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.
A

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

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

Aristotle of Stagira (384322 BC): Greek philosopher.
 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).
 astronomer:
A scientist engaged in the study things in space.

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

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

Bertrand, Joseph Louis Francois (18221900):
A 19th century mathematician perhaps best remembered for
Bertrand's theorem which concerns the types of central
forces lead to closed orbits.

big bang: (a) The
singularity
of infinite density that
occurs at the universal time zero of
FriedmannLemaitreLambda 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
(No532): Hoyle was
a coinventor of the
steadystate universe
(Bo140ff,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
(Bo85,181).
(b) The short time after the singularity in which the light
elements are sysnthesized.

big bang cosmological model: A particular
FriedmannLemaitreLambda 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.

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
FriedmannLemaitreLambda 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 FriedmannLemaitreLambda model at very early times
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C

Carnot, Sadi (17961832):
A French scientist noted for his theoretical discovery of the Carnot
engine: the most efficient heat engine that the laws of thermodynamics
allows.

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

center of mass: The massweighted average position of a body.

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.

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.

compact object:
Superdense astrobodies: white dwarfs (WDs), neutron stars (NSs),
and black holes (BHs).

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.

Copernican principle:
This principle is really an assumption: it states that
we occupy no special place in the universe
(Bo13;
CL4).
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.

Copernicus, Nicolaus (14731543): PolishGerman astronomer
who was the first modern proposer of the heliocentric model of
the solar system.

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

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.
 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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D
 Descartes, Rene (15961650):
French philosopher often associated with his famous principle.
cogito ergo sum: I think therefore I am.
He was also astronomical speculator.
 disaster: An unfortunate event.
From Italian for not having a lucky star: derived from disastro
and disastrato.
 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.

Doppler, Johann Christian (18031853)
Doppler was an Austrian physicist famous for his discovery of the
frequency shifting property of relative motion on wave phenomena.
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E
 E=mc**2:
See Einstein equation.
 Earth:
The 3rd planet from the Sun: home of humankind.

Einstein, Albert (18791955):
GermanSwissAmerican physicist most famous for his discoveries
of special relativity (SR) and general relativity (GR).

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">
Law46.
It is often used to show the energy possessed by
rest mass.

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 3dimensional surface of a 4dimensional sphere), and static
(No520;
Bo97).
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.
 ellipse:
 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.
 ephemeris:

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
FriedmannLemaitreLambda models.
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F

Faraday, Michael (17911867):
 Fontenelle,
Bernard le Bouyer de Fontenelle (16571757):
 Franklin, Ben (17061790):
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G
 galaxy:
 Galileo Galilei (15641642)
 Gamow, George (19041968):
Probably the person best credited as the main originator of the
big bang cosmology.

general relativity (GR):
GR is Einstein's theory of gravity. In GR
massenergy
distorts
spacetime and these
distortions tell massenergy how to move.
GR is summarized in Einstein's field equations
(CL9;
Bo95).
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
(ST110).
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.

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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H
 Harriot, Thomas (15601621):

Hertz, Heinrich (18571894):
The inventor of radio.
 Hoyle, Sir Fred (19152001):
An astrophysicist famous among many other things as coinventor
of the
Hubble, Edwin Powell (18891953):

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.

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
(FK653.

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
(CL47)
and so consitutes a characteristic size for the observable universe.

Hubble time:
This is the reciprocal of the Hubble constant.
Using the
best modern Hubble constant value 71 (km/s)/Mpc
(FK653), 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
FriedmannLemaitreLambda 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.
 Huygens, Christian (16291695):
 hydrodynamics:
The science of fluid motion.
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I

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.

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.

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

Jupiter
The 5th planet from the Sun. The largest planet.
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K
 Kelvin, Lord Kelvin (William Thompson)
(18241907):

Kelvin or Absolute Temperature Scale
 Kepler: Johannes Kepler (15711630)
 Khayyam, Omar
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L

largescale structure:
The pattern of galaxies and larger galaxy structures:
clusters, superclusters, filaments, sheets, and voids.
 logarithms:
 lookback time:
Because of the finite speed of light when you look at light
from a source, you look back in time.
The lookback time is the travel time for light from a source.
For relatively nearby cosmological objects the lookback time
is to good approximation just the measured distance
divided by the speed of light c.
If the distance is given in lightyears, the lookback time
is the same value with units of years.
For larger cosmological distances, the lookback time
depends on cosmological model our universe domain actually
obeys.
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M
 magnitudes:
 Mars: The 4th planet
from the Sun.
 massenergy:
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 massenergy to impress the layperson.
 Maxwell, James Clark (18311879):
 Mercury: The 1st planet from the
Sun.
 meteorite:
 metric prefixes:
 metric system or SI:
 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.
 Moon: The Earth's only natural
satellite.
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N
 naked eye star:
 nebula, pl. nebulae:
 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
(No428).
 Newton: Sir Isaac Newton (16431727):
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O
 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.
 Omar Khayyam:
 Orion Nebula:
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P
 planet: A larger body orbiting
the Sun or any star.
 Pluto: The 9th planet from the Sun.
 polarization:
 Popper,
Sir Karl Raimund: A philosopher of science.
 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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Q
 quantum mechanics:
The physics of microscopic systems: i.e., atomics scale and smaller.
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R

radiative transfer:
The transfer of energy and information by electromagnetic radiation.
 rainbow:
 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 ``restmassless.''
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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S

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.

Schroedinger, Erwin (18871961): The codiscoverer
of quantum mechanics. He formulate the Schroedinger equation
which is the basic law of motion of nonrelativistic quantum
mechanics.

Schwarzschild, Karl (18731916): The discoverer of
Schwarzschild solution in
general relativity. See also
St. Andrews: Karl Schwarzschild">
 science fiction:
Fiction that makes explicit use of scientific conceptions or that
it is set in the future or fictional worlds.
 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.
 sky map:
A map of the sky locating stars, etc.
 Slipher, Vesto
Melvin (18751969): The discovery of the tendency of the
spiral nebulae to be redshifted. He worked at Lowell Observatory
in Flagstaff, Arizona
(No522523).

solar system:
The Sun and the astrobodies that are gravitationally bound to the
sun.

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.

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.
 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).

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

star evolution:
The temporal development of stars.
 steadystate 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)
(Bo140ff, 152ff).
The steadystate 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 steadystate 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 steadystate universe untenable
without numerous new
ad hoc hypotheses which few accepted.
Most people then dismissed the steadystate 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 ``steadystate universe'' is
offered by
eternal inflation which is a contemporary contender.

Stonehenge:
A neolithic site from 30001000 BCE ????
consisting of stone rings and other artifacts.
The design includes astronomical information: alignment astronomy.
 Sun:
A G2 V star to which the Earth is gravitationally bound.
 supernova (SN):
The giant explosion of a star.

synodic period:
The time it takes for an astrobody to return to the
same angular position relative to the Sun.
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T

Tacoma Narrows Bridge: The bridge that
collapsed due to winddriven resonances.

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.

Tycho Brahe (15461601):
A Danish astronomer famous for his detailed and copious pretelescopic
observations.
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U
 unit:
A standard amount of any quantifiable thing.
 universe:
Also cosmos.
(a) Everything. (b) The part of everything that is similar
to the
observable universe. (c) The observable universe.

universe domain: Also universe patch or pocket universe???.
A region of the universe that has
the same laws of physics and the same largescale 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.

Uranus: The 7th planet
from the Sun. It has blue color due to methane in its atmosphere.
It was discovered 1781mar13 by William Herschel
(No399).
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 nonstar had also been
made before Herschel
(No427).
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V
 Venus
The 2nd planet from the Sun. It is the historical Evening
and Morning Star.
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W
 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
nonphysics definition is not far removed from the physics
definition.
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X
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Y
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Z
 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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