Earth

Question

This piglix contains articles or sub-piglix about Earth

Answer 1

Planetarium (board game)

Planetarium is a strategy board game designed by Stéphane Vachon. The theme of the game is planetary formation and the birth of a new solar system.

In 2015, the game won the Le Plateau d'or, a game design contest presented during Les journées Ludique de Québec, an annual gaming convention held in Canada. In July 2015, the publishing rights to the game were acquired by tabletop game publisher Game Salute, and for one year Stéphane Vachon and Dann May (Chief Creative Officer at Game Salute) worked together to modify the game design, develop the game play and redesign the components. In June 2016, Planetarium launched on Kickstarter, in a campaign that ran for 3 weeks. 3,000 backers supported the project with funding raised of $126,815. The game is scheduled to be delivered to Kickstarter backers in March 2017 and will be available in the retail market 1 or 2 months later.

Matter swirls around a new born star, coalescing on the planetoids that orbit it. Planets evolve, grow and migrate in their orbits, forming a unique solar system by the end of every game. Planetarium is a game of creation, chaos and terraforming on the grandest scale.

Players are competing to crash combinations of elements onto planets that then allow them to play cards to evolve the planets in a variety of ways, with each player looking to evolve planets in the system to suit their own secret endgame goals.

On a turn a player will firstly move a matter or planet token in a clockwise direction around the star. The board is mapped with a series of lines, tracing orbits around the star, and it is along these lines that the tokens are moved. If a matter token moves onto a space occupied by planet token then the matter token is placed on the player's mat (on the respective planet). In the same way, planets can also be moved onto matter tokens, placing the matter tokens on the player's mat.

...

Wikipedia

Answer 2

Precession

Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called nutation. In physics, there are two types of precession: torque-free and torque-induced.

In astronomy, precession refers to any of several slow changes in an astronomical body's rotational or orbital parameters. An important example is the steady change in the orientation of the axis of rotation of the Earth, known as the precession of the equinoxes. (See section Astronomy below.)

Torque-free precession implies that no external moment (torque) is applied to the body. In torque-free precession, the angular momentum is a constant, but the angular velocity vector changes orientation with time. What makes this possible is a time-varying moment of inertia, or more precisely, a time-varying inertia matrix. The inertia matrix is composed of the moments of inertia of a body calculated with respect to separate coordinate axes (e.g. x, y, z). If an object is asymmetric about its principal axis of rotation, the moment of inertia with respect to each coordinate direction will change with time, while preserving angular momentum. The result is that the component of the angular velocities of the body about each axis will vary inversely with each axis' moment of inertia.

...

Wikipedia

Answer 3

Earth radius

Earth radius is the distance from the Earth's center to its surface, about 6,371 km (3,959 mi). This length is also used as a unit of distance, especially in astronomy and geology, where it is usually denoted by R_⊕.

This article deals primarily with spherical and ellipsoidal models of the Earth. See Figure of the Earth for a more complete discussion of the models. The Earth is only approximately spherical, so no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km (3,947–3,968 mi). Several different ways of modeling the Earth as a sphere each yield a mean radius of 6,371 km (3,959 mi).

While "radius" normally is a characteristic of perfect spheres, the term as used in this article more generally means the distance from some "center" of the Earth to a point on the surface or on an idealized surface that models the Earth. It can also mean some kind of average of such distances, or of the radius of a sphere whose curvature matches the curvature of the ellipsoidal model of the Earth at a given point.

An early report on the circumference of the Earth was given by Aristotle at 400,000 stadia, which exceeds the actual mean value by a factor of ten. The first scientific estimation of the radius of the Earth was given by Eratosthenes about 240 BC. Estimates of the accuracy of Eratosthenes’s measurement range from within 2% to within 15%. As with Aristotle's report, uncertainty in the accuracy of his measurement is due to modern uncertainty over which stadion definition he used.

Earth's rotation, internal density variations, and external tidal forces cause its shape to deviate systematically from a perfect sphere. Local topography increases the variance, resulting in a surface of profound complexity. Our descriptions of the Earth's surface must be simpler than reality in order to be tractable. Hence, we create models to approximate characteristics of the Earth's surface, generally relying on the simplest model that suits the need.

...

Wikipedia

Answer 4

Rare Earth hypothesis

In planetary astronomy and astrobiology, the Rare Earth Hypothesis argues that the origin of life and the evolution of biological complexity such as sexually reproducing, multicellular organisms on Earth (and, subsequently, human intelligence) required an improbable combination of astrophysical and geological events and circumstances. The hypothesis argues that complex extraterrestrial life is a very improbable phenomenon and likely to be extremely rare. The term "Rare Earth" originates from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist, both faculty members at the University of Washington.

An alternative view point was argued in the 1970s & 1980s by Carl Sagan and Frank Drake, among others. It holds that Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred-spiral galaxy. Given the principle of mediocrity (in the same vein as the Copernican principle), it is probable that the universe teems with complex life. Ward and Brownlee argue to the contrary: that planets, planetary systems, and galactic regions that are as friendly to complex life as are the Earth, the Solar System, and our region of the Milky Way are very rare.

...

Wikipedia

Answer 5

Earth orientation parameters

Earth orientation parameters (EOP) are a collection of parameters that describe irregularities in the rotation of the Earth.

The Earth's rotation is not even. Any motion in/on the Earth causes a slowdown or speedup of the rotation, or a change of rotation axis. Most of them can be ignored, but movements of very large mass, like sea current or tide can produce discernible changes and cause error to very precise astronomical observations.

A single parameter can be used to describe one phenomenon. The collection of earth orientation parameters is fitted to describe the rotation irregularities all together. Technically, they provide the rotation transforming the International Terrestrial Reference System (ITRS) to the International Celestial Reference System (ICRS), or vice versa, as a function of time.

Universal time (UT1) tracks the Earth's rotation in time, which performs one revolution in about 24 hours. The Earth's rotation is uneven, so UT is not linear with respect to atomic time. It is practically proportional to the sidereal time, which is also a direct measure of Earth rotation. The excess revolution time is called length of day (LOD).

Due to the very slow pole motion of the Earth, the Celestial Ephemeris Pole (CEP, or celestial pole) does not stay still on the surface of the Earth. The Celestial Ephemeris Pole is calculated from observation data, and is averaged, so it differs from the instantaneous rotation axis by quasi-diurnal terms, which are as small as under 0.01" (see ). In setting up a coordinate system, a static terrestrial point called the IERS Reference Pole, or IRP, is used as the origin; the x-axis is in the direction of IRM, the IERS Reference Meridian; the y-axis is in the direction 90 degrees West longitude. x and y are the coordinates of the CEP relative to the IRP.

...

Wikipedia

Answer 6

Earth%27s rotation

...

Wikipedia

Answer 7

Earth%27s shadow

...

Wikipedia

Answer 8

Spherical Earth

The earliest reliably documented mention of the spherical Earth concept dates from around the 6th century BC when it appeared in ancient Greek philosophy but remained a matter of speculation until the 3rd century BC, when Hellenistic astronomy established the spherical shape of the Earth as a physical given. The paradigm was gradually adopted throughout the Old World during Late Antiquity and the Middle Ages. A practical demonstration of Earth's sphericity was achieved by Ferdinand Magellan and Juan Sebastián Elcano's expedition's circumnavigation (1519−1522).

The concept of a spherical Earth displaced earlier beliefs in a flat Earth: In early Mesopotamian mythology, the world was portrayed as a flat disk floating in the ocean and surrounded by a spherical sky, and this forms the premise for early world maps like those of Anaximander and Hecataeus of Miletus. Other speculations on the shape of Earth include a seven-layered ziggurat or cosmic mountain, alluded to in the Avesta and ancient Persian writings (see seven climes).

The realization that the figure of the Earth is more accurately described as an ellipsoid dates to the 17th century, as described by Isaac Newton in Principia. In the early 19th century, the flattening of the earth ellipsoid was determined to be of the order of 1/300 (Delambre, Everest). The modern value as determined by the US DoD World Geodetic System since the 1960s is close to 1/298.25.

...

Wikipedia

Answer 9

Subsolar point

The subsolar point on a planet is where its sun is perceived to be directly overhead (in zenith); that is where the sun's rays are hitting the planet exactly perpendicular to its surface. It can also mean the point closest to the sun on an object in space, even though the sun might not be visible.

For planets with an orientation and rotation similar to the Earth's, the subsolar point will move westward, circling the globe once a day, but it will also move north and south between the tropics over the course of a year. The December solstice occurs when the subsolar point is on the Tropic of Capricorn and the June solstice is at the instant when the subsolar point is on the Tropic of Cancer. The March and September equinoxes occur when the subsolar point crosses the equator.

When the point passes through Hawaii, it is known as Lahaina Noon.

...

Wikipedia

Answer 10

Substorm

A substorm, sometimes referred to as a magnetospheric substorm or an auroral substorm, is a brief disturbance in the Earth's magnetosphere that causes energy to be released from the "tail" of the magnetosphere and injected into the high latitude ionosphere. Visually, a substorm is seen as a sudden brightening and increased movement of auroral arcs. Substorms were first described in qualitative terms by Kristian Birkeland which he called polar elementary storms. Sydney Chapman used the term substorm about 1960 which is now the standard term. The morphology of aurora during of a substorm was first described by Syun-Ichi Akasofu in 1964 using data collected during the International Geophysical Year.

Substorms are distinct from geomagnetic storms in that the latter take place over a period of several days, are observable from anywhere on Earth, inject a large number of ions into the outer radiation belt, and occur once or twice a month during the maximum of the solar cycle and a few times a year during solar minimum. Substorms, on the other hand, take place over a period of a few hours, are observable primarily at the polar regions, do not inject many particles into the radiation belt, and are relatively frequent — often occurring only a few hours apart from each other. Substorms can be more intense and occur more frequently during a geomagnetic storm when one substorm may start before the previous one has completed. The source of the magnetic disturbances observed at the Earth's surface during geomagnetic storms is the ring current, whereas the sources of magnetic disturbances observed on the ground during substorms are electric currents in the ionosphere at high latitudes.

Substorms can cause magnetic field disturbances in the auroral zones up to a magnitude of 1000 nT, roughly 2% of the total magnetic field strength in that region. The disturbance is much greater in space, as some geosynchronous satellites have registered the magnetic field dropping to half of its normal strength during a substorm. The most visible indication of a substorm is an increase in the intensity and size of polar auroras. Substorms can be divided into three phases: the growth phase, the expansion phase, and the recovery phase.

...

Wikipedia