Prograde motion is the rotational or orbital motion of a body in a direction similar to that of other bodies within a given system, and is sometimes called direct motion. Retrograde motion is in the contrary direction. The word 'retrograde' derives from the Latin words retro, backwards, and gradus, step. Prograde and retrograde motion - Two notations. The north orbital pole of a celestial body is defined by the right-hand rule: If you curve the fingers of your right hand a ...

Including:

• Prograde and retrograde motion - Two notations
• Prograde and retrograde motion - Retrograde orbits
• Prograde and retrograde motion - Retrograde rotation
• Prograde and retrograde motion - Retrogradation or apparent retrograde motion
• Prograde and retrograde motion - Examples
• Prograde and retrograde motion - Reference

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The Moon as seen from Earth Ammonia Carbon dioxide The Moon is the planet Earth's only natural satellite. It has no formal name other than "The Moon", although in English it is occasionally called Luna (Latin for moon), or Selene, to distinguish it from the generic "moon" (natural satellites of other planets are also called moons). Its symbol is a crescent (Unicode: ☾). The terms lunar, selene/seleno-, and cynthion (from the Lunar deities Selene and Cynthia) refer to the Moon (apo ...

Including:

• Moon - The two sides
• Moon - Orbit
• Moon - Earth & Moon
• Moon - Origin and history
• Moon - Physical characteristics
• Moon - Composition
• Moon - Selenography
• Moon - Presence of water
• Moon - Magnetic field
• Moon - Atmosphere
• Moon - Eclipses
• Moon - Occultation of stars
• Moon - Observation of the Moon
• Moon - Exploration of the Moon
• Moon - Human understanding of the Moon
• Moon - Myth and folk culture
• Moon - The Moon as muse
• Moon - Astrology
• Moon - Scientific understanding
• Moon - Meteor impact on the Moon
• Moon - Legal status
• Moon - Satellites
• Moon - Surface installations
• Moon - Lunar location listings

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(332,950 Earths) (27.9 g) The Sun (or Sol) is the star at the center of our Solar system. Earth orbits the Sun, as do many other bodies, including other planets, asteroids, meteoroids, comets and dust. Its heat and light support almost all life on Earth. The Sun is a ball of plasma with a mass of about 2×1030 kg, which is somewhat higher than that of an average star. About 74% of its mass is hydrogen, with 25% helium and the rest made up of trace quantities of heavier elements. It is thou ...

Including:

• Sun - General information
• Sun - Structure
• Sun - Core
• Sun - Radiation zone
• Sun - Convection zone
• Sun - Photosphere
• Sun - Temperature minimum
• Sun - Chromosphere
• Sun - Corona
• Sun - Theoretical problems
• Sun - Solar neutrino problem
• Sun - Coronal heating problem
• Sun - Faint young sun problem
• Sun - Magnetic field
• Sun - Position of the Sun through the year
• Sun - Solar space missions
• Sun - History and future of the Sun
• Sun - Human understanding of the Sun
• Sun - The Sun as a power source
• Sun - Sun and eye damage

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Zecharia Sitchin was born in Russia and raised in Palestine, where he acquired a knowledge of modern and ancient Hebrew, other Semitic and European languages, the Old Testament, and the history and archaeology of the Near East. Sitchin attended and graduated from the University of London, majoring in economic history. A journalist and editor in Israel for many years, he now lives and writes in New York. His books have been widely translated, converted to Brail ...

Including:

• Zecharia Sitchin - Theories
• Zecharia Sitchin - Tiamat Planet Theory
• Zecharia Sitchin - Annunaki
• Zecharia Sitchin - Enki
• Zecharia Sitchin - Anu
• Zecharia Sitchin - Works

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The English suffix -ology or -logy denotes a field of study or academic discipline, and -ologist describes a person who studies that field. -logy - Etymology. The word ology is a back-formation from the names of these disciplines. "-logy" basically means "the study of ____". Such words are formed from Greek or Latin roots with the terminal -logy derived from the Greek suffix -λογια (-logia), speaking, from λεγειν (legein), ...

Including:

• -logy - Etymology
• -logy - Other words ending in ology
• -logy - Usage
• -logy - List of -ologies
• -logy - A
• -logy - B
• -logy - C
• -logy - D
• -logy - E
• -logy - F
• -logy - G
• -logy - H
• -logy - I
• -logy - J
• -logy - K
• -logy - L
• -logy - M
• -logy - N
• -logy - O
• -logy - P
• -logy - R
• -logy - S
• -logy - T
• -logy - U
• -logy - V
• -logy - X
• -logy - Z
• -logy - Ologies that are not fields of study

Read more here: » -logy: Encyclopedia - -logy

The Abenaki (also Wabanaki) are a Native American tribe located in the northeastern United States. Religious ceremonies are led by shamans, called Medeoulin (Mdawinno). The history of the Abenaki people is divided into three time periods. In the first, the Ancient Age, humanity and animal-life are undifferentiated. In the second, the Golden Age, animals are still humans, but quantitatively different. In the third, the Present Age, animals and humanity are totally differentiated. Abenaki mythology - Beings of the Golden A ...

Including:

• Abenaki mythology - Beings of the Golden Age
• Abenaki mythology - Gluskab and Malsumis
• Abenaki mythology - Beings of the Present Age
• Abenaki mythology - External link

Read more here: » Abenaki mythology: Encyclopedia - Abenaki mythology

In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us. It allows the overall brightnesses of objects to be compared without regard to distance. The absolute magnitude uses the same convention as the visual magnitude, with a ~2.512 difference in brightness between step rates (because 2.5125 ≈ 100). The Milky Way, for example, has an absolute magnitude of about −20.5. So a quasar at an absolute magnitude of −25.5 is 100 ...

Including:

• Absolute magnitude - Absolute Magnitude for stars and galaxies M
• Absolute magnitude - Computation
• Absolute magnitude - Apparent magnitude
• Absolute magnitude - Absolute Magnitude for planets H
• Absolute magnitude - Calculations
• Absolute magnitude - Apparent magnitude

Read more here: » Absolute magnitude: Encyclopedia - Absolute magnitude

The solar system comprises our Sun and the retinue of celestial objects gravitationally bound to it. Traditionally, this is said to consist of the Sun, nine planets and their 158 currently known moons; however, a large number of other objects, including asteroids, meteoroids, planetoids, comets, and interplanetary dust, orbit the Sun as well. Although the term "solar system" is frequently applied to other star systems and the planetary systems which may comprise them, it should strictly refer to our system specifically: the wor ...

Including:

• Solar system - Structure and layout of the solar system
• Solar system - Origin and age of the solar system
• Solar system - Regions of the solar system
• Solar system - Interplanetary medium
• Solar system - The inner planets
• Solar system - The asteroid belt
• Solar system - The outer planets
• Solar system - The trans-Neptunian region
• Solar system - And beyond
• Solar system - Galactic orbit of the solar system
• Solar system - Planetary system formation
• Solar system - Discovery of the solar system
• Solar system - Exploration of the solar system
• Solar system - Attributes of major planets
• Solar system - Attributes of the largest minor planets
• Solar system - Other facts

Read more here: » Solar system: Encyclopedia - Solar system

The Torino Scale uses a scale from 0 to 10. A 0 indicates an object has a negligibly small chance of collision with the Earth, compared with the usual "background noise" of collision events, or is too small to penetrate the Earth's atmosphere intact. A 10 indicates that a collision is certain, and the impacting object is large enough to precipitate a global disaster. There are no fractional values or decimal values used. An object is assigned a 0 to 10 value based on its collision probability and its kinetic energy (expressed in megat ...

See also:

Torino Scale, Torino Scale - Overview, Torino Scale - History, Torino Scale - Current Torino scale, Torino Scale - Objects with high Torino ratings

Read more here: » Torino Scale: Encyclopedia II - Torino Scale - Overview

Trajectory - Uniform gravity no drag or wind. The case of uniform gravity, disregarding drag and wind, yields a trajectory which is a parabola. To model this, one chooses V = mgz, where g (gee) is the so-called acceleration of gravity. This gives the equations of motion Simplifications are made for the sake of studying the basics. The actual situation, at least on the surface of Earth ...

See also:

Trajectory, Trajectory - Physics of trajectories, Trajectory - Examples, Trajectory - Uniform gravity no drag or wind, Trajectory - Uphill/downhill in uniform gravity in a vacuum, Trajectory - Orbitting objects

Read more here: » Trajectory: Encyclopedia II - Trajectory - Examples

Under standard assumptions the orbital velocity () of a body traveling along elliptic orbit can be computed as: where: is standard gravitational parameter, is radial distance of orbiting body from central body, is length of semi-major axis. Conclusion: Velocity does not depend on eccentricity but is determined by length of semi-major axis (), Velocity equation is similar to that for hyperbolic trajectory with the differenc ...

See also:

Elliptic orbit, Elliptic orbit - Velocity, Elliptic orbit - Orbital period, Elliptic orbit - Energy, Elliptic orbit - Flight path angle, Elliptic orbit - Equation of motion, Elliptic orbit - Orbital parameters, Elliptic orbit - Solar system

Read more here: » Elliptic orbit: Encyclopedia II - Elliptic orbit - Velocity

In the past 600 million years there have been five major mass extinctions that on average extinguished half of all species. The largest mass extinction to have affected life on Earth was the Permian-Triassic one that ended the Permian period 250 million years ago and killed off 90% of all species. The last such mass extinction led to the demise of the dinosaurs and has been found to have coincided with a large asteroid impact; this is the Cretaceous-Tertiary (K-T) extinction event. There is no solid evidence of impacts leading to the four other major mass extinctions, though many ...

See also:

Impact event, Impact event - The geology of Earth-impact events, Impact event - Mass extinctions and impacts, Impact event - Recent pre-historic impact events, Impact event - Modern impact events, Impact event - End of civilization, Impact event - Impact Events in Fiction, Impact event - Notes

Read more here: » Impact event: Encyclopedia II - Impact event - Mass extinctions and impacts

After methane, ethane is the second-largest component of natural gas. Natural gas from different gas fields varies in ethane content from less than 1% to over 6% by volume. Prior to the 1960s, ethane was typically not separated from the methane component of natural gas, but simply burnt along with the methane as a fuel. Today, however, ethane is an important petrochemical feedstock, and ethane is separated from the other components of natural gas in most well-developed gas fields. Ethane and heavier hydrocarbons can be separated from ...

See also:

Ethane, Ethane - History, Ethane - Chemistry, Ethane - Ocurrence and use, Ethane - Extraterrestrial ethane

Read more here: » Ethane: Encyclopedia II - Ethane - Ocurrence and use

There are currently six methods of detecting extrasolar planets which are too faint relative to their much brighter host stars to be directly detected by present conventional optical means. The planned Space Interferometry Mission, Terrestrial Planet Finder and Darwin would all try to examine planets in a more direct fashion. Extrasolar planet - Pulsar timing. The first method used to discover extra-solar planets was to observe anomalies in the regularity of pulses from a pulsar. This led to the 'di ...

See also:

Extrasolar planet, Extrasolar planet - History of detection, Extrasolar planet - Methods of detection, Extrasolar planet - Pulsar timing, Extrasolar planet - Astrometry, Extrasolar planet - Radial velocity, Extrasolar planet - Gravitational microlensing, Extrasolar planet - Transit method, Extrasolar planet - Circumstellar disks, Extrasolar planet - Direct observation, Extrasolar planet - Solar system formation processes, Extrasolar planet - Notable extrasolar planets, Extrasolar planet - Table of extremes

Read more here: » Extrasolar planet: Encyclopedia II - Extrasolar planet - Methods of detection

Colonization of the outer solar system - Europa. The Artemis Project designed a plan to colonize Europa. Scientists were to inhabit igloos and drill down into the Europan ice crust, exploring any sub-surface ocean. It also discusses use of "air pockets" for human inhabitation. This Europa surface base would also make use of inflatable structures. Exploration of Europa and its subglacial ocean would be carried out with submarines. There are some difficulties related to colonization of Europa; one significan ...

See also:

Colonization of the outer solar system, Colonization of the outer solar system - Asteroids, Colonization of the outer solar system - Ceres, Colonization of the outer solar system - Jupiter, Colonization of the outer solar system - Europa, Colonization of the outer solar system - Ganymede, Colonization of the outer solar system - Callisto, Colonization of the outer solar system - Saturn, Colonization of the outer solar system - Titan, Colonization of the outer solar system - Uranus, Colonization of the outer solar system - Neptune, Colonization of the outer solar system - Kuiper Belt and Oort Cloud

Read more here: » Colonization of the outer solar system: Encyclopedia II - Colonization of the outer solar system - Jupiter

The Roche limit depends on the rigidity of the satellite. At one extreme, a rigid satellite will maintain its shape until tidal forces break it apart. At the other extreme, a highly fluid satellite gradually deforms with increasing tidal forces until it breaks apart. For a rigid spherical satellite, the cause of the rigidity is neglected, in that the material constituting the satellite is still treated as though held together only by its own self-gravity. Other effects are also neglected, such as tidal deformation of the primary, and rotation of the satellite. The Roche limit, < ...

See also:

Roche limit, Roche limit - Determining the Roche limit, Roche limit - Rigid satellites, Roche limit - Fluid satellites, Roche limit - Roche limits for selected examples, Roche limit - External link

Read more here: » Roche limit: Encyclopedia II - Roche limit - Determining the Roche limit

The Planck Epoch covers the time from 10-43 to 10-35 seconds after the Big Bang. The temperature during this epoch is estimated to decrease from 1032 K to 1027 K. 10-43 seconds A length of 10-43 seconds is known as Planck time. At this point, the force of gravity separated from the other three forces, collectively known as the electronuclear force. A complete theory of quantum gravity such as superstring theory is needed to understand these very earl ...

See also:

Timeline of the Big Bang, Timeline of the Big Bang - Introduction, Timeline of the Big Bang - Overview, Timeline of the Big Bang - The Big Bang and matter formation, Timeline of the Big Bang - The Primordial Age - from 0 years to 379000 years, Timeline of the Big Bang - Planck Epoch, Timeline of the Big Bang - Galaxy and star formation, Timeline of the Big Bang - The Stelliferous Age - from 10⁶ to 10¹⁴ years, Timeline of the Big Bang - Near-term future of the Universe - three different scenarios, Timeline of the Big Bang - Scenario A: The Big Rip, Timeline of the Big Bang - Scenario B: The Heat death of the Universe, Timeline of the Big Bang - Scenario C: The Big Crunch, Timeline of the Big Bang - Long-term future for a long-lived Universe, Timeline of the Big Bang - The Degenerate Age - from 10¹⁴ to 10⁴⁰ years, Timeline of the Big Bang - The Black Hole Age - from 10⁴⁰ years to 10¹⁰⁰ years, Timeline of the Big Bang - Ultimate fate for a long-lived Universe, Timeline of the Big Bang - The Dark Age - from 10¹⁰⁰ years until 10¹⁵⁰ years, Timeline of the Big Bang - The Photon Age - from 10¹⁵⁰ years until the Distant Future

Read more here: » Timeline of the Big Bang: Encyclopedia II - Timeline of the Big Bang - Planck Epoch

Most TNOs are lumps of ice with some organic (carbon-containing) material such as tholin, detected using spectroscopy. They are of the same composition as comets and many astronomers believe them to be just comets. The distinction between comet and asteroid is not yet clear and there is a substantial uncertainty, inhabited by such objects as 2060 Chiron. It is difficult to estimate the diameter of TNOs. For objects with very well known orbital elements (namely, Pluto and Charon ...

See also:

Trans-Neptunian object, Trans-Neptunian object - Notable trans-Neptunian objects, Trans-Neptunian object - Size and composition, Trans-Neptunian object - Largest discoveries

Read more here: » Trans-Neptunian object: Encyclopedia II - Trans-Neptunian object - Size and composition

The classical "Big Five" mass extinctions identified by Raup and Sepkoski (1982) are widely agreed upon as some of the most significant: End Ordovician, Late Devonian, End Permian, End Triassic, and End Cretaceous. These and a selection of other extinction events are highlighted below: 488 million years ago — a series of mass extinctions at the Cambrian-Ordovician transition (the Cambrian-Ordovician extinction events) eliminated many brachiopods and conodonts an ...

See also:

Extinction event, Extinction event - Extinction events, Extinction event - Causes for Mass Extinction, Extinction event - Postulated extinction cycles

Read more here: » Extinction event: Encyclopedia II - Extinction event - Extinction events

While there is much disagreement between current definitions of "planet", most focus on three general criteria: that it must orbit a star, be above a certain size (usually large enough to be rounded by its own gravity), and yet not be large enough to commence nuclear fusion. Each of these criteria has been challenged by various discoveries, outlined below. Definition of planet - Minor planets. Within our planetary system, the nine objects currently accepted as planets orbit the Sun wit ...

See also:

Definition of planet, Definition of planet - History and etymology, Definition of planet - Issues and controversies, Definition of planet - Minor planets, Definition of planet - Double planets, Definition of planet - Rogue planets or sub-stars, Definition of planet - History vs. science

Read more here: » Definition of planet: Encyclopedia II - Definition of planet - Issues and controversies

Terraforming - Mars. There is some scientific debate over whether it would even be possible to terraform Mars, or how stable its climate would be once terraformed. It is possible that over geological timescales - tens or hundreds of millions of years—Mars could lose its water and atmosphere again, possibly to the same processes that reduced it to its current state. Indeed, it is thought that Mars once did have a relatively Earthlike environment early in its history, with a thicker atmosphere and abundant ...

See also:

Terraforming, Terraforming - History of scholarly study, Terraforming - Ethical issues, Terraforming - Theoretical methods of terraforming, Terraforming - Mars, Terraforming - Venus, Terraforming - Other worlds, Terraforming - Paraterraforming, Terraforming - In fiction, Terraforming - Prose, Terraforming - Television and film, Terraforming - Miscellanea

Read more here: » Terraforming: Encyclopedia II - Terraforming - Theoretical methods of terraforming

Cretaceous-Tertiary extinction event - Alvarez hypothesis. In 1980, a team of researchers led by Nobel-prize-winning physicist Luis Alvarez, his son, geologist Walter Alvarez, and a group of colleagues discovered that fossilized sedimentary layers found all over the world at the Cretaceous-Tertiary boundary, 65.5 million years ago contain a concentration of iridium hundreds of times greater than normal. The end of the Cretaceous coincided with the end of the dinosaurs. It was in general a period of extraordinary ...

See also:

Cretaceous-Tertiary extinction event, Cretaceous-Tertiary extinction event - Casualties of the extinction, Cretaceous-Tertiary extinction event - Theories, Cretaceous-Tertiary extinction event - Alvarez hypothesis, Cretaceous-Tertiary extinction event - Chicxulub crater, Cretaceous-Tertiary extinction event - Deccan traps, Cretaceous-Tertiary extinction event - Multiple impact event, Cretaceous-Tertiary extinction event - Supernova hypothesis, Cretaceous-Tertiary extinction event - Further skepticism, Cretaceous-Tertiary extinction event - Other mass extinctions

Read more here: » Cretaceous-Tertiary extinction event: Encyclopedia II - Cretaceous-Tertiary extinction event - Theories