countries, but may nonetheless be treated as a separate “country of origin” in international trade, as Hong Kong is.

Several organizations seek to identify trends in order to produce country classifications. Countries are often distinguished as developing countries or developed countries.

The UN additionally recognizes multiple trends that impact the developmental status of countries in the World Economic Situation and Prospects. The report highlights fuel-exporting and fuel-importing countries, as well as small island developing states and landlocked developing countries. It also identifies heavily indebted poor countries.

There are a number of different theories and hypotheses regarding early state formation that seek generalizations to explain why the state developed in some places but not others. Other scholars believe that generalizations are unhelpful and that each case of early state formation should be treated on its own.

Voluntary contend groups theories of people came together to form states as a result of some shared rational interest.

Conflict theories of state formation regard conflict and dominance of some population over another population as key to the formation of states.

The first states of sorts were those of early dynastic Sumer and early dynastic Egypt, which arose from the Uruk period and Predynastic Egypt respectively around approximately 3000 BCE.

Although state-forms existed before the rise of the Ancient Greek empire, the Greeks were the first people known to have explicitly formulated a political philosophy of the state, and to have rationally analyzed political institutions. Prior to this, states were described and justified in terms of religious myths.

Several important political innovations of classical antiquity came from the Greek city-states (polis) and the Roman Republic. The Greek city-states before the 4th century granted citizenship rights to their free population; in Athens these rights were combined with a directly democratic form of government that was to have a long afterlife in political thought and history.

Political globalization began in the 20th century through intergovernmental organizations and supranational unions. The League of Nations was founded after World War I, and after World War II it was replaced by the United Nations. Various international treaties have been signed through it. Regional integration has been pursued by the African Union, ASEAN, the European Union, and Mercosur. International political institutions on the international level include the International Criminal Court, the International Monetary Fund, and the World Trade Organization.

The history of the world is commonly understood as the history of humanity spanning the major geopolitical developments of about five millennia, from the first civilizations to the present. In terms such as world religion, world language, world government, and world war, the term world suggests an international or intercontinental scope without necessarily implying participation of every part of the world.

The world population is the sum of all human populations at any time; similarly, the world economy is the sum of the economies of all societies or countries, especially in the context of globalization:

• Terms such as “world championship”, “gross world product”, and “world flags” imply the sum or combination of all sovereign states.
• While the Germanic word thus reflects a mythological notion of a domain of Man compare Midgard.
• The corresponding word in Latin is mundus literally “clean, elegant” as an act of establishing order out of chaos.

Itself a loan translation of Greek cosmos “orderly arrangement.” presumably as opposed to the divine sphere on the one hand and the chthonic sphere of the underworld on the other, the Greco-Latin term expresses a notion of creation.

“World” distinguishes the entire planet or population from any particular country or region: world affairs pertain not just to one place but to the whole world, and world history is a field of history that examines events from a global (rather than a national or a regional) perspective. Earth, on the other hand, refers to the planet as a physical entity, and distinguishes it from other planets and physical objects.

Was also classically used to mean the material universe, or the cosmos: “The worlde is an apte frame of heauen and earthe, and all other natural thinges contained in them.”

The term can also be used attributively, to mean “global”, or “relating to the whole world”, forming usages such as world community or world canonical texts.

By extension, a world may refer to any planet or heavenly body, especially when it is thought of as inhabited, especially in the context of science fiction or futurology.

In philosophy, the term world has several possible meanings. In some contexts, it refers to everything that makes up reality or the physical universe. In others, it can mean have a specific ontological sense. While clarifying the concept of world has arguably always been among the basic tasks of Western philosophy, this theme appears to have been raised explicitly only at the start of the twentieth century and has been the subject of continuous debate. The question of what the world is has by no means been settled.

The traditional interpretation of Parmenides work is that he argued that the everyday perception of reality of the physical world is mistaken, and that the reality of the world is One Being: an unchanging, ungenerated, indestructible whole.