Astronomy, Physics, Science, Technology Glossary

This is primarily an astronomy glossary, but other related and/or useful terms creep in as well.

I've attempted to keep the definitions and explanations brief with few or no references.

In many cases I've just left links to definitions longer definitions, discussions, references, examples, images, lists, etc. Some links do not exist yet. The links may be to pages of other persons.

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  1. acronym: (a) A word formed from the initial letters of other words. (b) Multi-word designations abbreviated by the first letters of the words.

  2. Aristarchos of Samos (3rd century BC): Greek mathematician and astronomy: the first known proposer of the heliocentric model of the solar system.

  3. Aristotle of Stagira (384--322 BC): Greek philosopher.

  4. asteroid: A rocky body in space larger than of order 10 meters, but not classified as a moon or as planet (i.e., it is too small to be a planet).

  5. astronomer: A scientist engaged in the study things in space.

  6. astronomical unit (AU): The mean Earth-Sun distance treated as a unit: 1 AU = 1.4959787*10**13 cm = 1.496*10**13 cm = 1.5*10**13 cm.

  7. Avebury: A Neolithic stone circle in Wiltshire, England, not far from Stonehenge.

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  1. Bertrand, Joseph Louis Francois (1822-1900): A 19th century mathematician perhaps best remembered for Bertrand's theorem which concerns the types of central forces lead to closed orbits.

  2. big bang: (a) The singularity of infinite density that occurs at the universal time zero of Friedmann-Lemaitre-Lambda models of the universe or similar models. The name was originally derisively and was first used by Fred Hoyle in a BBC radio program in 1950 (No-532): Hoyle was a coinventor of the steady-state universe (Bo-140ff,152ff) which was a serious rival of big bang cosmology up to the early 1960s. The singularity was sometimes called the point origin in earlier work (Bo-85,181). (b) The short time after the singularity in which the light elements are sysnthesized.

  3. big bang cosmological model: A particular Friedmann-Lemaitre-Lambda model of the universe or similar universe model that begins from a singularity of infinite density: i.e., a big bang. The model can be considered as starting in actuality from a later time than the singularity.

  4. big bang cosmology: The theory that the universe or our universe domain began from a hot dense state in which the light elements were synthesized and then that the universe evolved according to a Friedmann-Lemaitre-Lambda model of the universe or similar universe model. Such models formally have a time zero singularity of infinite density (i.e., the big bang), but the singularity itself does not have to be included in big bang cosmology. One just as the universe track into a Friedmann-Lemaitre-Lambda model at very early times

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  1. Carnot, Sadi (1796--1832): A French scientist noted for his theoretical discovery of the Carnot engine: the most efficient heat engine that the laws of thermodynamics allows.

  2. Celestial Sphere: A large imaginary sphere center on the Earth. On this sphere the astro-bodies can be located uniquely for a terrestrial observer if all parallax effects can be neglected.

  3. center of mass: The mass-weighted average position of a body.

  4. Chaucer, Geoffrey (1343?--1400): English poet, most famous for The Canterbury Tales. By the standards of his day he was exceptionally knowledgeable about astronomy and often included astronomical details in his works.

  5. comet: (a) An icy/rocky body typically a few kilometers or tens of kilometers in size scale in a highly elliptical orbit about the Sun. When close to the Sun the ices explosively evaporated and create the large cometary head and tail. (b) The comet head and tail.

  6. compact object: Super-dense astro-bodies: white dwarfs (WDs), neutron stars (NSs), and black holes (BHs).

  7. constellation: (a) A recognized grouping in angle of stars on the sky. (b) Any of the 88 IAU recognized constellations and their defined surrounding angular areas on the sky.

  8. Copernican principle: This principle is really an assumption: it states that we occupy no special place in the universe (Bo-13; CL-4). It is one of the simplifying guiding principles of cosmology: i.e., it guides us in constructing cosmological model. There is no observational evidence or broadly accepted theoretical reason to suggest the Copernican principle is false. In fact, as far as we can tell it seems true.
  9. Copernicus, Nicolaus (1473--1543): Polish-German astronomer who was the first modern proposer of the heliocentric model of the solar system.

  10. copyright: The ownership of some intangible, conceptual property: e.g, book, image, design.

  11. Cosmological principle: This is the glorified expression for the simplifying assumption that the universe averaged over sufficiently large scales is homogeneous (i.e., the same everywhere at one time) and isotropic (i.e, the same in all directions). This assumption is still usually used today for developing models of our universe domain. The Cosmological principle was used by Einstein in developing the Einstein universe, but I don't know if he used the term himself or can be considered the inventor of the assumption.

  12. cosmology: (a) The science of the universe as a whole. Also modern cosmology. (b) The philosophy or myth of the universe as a whole.

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  1. Descartes, Rene (1596--1650): French philosopher often associated with his famous principle. cogito ergo sum: I think therefore I am. He was also astronomical speculator.

  2. disaster: An unfortunate event. From Italian for not having a lucky star: derived from disastro and disastrato.

  3. dissipation: (1) In physics jargon, dissipation is often used to mean the conversion of energy to heat, often waste heat that is of no further use to the system. (2) Scattering, dispersing, wasting, squadering.

  4. Doppler, Johann Christian (1803--1853) Doppler was an Austrian physicist famous for his discovery of the frequency shifting property of relative motion on wave phenomena.

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  1. E=mc**2: See Einstein equation.

  2. Earth: The 3rd planet from the Sun: home of humankind.

  3. Einstein, Albert (1879--1955): German-Swiss-American physicist most famous for his discoveries of special relativity (SR) and general relativity (GR).

  4. Einstein equation E=mc**2: This equation follows from in relativistic physics (special relativity and general relativity). Its primary meaning is that all forms of energy have mass (i.e., resistance to acceleration): the mass of energy E is m=E/c**2 (a href="../../writer/ref.html#lawden"> Law-46. It is often used to show the energy possessed by rest mass.

  5. Einstein universe: This is general relativistic model of the universe proposed by Einstein in 1917: it is homogenous and isotropic, hyperspherical (i.e., a finite but bounded 3-dimensional surface of a 4-dimensional sphere), and static (No-520; Bo-97). The model is suppose to represent the average behavior of an actual static universe. To make his universe static, Einstein invented the cosmological constant since pure general relativity failed to give a static universe. He renounced the cosmological constant after the expansion of the universe was discovered: he called it the biggest blunder of his life since introducing it precluded predicting the expansion of the universe himself.

  6. ellipse:

  7. energy: Energy really requires an explanation rather than a definition: no one line definition seems to be adequate. But one can say energy is something like the basic stuff of the universe: it comes in many forms and all forms can be converted to any other form in principle although not necessarily easily in practice. Forces are the agent of conversion and at least at the macroscopic level the process of conversion is often called work or if to heat energy dissipation. Energy is never created or destroyed: i.e., energy is conserved.

  8. ephemeris:

  9. expansion of the universe: The ongoing increase in distance between the largest gravitionally bound systems in the observable universe. The observed expansion is consistent with the general relativistic Friedmann-Lemaitre-Lambda models.

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  1. Faraday, Michael (1791--1867):

  2. Fontenelle, Bernard le Bouyer de Fontenelle (1657--1757):

  3. Franklin, Ben (1706--1790):

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  1. galaxy:

  2. Galileo Galilei (1564--1642)

  3. Gamow, George (1904--1968): Probably the person best credited as the main originator of the big bang cosmology.

  4. general relativity (GR): GR is Einstein's theory of gravity. In GR mass-energy distorts spacetime and these distortions tell mass-energy how to move. GR is summarized in Einstein's field equations (CL-9; Bo-95). The field equations are extremely difficult to solve, but they have passed all tests since Einstein presented them in 1915???. Over any small enough region of spacetime special relativity applies, and thus special relativity becomes a special case of general relativity as one would expect from the names (ST-110). In the limit of weak gravity, they yield Newtonian gravity as they should since Newtonian gravity is extremely well verified in this limit: weak gravity pertains to most systems aside from, for example, black holes and the universe as a whole. Nevertheless, almost all physicists would agree GR cannot be the fundamental theory of gravity because it is not consistent quantum mechanics. The fundamental theory of gravity should be a quantum theory. There are ideas about such a theory, but no absolute favorite has emerged.

  5. gravity: The force of attraction between masses. In general relativity, gravity manifests itself not as a Newtonian force, but as distortion of spacetime

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  1. Harriot, Thomas (1560--1621):

  2. Hertz, Heinrich (1857--1894): The inventor of radio.

  3. Hoyle, Sir Fred (1915--2001): An astrophysicist famous among many other things as co-inventor of the Hubble, Edwin Powell (1889--1953):

  4. Hubble diagram: A plot of velocity or spectral shift of galaxies or other extragalactic objects versus distance. The systematic relationship is a straight line through the origin. The line is representation of the Hubble law v=Hd.

  5. Hubble's law: This is the linear relationship between recessional velocity and distance for extragalactic objects participating in the expansion of the universe. The law is v=Hd, where v is recessional velocity, d is distance, and H is the Hubble constant: H=71 (+4/-3) (km/s)/Mpc is a good modern value (FK-653.

  6. Hubble length: This is the distance that light travels in a Hubble time. If one could freeze the expansion of the universe right now, the Hubble length would be the radius of sphere centered on us that could signal to us within a Hubble time. The Hubble length is of order (and possibley within a factor of 2 of) the size of the radius of the observable universe in big bang cosmology (CL-47) and so consitutes a characteristic size for the observable universe.

  7. Hubble time: This is the reciprocal of the Hubble constant. Using the best modern Hubble constant value 71 (km/s)/Mpc (FK-653), one finds t_H=1/H=4.35*10*17 s= 13.8 Gyr. The Hubble time is a characteristic time for the expansion of the universe and assuming Friedmann-Lemaitre-Lambda models (which we discuss below) or similar universe models, 1/H should be order of the age of the universe since the big bang, and thus 1/H is a characteristic age for the universe for such models.

  8. Huygens, Christian (1629--1695):

  9. hydrodynamics: The science of fluid motion.

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  1. inertial frame: Newton's laws of motion are obeyed with respect to inertial frames. Inertial frames are a special class of mutually unaccelerated frames. Many frames are approximately inertial for certain purposes: e.g., the Earth's surface is sufficiently inertial for many, but not all calculations. Newton himself probably regarded the fixed stars as defining an absolute, fiducial inertial frame. Nowadays we think (I think) that absolute, fiducial inertial frames are those that participate in the mean expansion of the universe. We still believe in absolute inertial frames even though Newton's laws are themselves known to be only approximations.

  2. inflation: The concept that the universe or universe domains undergo sudden enormous expansions from microscopic regions. The expansions flatten the geometry of domain. After inflation in our domain at least, ordinary big bang cosmology begins.

  3. interstellar medium (ISM): The gas and dust between the stars inside of galaxies.

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  1. Jupiter The 5th planet from the Sun. The largest planet.

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  1. Kelvin, Lord Kelvin (William Thompson) (1824--1907):

  2. Kelvin or Absolute Temperature Scale

  3. Kepler: Johannes Kepler (1571--1630)

  4. Khayyam, Omar

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  1. large-scale structure: The pattern of galaxies and larger galaxy structures: clusters, superclusters, filaments, sheets, and voids.

  2. logarithms:

  3. look-back time: Because of the finite speed of light when you look at light from a source, you look back in time. The look-back time is the travel time for light from a source. For relatively nearby cosmological objects the look-back time is to good approximation just the measured distance divided by the speed of light c. If the distance is given in light-years, the look-back time is the same value with units of years. For larger cosmological distances, the look-back time depends on cosmological model our universe domain actually obeys.

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  1. magnitudes:

  2. Mars: The 4th planet from the Sun.

  3. mass-energy: This is essentially a synonym for energy that merely emphasizes that in relativistic physics (special relativity and general relativity) all energy has mass (i.e., resistance to acceleration) and that rest mass is a form of energy. One can use mass-energy to impress the layperson.

  4. Maxwell, James Clark (1831-1879):

  5. Mercury: The 1st planet from the Sun.

  6. meteorite:

  7. metric prefixes:

  8. metric system or SI:

  9. Milky Way: Also the Galaxy. (a) The galaxy of humankind and the Sun. (b) The band of whitish luminosity on the sky that straddles a great circle at about 60 degrees from the Celestial Equator. This band is the appearance of the disk of Milky Way from inside the disk.

  10. Moon: The Earth's only natural satellite.

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  1. naked eye star:

  2. nebula, pl. nebulae:

  3. Neptune: The 8th planet from the Sun. A mathematical prediction by French and English astronomers in 1846 led to the discovery of Neptune within 1 degree of predicted location by the Berlin Observatory on 1846sep23 (No-428).

  4. Newton: Sir Isaac Newton (1643--1727):

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  1. observable universe: In big bang cosmology there is a finite time since the initial singularity (i.e., big bang) or the time since our universe domain universe domain tracked into the a big bang cosmological model (i.e., at the end of inflation). This means that there are regions of the universe too far away to have signaled us by the fastest means (i.e., by light) since the effective time zero of our universe domain. Those regions are beyond our observable universe. The regions that could signal us constitute the formal observable universe: this would be a sphere centered on the Earth. The observable universe so defined may not be completely observable (IAWL: Cosmology), but it mostly is. The radius of the observable universe sphere is NOT itself directly observable: it depends on the actual cosmological model our universe domain obeys. The characteristic size of the observable universe is the Hubble length.

  2. Omar Khayyam:

  3. Orion Nebula:

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  1. planet: A larger body orbiting the Sun or any star.

  2. Pluto: The 9th planet from the Sun.

  3. polarization:

  4. Popper, Sir Karl Raimund: A philosopher of science.

  5. proper distance: In general relativistic cosmological models, the proper distance is the distance measured by rigid rulers at one instant in cosmic time. In fact, such measurements cannot be done and proper distances must determined from the cosmological model one adopts and thus have the uncertainty of that model.

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  1. quantum mechanics: The physics of microscopic systems: i.e., atomics scale and smaller.

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  1. radiative transfer: The transfer of energy and information by electromagnetic radiation.

  2. rainbow:

  3. rest mass: Rest mass is the mass (i.e., resistance to acceleration) that a body has as measured in an inertial frame at which it is at rest. The concept arises from relativistic physics (special relativity and general relativity). Some objects such as photons have no rest mass. When we say that light is ``massless'' this is really an abbreviation for ``rest-massless.'' Rest mass is a form of energy, and so can be converted to any other form of energy in principle. The energy of any rest mass is given by the Einstein equation E=mc**2.

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  1. Saturn: The 6th planet from the Sun. It is a gas giant planet and has by far the most prominent ring system in the solar system.

  2. Schroedinger, Erwin (1887--1961): The co-discoverer of quantum mechanics. He formulate the Schroedinger equation which is the basic law of motion of non-relativistic quantum mechanics.

  3. Schwarzschild, Karl (1873--1916): The discoverer of Schwarzschild solution in general relativity. See also St. Andrews: Karl Schwarzschild">

  4. science fiction: Fiction that makes explicit use of scientific conceptions or that it is set in the future or fictional worlds.

  5. singularity: (1) Mathematically the infinity of a function. (2) Physically an infinity of a physical quantity which usually is regarded as an idealization of physical reality or as indicating a breakdown of physical theory. (3) The infinite density region of a black hole. (4) The infinite density time zero condition of big bang cosmology.
  6. sky map: A map of the sky locating stars, etc.

  7. Slipher, Vesto Melvin (1875--1969): The discovery of the tendency of the spiral nebulae to be redshifted. He worked at Lowell Observatory in Flagstaff, Arizona (No-522--523).

  8. solar system: The Sun and the astro-bodies that are gravitationally bound to the sun.

  9. spacetime: In relativistic physics (special relativity and general relativity) space and time become coupled (i.e., they are interdependent), and thus it is convenient to have a single word to refer to them both simultaneously: i.e., spacetime. In Newtonian physics space and time are mostly uncoupled: ``mostly'' because it is impossible to know time without motions in space.

  10. special relativity (SR): Einstein's theory of spacetime and motion. The theory has two primary postulates: (1) the laws of physics are the same in all inertial frames (i.e., the relativity postulate) and (2) the vacuum speed of light is the same for all observers and is the highest physical speed. SR predicts among other things that time flow and length are frame dependent and that all energy has mass. The latter result is summarized in the Einstein equation E=mc**2. SR is extremely well verified and I would call it an approximate true theory: i.e., it is completely true within in its realm of validity.

  11. star: (a) A large sphere of gas in space that is generating energy by nuclear fusion. (b) A compact object that used to be a star by the first definition and still has star in its name: white dwarfs (WDs) and neutron stars (NSs).

  12. star cluster: A gravitationally bound group of stars that is smaller than a galaxy and is itself bound to a galaxy.

  13. star evolution: The temporal development of stars.

  14. steady-state universe: This is a model of the universe first put forward by Bondi & Gold (1948) and then based on different axioms by Fred Hoyle (1948, 1949) (Bo-140ff, 152ff). The steady-state universe assumes what is grandly called the perfect cosmological principle which is the plus the notion that the universe does not change with time on average. The universe in question was one that is just the observable universe extended to infinity. The steady-state universe was a serious rival of big bang cosmology up to the early 1960s. The discovery of quasars (which showed the universe was evolving) and the cosmic microwave background (which was naturally explained in big bang cosmology), made the steady-state universe untenable without numerous new ad hoc hypotheses which few accepted. Most people then dismissed the steady-state universe to the pile of discarded theories. It was, however, in many ways a good theory: it explained many things and offered many falsifiable predictions. It was perhaps too simple: ``A theory should be as simple as possible, but not simpler." ( Einstein approximately). In fact a rather different ``steady-state universe'' is offered by eternal inflation which is a contemporary contender.

  15. Stonehenge: A neolithic site from 3000--1000 BCE ???? consisting of stone rings and other artifacts. The design includes astronomical information: alignment astronomy.

  16. Sun: A G2 V star to which the Earth is gravitationally bound.

  17. supernova (SN): The giant explosion of a star.

  18. synodic period: The time it takes for an astro-body to return to the same angular position relative to the Sun.

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  1. Tacoma Narrows Bridge: The bridge that collapsed due to wind-driven resonances.

  2. telescope: An optical device whose essential components are lenses and/or mirrors. It is used for seeing objects at optical infinity and is particularly used in astronomy.

  3. Tycho Brahe (1546--1601): A Danish astronomer famous for his detailed and copious pre-telescopic observations.

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  1. unit: A standard amount of any quantifiable thing.

  2. universe: Also cosmos. (a) Everything. (b) The part of everything that is similar to the observable universe. (c) The observable universe.

  3. universe domain: Also universe patch or pocket universe???. A region of the universe that has the same laws of physics and the same large-scale structure.

    Our domain which is larger than the observable universe (we think) may itself constitute the whole universe, but many people think it does not and that other domains, maybe infinitely many, exist.

  4. Uranus: The 7th planet from the Sun. It has blue color due to methane in its atmosphere. It was discovered 1781mar13 by William Herschel (No-399). Uranus is actually marginally observable with the unaided eye and probably had often been seen throughout history without noticing it was a planet. Telescopic observations and records of Uranus without recognizing it as a non-star had also been made before Herschel (No-427).

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  1. Venus The 2nd planet from the Sun. It is the historical Evening and Morning Star.

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  1. work: (1) In physics Work is the process of energy conversion. Often the term is restricted to conversions of macroscopic energy. (2) In general work is act of doing some task. Since actions almost always involve energy conversions, this non-physics definition is not far removed from the physics definition.

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  1. Zodiac: A band of about 16 degrees straddling the Celestial Sphere containing the 12 constellations of the Zodiac and the 12 signs of Zodiac which in precise astrology are 30 degree segments along the band.

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