- Absolute magnitude: In astronomy, absolute magnitude is the apparent magnitude an object would have if it were at a standard luminosity distance away from us, in the absence of interstellar extinction. It allows the overall brightness 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 steps in magnitude. 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 times brighter than our galaxy (because 2.5125 ? 100). If this particular quasar and our galaxy could be seen side by side at the same distance, the quasar would be 5 magnitudes (or 100 times) brighter than our galaxy.
- Astrolabe: It is a historical astronomical instrument used by classical astronomers, navigators, and astrologers. Its many uses included locating and predicting the positions of the Sun, Moon, planets and stars; determining local time given local longitude and vice-versa; surveying; and triangulation. It is not the same as the mariner's astrolabe. While the astrolabe could be useful for determining latitude on land, it was an awkward instrument for use on the heaving deck of a ship or in wind. The mariner's astrolabe was developed to address these issues.
- Atmosphere: It is a layer of gases that may surround a material body of sufficient mass. The gases are attracted by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low. Some planets consist mainly of various gases, and therefore have very deep atmospheres. The term stellar atmosphere is used for the outer region of a star, and typically includes the portion starting from the opaque photosphere outwards.
- Auroras (North/South Polar Lights; or aurorae: They are natural coloured light displays, which are usually observed in the night sky, particularly in the polar zone. The aurora typically occurs in the ionosphere. Some scientists call them "polar auroras" (or "aurorae polares"). In northern latitudes, it is known as the aurora borealis (or the northern lights. It often appears as a greenish glow or sometimes a faint red, as if the sun was rising from an unusual direction. The aurora borealis is also called the northern polar lights, as it is only visible in the North sky from the Northern Hemisphere. Its southern counterpart, the aurora australis/southern polar lights, has similar properties.
- Baryogenesis: It is the generic term for hypothetical physical processes that produced an asymmetry between baryons and anti-baryons in the very early universe, resulting in the substantial amounts of residual matter that make up the universe today.
- Baryonic matter: It is matter composed mostly of baryons, which includes atoms of any sort (nearly all matter that we may encounter everyday, including our bodies). The distinction between baryonic and non-baryonic matter is important in cosmology, because Big Bang nucleosynthesis models set tight constraints on the amount of baryonic matter present in the early universe. The existence of baryons is a significant issue in cosmology, since it is assumed that the Big Bang produced a state with equal amounts of baryons and anti-baryons. The process by which baryons come to outnumber their antiparticles is called baryogenesis.
- Black holes
How are they created? Take a star with a mass about 10 times the mass of
the sun; during its lifetime-billions of years- heat will be generated by
fusion of its hydrogen to produce helium. The energy released will create
a pressure to counterbalance the gravity effect and the star -with a radius
of five times the sun radius- will be stable. When the hydrogen is exhausted,
the outwards pressure will fall and the star will collapse on itself due
to its own gravity to a sphere of radius around 30 kilometres. The escape
velocity at its surface that was about 1,000 kilometres per second will
increase to something like 300,000 kilometres per second, which is the velocity
of light. At that point nothing, no even light, can escape from the collapsed
star.
The star is now a black hole that is a region of space-time from which nothing can escape and its boundary is called the "event horizon". Black holes of that size have been detected experimentally. Many smaller ones resulting not of the collapse of big stars but by the collapse of highly compressed regions in the hot, dense medium that have existed moment after the Big bang. Such a black hole weighting a billion tons (the mass of a mountain) would have a radius of 10exp-13 centimetre -the size of a proton or neutron.
There is a connection between black holes and thermodynamics in the sense that the surfaces of their event horizons always increase when additional matter or radiation fall into the black holes. Moreover if two black holes merge, the area of the event horizon of the new black hole is greater that the sum of the surfaces of the original black holes
- Bolometric magnitude: In defining absolute magnitude it is necessary to specify the type of electromagnetic radiation being measured. When referring to total energy output, the proper term is bolometric magnitude. The bolometric magnitude can be computed from the visual magnitude plus a bolometric correction, Mbol = MV + BC. This correction is needed because very hot stars radiate mostly ultraviolet radiation, while very cool stars radiate mostly infrared radiation. The dimmer an object (at a distance of 10 parsecs) would appear, the higher its absolute magnitude. The lower an object's absolute magnitude, the higher its luminosity. A mathematical equation relates apparent magnitude with absolute magnitude, via parallax.
- Chaotic inflation theory or bubble universe: This model is a variant of the inflationary model of the big bang. This model postulates that our universe is one of many that grew from a multiverse consisting of vacuum that had not decayed to its ground state.
- Chromosphere (colour sphere): It is a thin layer of the Sun's atmosphere just above the photosphere, roughly 10,000 kilometres deep. The chromosphere is more visually transparent than the photosphere. The coloration may be seen directly with the naked eye only during a total solar eclipse, where the chromosphere is briefly visible as a flash of colour just as the visible edge of the photosphere disappears behind the Moon. The temperature of the chromosphere is hotter than that of the photosphere. The photosphere is closer to the surface of the sun and its temperature is around 4000K to 6400K but the chromosphere is about 4500K to as high as 20,000K.
- Corona: It is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometres into space, most easily seen during a total solar eclipse, but also observable in a coronagraph. The corona is divided into three parts. The K-corona (K for continuum) interfaces directly with the chromosphere and is created by sunlight scattering off electrons. The E-corona (E for emission) contains abundant calcium and iron. The F-corona (F for Fraunhofer) is created by sunlight bouncing off dust particles.
- Coronal loops: They form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle, it is for this reason coronal loops are often found with sunspots at their footpoints. The upwelling magnetic flux pushes through the photosphere, exposing the cooler plasma below. The contrast between the photosphere and solar interior gives the impression of dark spots, or sunspots.
- Coronal mass ejection (CME): it is an ejection of material from the solar corona. The ejected material is a plasma consisting primarily of electrons and protons (in addition to small quantities of heavier elements such as helium, oxygen, and iron), plus the entrained coronal magnetic field.
- Cosmic Censorship Hypotheses (weak and the strong): These are two mathematical conjectures about the structure of singularities arising in general relativity. Singularities that arise in the solutions of Einstein's equations are hidden within event horizons, and therefore cannot be seen from the rest of spacetime. Singularities which are not so hidden are called naked. The weak cosmic censorship hypothesis conjectures that no naked singularities other than the Big Bang singularity exist in the universe. The hypothesis was conceived by Roger Penrose in 1969.
- Cosmic microwave background radiation (most often abbreviated CMB: It is a form of electromagnetic radiation that fills the entire universe. It has a thermal black body spectrum at a temperature of 2.725 Kelvin. Thus the spectrum peaks in the microwave range at a frequency of 160.2 GHz, corresponding to a wavelength of 1.9 mm. Most cosmologists consider this radiation to be the best evidence for the Big Bang model of the universe.
Gamov also thought that some of the residual heat produced by the big bang is still in circulation in the universe today. If this were true it would be the second proof of the Big Bang Theory. The first calculations showed that today's temperature of this radiation should be around5 degree Kelvin (5degrees above absolute 0). Later measurements showed that is was 2.7 degrees. This radiation is in the micro range and is similar to a "Black Body Radiation".
- Cosmological horizon: It marks a limit to observability, and marks the boundary of a region that an observer cannot see into directly due to cosmological effects. The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model being discussed. The cosmological horizon is a maximal limit of perception and not an actual boundary. This is sometimes referred to as the "observable universe", and it has been said that the observable universe is many orders of magnitude smaller than the greater universe that lies beyond the limits of our perception. Imagine that the entire cosmological horizon is modelled by a sphere that is the diameter of a quarter. If Alan Guth's inflationary model of early era cosmology is correct, the universe that lies beyond this "quarter-sized" horizon would conservatively be a sphere as large as the Earth globe itself.
- Cosmological time: This is the timeline of the Big Bang that describes the events according to the scientific theory of the Big Bang, using the cosmological time parameter of comoving coordinates. Observations suggest that the universe as we know it began around 13.7 billion years ago.
- Curved space: It refers to a spatial geometry which is not "flat" where a flat space is described by Euclidean Geometry. Curved spaces can generally be described by Riemannian Geometry though some simple cases can be described in other ways. Curved spaces play an essential role in General Relativity where gravity is often visualized as curved space. The Friedmann-Lemaître-Robertson-Walker metric is a curved metric which forms the current foundation for the description of the expansion of space and shape of the universe
- Cold dark matter (or CDM): It is a refinement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation and hence is dark while at the same time the particles making up this matter are slow and hence are cold. As of 2006, most cosmologists favour the cold dark matter theory as a description of how the universe went from a smooth initial state at early times (as shown by the cosmic microwave background radiation), to the lumpy distribution of galaxies and their clusters we see today - the large-scale structure of the universe. In the cold dark matter theory, structure grows hierarchically, with small objects collapsing first and merging in a continuous hierarchy to form more and more massive objects.
- Direct motion: It is the motion of a planetary body in a direction similar to that of other bodies within its system, and is sometimes called prograde motion. Retrograde motion is motion in the opposite direction. While the terms direct and prograde are equivalent in this context, the former is the traditional term in astronomy.
- Earth's atmosphere: It consists, from the ground up, of the troposphere (which includes the planetary boundary layer or peplosphere as lowest layer), stratosphere, mesosphere, ionosphere (or thermosphere), exosphere and the magnetosphere. Three quarters of the atmosphere lies within the troposphere, and the depth of this layer varies between 17 km at the equator and 7 km at the poles. The ozone layer, which absorbs ultraviolet energy from the Sun, is located primarily in the stratosphere, at altitudes of 15 to 35 km. The Kármán line, located within the thermosphere at an altitude of 100 km, is commonly used to define the boundary between the Earth's atmosphere and outer space. However, the exosphere can extend from 500 up to 10,000 km above the surface, where it interacts with the planet's magnetosphere.
. Troposphere: It is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and almost all of its water vapour and aerosols. The average depth of the troposphere is about 11 km in the middle latitudes. It is deeper in the tropical regions (up to 20 km) and shallower near the poles (about 7 km in summer, indistinct in winter).
. Stratosphere: It is the second layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler higher up and warmer farther down. The border of the troposphere and stratosphere, the tropopause, is marked by where this inversion begins. The stratosphere is situated between about 10 km and 50 km altitude above the surface at moderate latitudes, while at the poles it starts at about 8 km altitude. The stratosphere is layered in temperature because it is heated from above by absorption of ultraviolet radiation from the Sun. The top of the stratosphere has a temperature of about 270 K (?3°C). The heating is caused by an ozone layer that absorbs solar ultraviolet radiation, heating the upper layers of the stratosphere. The base of the stratosphere occurs where heating by conduction from above and heating by convection from below (through the troposphere) balance out; hence, the stratosphere begins at lower altitudes near the poles due to the lower ground temperature there.
. Mesosphere: it is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. The mesosphere is located from about 50 km to 80-90 km altitude above Earth's surface. Within this layer, temperature decreases with increasing altitude. The main dynamical features in this region are atmospheric tides, internal atmospheric gravity waves (usually just called "gravity waves") and planetary waves. Most of these waves and tides are excited in the troposphere and lower stratosphere and propagate upward to the mesosphere. Temperatures in the upper mesosphere fall as low as -100°c, varying according to latitude and season. Millions of meteors burn up daily in the mesosphere as a result of collisions with the gas particles contained there; this creates enough heat to vaporize almost all of the falling objects long before they reach the ground, resulting in a high concentration of iron and other metal atoms there.
. Thermosphere: It is the layer of the earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. It is the fourth atmospheric layer from earth. It begins about 80 km above the earth. At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation by the small amount of residual oxygen still present. Temperatures are highly dependent on solar activity, and can rise to 2,000°C. Radiation causes the air particles in this layer to become electrically charged, enabling radio waves to bounce off and be received beyond the horizon. At the exosphere, beginning at 500 to 1,000km above the earth's surface, the atmosphere blends into space.
. Exosphere: It is the uppermost layer of the atmosphere. Its lower boundary at the edge of the thermosphere is estimated to be 500 km to 1000 km above the Earth's surface, and its upper boundary at about 10,000 km. It is only from the exosphere that atmospheric gases, atoms, and molecules can, to any appreciable extent, escape into space. The main gases within the exosphere are the lightest gases, mainly hydrogen, with some helium, carbon dioxide, and atomic oxygen near the exobase. The exosphere is the last layer before space.
We also have:
. Ignorosphere: Because it lies between the maximum altitude for aircraft
and the minimum altitude for orbital spacecraft, this region of the atmosphere
has only been accessed through the use of sounding rockets. As a result,
it is the most poorly understood part of the atmosphere. This has led the
mesosphere and the lowest thermosphere to be despairingly referred to by
scientists as the ignorosphere
. Ionosphere: It is the uppermost part of the atmosphere, distinguished
because it is ionized by solar radiation. It is the inner edge of the magnetosphere.
It has practical importance because, among other functions, it influences
radio propagation to distant places on the Earth. It is located in the Thermosphere.
. Magnetosphere: It is the region around an astronomical object in which
phenomena are dominated or organized by its magnetic field. Earth is surrounded
by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn,
Uranus and Neptune. Jupiter's moon Ganymede is magnetized, but too weakly
to trap solar wind plasma. Mars has patchy surface magnetization. The term
"magnetosphere" has also been used to describe regions dominated
by the magnetic fields of celestial objects, e.g. pulsar magnetospheres.
. Peplosphere: The planetary boundary layer (PBL), also known as the atmospheric
boundary layer (ABL) or peplosphere, is the lowest part of the atmosphere
and its behaviour is directly influenced by its contact with a planetary
surface.
. Photosphere of an astronomical object is the region from which externally
received light comes. It extends into a star's surface until the gas becomes
opaque. The photosphere is the region where an object stops being transparent
to ordinary light.
. Tropopause: It is the boundary between the troposphere and the stratosphere.
- Eclipse (Ancient Greek "I vanish." "I leave" is an astronomical event that occurs when one celestial object moves into the shadow of another. For instance a solar eclipse, when the Moon's shadow crosses the Earth's surface, or a lunar eclipse occurs when the Moon moves into the shadow of Earth. However, it can also refer to such events beyond the Earth-Moon system: for example, a planet moving into the shadow cast by one of its moons, a moon passing into the shadow cast by its parent planet, or a moon passing into the shadow of another moon.
- Ecliptic: It is the apparent path that the Sun traces out in the sky, as it appears to move in the sky in relation to the stars, this apparent path aligns with the planets throughout the course of the year. It should be distinguished from the invariable ecliptic plane, which is the vector sum of the angular momenta of all planetary orbital planes, to which Jupiter is the main contributor.
- Ecliptic plane: It is the geometric plane containing the mean orbit of the Earth around the Sun.
- Energy density: It is the amount of energy stored in a given system or region of space per unit volume, or per unit mass, depending on the context. Energy density per unit volume has the same physical units as pressure, and in many circumstances is an exact synonym: for example, the energy density of the magnetic field may be expressed as (and behaves as) a physical pressure, and the energy required to compress a gas may be determined by multiplying the pressure of the compressed gas times its change in volume.
- Exobase, also called the critical level, the lowest altitude of the exosphere,
is defined in one of two ways:
1. The height above which there are negligible atomic collisions between
the particles and
2. The height above which the constituent atoms are on purely ballistic
trajectories.
- Galactic Centre: It is the rotational centre of the Milky Way galaxy. It is located about 24,800 light years away from the Earth, in the direction of the constellation Sagittarius, where the Milky Way appears brightest. Scientists hypothesize that a supermassive black hole lies in the Galactic Centre of the Milky Way, and most (if not all) other galaxies.
- Gegenschein: (German for counter shine), is a faint brightening of the night sky in the region of the ecliptic directly opposite the Sun.
- Gravitational singularity (sometimes spacetime singularity): It is a place where quantities which are used to measure the gravitational field become infinite. Such quantities include the curvature of spacetime or the density of matter. The two most important types of spacetime singularities are curvature singularities and conical singularities. Singularities can also be divided according to whether they are covered by an event horizon or not (naked singularities). According to general relativity, the initial state of the universe, at the beginning of the Big Bang, was a singularity. Another type of singularity predicted by general relativity is inside a black hole.
- Halo: The term galactic halo is used to denote an extended, roughly spherical
component of a galaxy, which extends beyond the main, visible component.
It can refer to any of several distinct components which share these properties:
. The galactic spheroid (stars): A spiral galaxy is a galaxy belonging to
one of the three main classes of galaxy originally described by Edwin Hubble.
Spiral galaxies consist of a flat, rotating disk of stars, gas and dust,
and a central concentration of stars known as the bulge. These are surrounded
by a much fainter halo of stars, many of which reside in globular clusters.
. Most of the mass of any galaxy is dominated by a component concentrated
at the centre of the galaxy but dominating its dynamics throughout, known
as the dark matter halo.
The distinction between the halo and the main body of the galaxy is clearest in spiral galaxies, where the spherical shape contrasts with the flat disc. In an elliptical galaxy, there is no sharp transition between the body of the galaxy and the halo.
- Hubble Constant: The value of the Hubble constant is estimated by measuring the redshift of distant galaxies and then determining the distances to the same galaxies (by some other method than Hubble's law). Uncertainties in the physical assumptions used to determine these distances have caused varying estimates of the Hubble constant. For most of the second half of the 20th century the value of H0 was estimated to be between 50 and 90 (km/s)/Mpc.
- Interstellar medium (or ISM): It is the gas and dust that pervade interstellar space: the matter that exists between the stars within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic medium. The interstellar medium consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and (galactic) magnetic fields. The matter consists of about 99% gas and 1% dust by mass. Densities range from a few thousand to a few hundred million particles per cubic meter with an average value in the Milky Way Galaxy of a million particles per cubic meter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium by number of nuclei, with additional heavier elements present in trace amounts.
- Intrinsic energy: The simplest explanation for dark energy is that it is simply the "cost of having space": that is, a volume of space has some intrinsic, fundamental energy.
- Invariable ecliptic plane: It is the vector sum of the angular momenta of all planetary orbital planes, to which Jupiter is the main contributor.
- Kármán line: It lies at an altitude of 100 km above the Earth's surface, and is commonly used to define the boundary between the Earth's atmosphere and outer space. Around this altitude the Earth's atmosphere becomes too thin for aeronautic purposes. Also, there is an abrupt increase in atmospheric temperature and interaction with solar radiation.
- Light-year: It is a unit of measurement of length, specifically the distance
that light travels in a vacuum in one year. While there is no authoritative
decision on which year is used, the International Astronomical Union (IAU)
recommends the Julian year.
A light-year is equal to:
9,460,730,472,580.8 km (about 9.461 Pm)
5,878,625,373,183.61 statute miles
about 63,241 astronomical units
about 0.3066 parsecs
- Lunar eclipse: occurs whenever the Moon passes through some portion of the Earth's shadow. This can occur only when the sun, Earth, and moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, the Moon is always full near a lunar eclipse.
- Magnetic reconnection: It is the process whereby magnetic field lines from different magnetic domains are spliced to one another, changing their patterns of connectivity with respect to the sources. It is a violation of an approximate conservation law in plasma physics, and can concentrate mechanical or magnetic energy in both space and time. Solar flares, the largest explosions in the solar system, may involve the reconnection of large systems of magnetic flux on the Sun, releasing in minutes energy that is stored in the magnetic field over a period of hours to days. Magnetic reconnection in Earth's magnetosphere is one of the mechanisms responsible for the aurora, and it is important to the science of controlled nuclear fusion because it is one mechanism preventing magnetic confinement of the fusion fuel.
- Milky Way, sometimes referred as "the Galaxy", is a barred spiral galaxy visible from Earth as a band of light in the night sky. Although the Milky Way is one of billions of galaxies in the observable universe, the Galaxy has special significance to humanity as it is the home galaxy of the planet Earth.
- Mirror Universe (MU) is a fictional parallel universe in which the plots of several Star Trek television episodes take place. It is named for "Mirror, Mirror", the original series episode in which it first appeared.
- Missing mass problem: Some hard-to-detect baryonic matter makes a contribution to dark matter, but constitutes only a small portion. Determining the nature of this missing mass is o ne of the most important problems in modern cosmology and particle physics. The names "dark matter" and "dark energy" serve mainly as expressions of human ignorance.
- Multiverse: According to this theory a small part of a universe can inflate producing a "baby universe" which, in its turn, can produce other universes. This process can go on for ever, each universe giving birth to new universes. This, of course, means that big bangs happen all the time and all universes are like bubbles floating in a sea of bubbles. This is what is known as "multiverse" and the theory is called "Chaotic Inflation".
- Naked singularity: It is a gravitational singularity without an event horizon. The singularities inside black holes are always surrounded by an area which does not allow light to escape, and therefore cannot be directly observed. A naked singularity, by contrast, is observable from the outside. They important because their existence would mean that it would be possible to observe the collapse of an object to infinite density. It has been suggested that if loop quantum gravity is correct, then naked singularities could exist in nature implying that the cosmic censorship hypothesis does not hold.
- Natural satellite or moon: It is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify non-artificial satellites of planets, dwarf planets, and minor planets. (There are no known natural satellites of moons.) Two hundred and forty bodies, all in the Solar System, are classified as moons. They include 166 orbiting the eight planets, 4 orbiting dwarf planets, and dozens more orbiting small solar system bodies.
- Non-baryonic matter: It is the fundamental antithesis of baryonic matter that is any sort of matter that is not primarily composed of baryons. This might include such ordinary matter as neutrinos or free electrons; however, it may also include exotic species of non-baryonic dark matter, such as supersymmetric particles, axions or black holes.
- Nucleus: The solid, central part of a comet is known as the comet nucleus. The nucleus is a minor planet composed of rock, dust, and frozen gases. When heated by the sun, the gases sublimate or are ignited, and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The nucleus is generally up to 50km in diameter.
- Omega (?): It refers to the density of the universe, also called the density parameter.
- Orbital period: It is the time taken for a given object to make one complete
orbit about another object. Usually, in astronomy, this refers to the sidereal
period of an astronomical object, which is calculated with respect to the
stars. There are several kinds of orbital periods for objects around the
Sun:
.The sidereal period is the time that it takes the object to make one full
orbit around the Sun, relative to the stars.
.The synodic period is the time that it takes for the object to reappear
at the same point in the sky, relative to the Sun, as observed from Earth.
This is the time that elapses between two successive conjunctions with the
Sun and is the object's Earth-apparent orbital period.
.The draconitic period is the time that elapses between two passages of
the object at its ascending node, the point of its orbit where it crosses
the ecliptic from the southern to the northern hemisphere. It differs from
the sidereal period because the object's line of nodes typically precesses
or recesses slowly.
.The anomalistic period is the time that elapses between two passages of
the object at its perihelion, the point of its closest approach to the Sun.
It differs from the sidereal period because the object's semimajor typically
precesses or recesses slowly.
.The tropical period, finally, is the time that elapses between two passages
of the object at right ascension zero. It is slightly shorter than the sidereal
period because the vernal point precesses.
- Orbits: Within a planetary system, planets, dwarf planets, asteroids (a.k.a. minor planets), comets, and space debris orbit the central star in elliptical orbits. A comet in a parabolic or hyperbolic orbit about a central star is not gravitationally bound to the star and therefore is not considered part of the star's planetary system. To date, no comet has been observed in our solar system with a distinctly hyperbolic orbit. Bodies which are gravitationally bound to one of the planets in a planetary system, either natural or artificial satellites, follow orbits about that planet.
- Orionids: Meteor showers, such as the one named after the constellation Orion, are the fallen debris of a passing comet. Meteor showers are times when you can see many meteors or shooting stars in one night. There are several meteor showers that occur each year, such as the Leonids in November, the Geminids in December, or the Perseids in August. Usually, there is one night (possibly two) when you can see the most meteors. This night is called the peak of the meteor shower. The peak of the Orionid shower is around October 21st. The Orionid Meteor Showers are caused by the well known Halley's Comet.
- Oscillating universe theory: It is a cosmological model investigated briefly by Einstein in 1930 in which the universe undergoes a series of oscillations, each beginning with a big bang and ending with a big crunch. After the big bang, the universe expands for a while before the gravitational attraction of matter causes it to collapse back in and undergo a bounce.
- Ozone layer: It is a layer in Earth's atmosphere which contains relatively high concentrations of ozone (O3). This layer absorbs 97-99% of the sun's high frequency ultraviolet light, which is potentially damaging to life on Earth. Over 90% of ozone in earth's atmosphere is present here. "Relatively high" means a few parts per million-much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from approximately 15 km to 35 km above Earth's surface, though the thickness varies seasonally and geographically.
- Parallel Universes, Multiverse (or meta-universe): It is the hypothetical set of multiple possible universes (including our universe) that together comprise all of reality. The structure of the multiverse, the nature of each universe within it and the relationship between the various constituent universes, depend on the specific multiverse hypothesis considered. Multiverses have been hypothesized in cosmology, physics, astronomy, philosophy, theology, and fiction, particularly in science fiction and fantasy. The possibility of many universes raises various scientific and philosophical questions.
- Parsec: The parsec is a unit of length used in astronomy. The length of the parsec is based on the method of trigonometric parallax, one of the oldest methods for measuring the distances to stars. The name parsec stands for "parallax of one second of arc", and one parsec is defined to be the distance from the Earth to a star that has a parallax of 1 arcsecond. The actual length of a parsec is approximately 3.086×1016 m, 3.262 light-years or 19,176,075,967,324.937miles.
- Planetesimal: A widely accepted theory of planet formation states that planets form out of dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometre, then they can attract each other directly through their mutual gravity, aiding further growth into moon-sized protoplanets enormously. Bodies that are smaller than planetesimals must rely on Brownian motion or turbulent motions in the gas to cause the collisions that can lead to sticking. Many planetesimals may eventually break apart during violent collisions, but a few of the largest planetesimals can survive such encounters and continue to grow into protoplanets and later planets.
- Precession of the equinoxes: The precession of Earth's axis of rotation with respect to inertial space is called the precession of the equinoxes; the direction of the Earth's axis is changing. Because of this wobble, the position of the earth in its orbit around the sun at the moment of the equinoxes and solstices will also change. Currently, this annual motion is about 50.3 seconds of arc per year or 1 degree every 71.6 years. The process is slow, but cumulative. A complete precession cycle covers a period of approximately 25,765 years during which time the equinox regresses a full 360° through all twelve constellations of the zodiac.
- Retrograde motion: It is motion in the opposite direction of direct motion. (Counter-clockwise so to speak). In the case of celestial bodies, such motion may be real, defined by the inherent rotation or orbit of the body, or apparent, as seen in the skies from Earth.
- Satellite: In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon.
- Schwarzschild radius or gravitational radius: It is a characteristic radius associated with every mass. It is the radius for a given mass where, if that mass could be compressed to fit within that radius, no known force or degeneracy pressure could stop it from continuing to collapse into a gravitational singularity. The Schwarzschild radius of an object is proportional to the mass. Accordingly, the Sun has a Schwarzschild radius of approximately 3 km, while the Earth' is only about 9 mm. An object smaller than its Schwarzschild radius is called a black hole. The surface at the Schwarzschild radius acts as an event horizon in a non-rotating body. Neither light nor particles can escape through this surface from the region inside, hence the name "black hole".
- Solar eclipse: It occurs when the moon passes between earth and the sun, thereby totally or partially obscuring Earth's view of the Sun. disregarded.
- Solar flare: It is a violent explosion in the Sun's atmosphere releasing up to a total energy of 6 × 1025 Joules. Solar flares take place in the solar corona and chromosphere, heating plasma to tens of millions of Kelvin and accelerating electrons, protons and heavier ions to near the speed of light. They produce electromagnetic radiation across the electromagnetic spectrum at all wavelengths. Most flares occur in active regions around sunspots, where intense magnetic fields emerge from the Sun's surface into the corona. Flares are powered by the sudden release of magnetic energy stored in the corona. X-rays and UV radiation emitted by solar flares can affect Earth's ionosphere and disrupt long-range radio communications. Direct radio emission at decimetric wavelengths may disturb operation of radars and other devices operating at these frequencies.
- Space: Science considers space to be a fundamental quantity (a quantity which can not be defined via other quantities because other quantities -like force and energy- are already defined via space).
- Sunspot: This is a region on the Sun's surface (photosphere) that is marked by a lower temperature than its surroundings and has intense magnetic activity, which inhibits convection, forming areas of low surface temperature. Although they are blindingly bright at temperatures of roughly 4000-4500 K, the contrast with the surrounding material at about 5800 K leaves them clearly visible as dark spots. Sunspots are often related to intense magnetic activity such as coronal loops and reconnection. Most solar flares and coronal mass ejections originate in magnetically active regions around sunspot groupings.
- Superspace: It has had two meanings in physics.
. It describes the configuration space of general relativity.
. The second meaning refers to the coordinate space of a theory exhibiting
supersymmetry.
- Time machine: It is a device that allows time travel to the past or future. Only exists in science fiction.
- Transit or astronomical transit: This term has three meanings in astronomy:
.A transit is the astronomical event that occurs when one celestial body
appears to move across the face of another celestial body, as seen by an
observer at some particular vantage point.
.A transit occurs when a celestial body crosses the meridian due to the
Earth's rotation, about halfway between rising and setting. For instance,
the Sun transits the meridian at solar noon.
.The term star transit is used for the passage of a star through the eyepiece
of a telescope.
- T Tauri wind: It is a phenomenon indicative of the phase of stellar development between the accretion of material from the slowing rotating material of a solar nebula and the ignition of the hydrogen that has agglomerated into the protostar.
- Turbosphere (or homosphere): It is that portion of the atmosphere that is sufficiently well-stirred by atmospheric motion that long-lived gases are well-mixed and do not appreciably separate by weight. The turbosphere extends up to about 110 km and includes the troposphere, stratosphere, mesosphere, and lower thermosphere
- Van Allen Radiation Belt: It is a torus of energetic charged particles (plasma) around Earth, held in place by Earth's magnetic field. The earth's geomagnetic field is not uniformly distributed around its surface. On the sun side, it is compressed because of the solar wind and on the other side, it is elongated to around 3 earth radii. This creates a cavity called the Chapman Ferraro Cavity, in which the Van Allen radiation belts reside. The Van Allen belts are closely related to the polar aurora where particles strike the upper atmosphere and fluoresce.
- Wormhole: It is a hypothetical topological feature of spacetime that is basically a 'shortcut' through space and time. A wormhole has at least two mouths which are connected to a single throat or tube. If the wormhole is traversable, matter can 'travel' from one mouth to the other by passing through the throat. While there is no observational evidence for wormholes, spacetimes containing wormholes are known to be valid solutions in general relativity.
- Zodiacal light: This is a faint, roughly triangular, whitish glow seen
in the night sky which appears to extend up from the vicinity of the sun
along the ecliptic or zodiac. In mid-northern latitudes, the zodiacal light
is best observed in the western sky in the spring after the evening twilight
has completely disappeared, or in the eastern sky in the autumn just before
the morning twilight appears. It is so faint that it is completely masked
by either moonlight or light pollution. The zodiacal light decreases in
intensity with distance from the Sun, but on very dark nights it has been
observed in a band completely around the ecliptic. In fact, the zodiacal
light covers the entire sky, being responsible for 60% of the total skylight
on a moonless night. There is also a very faint, but still slightly increased,
oval glow directly opposite the Sun which is known as the gegenschein.