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Al-Farghani, Alfraganus, Amateur Astronomer, Amateurs, Anders Angstrom, Arecibo Observatory, Asteroid Belt, Asteroid,
Astronomical, Astronomy, Atacama, Bernhard Schmidt, Big Horn Medicine Wheel, Black Hole, Bolide, Brian Timmins,
Brians Timelines, brianstimelines, CADC, Cahokia Mounds, Caltech, June 2000, Cambridge University,
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Chronology, Clyde Tombaugh, Comet, Copernicus, CTIO, DAO, Dark Matter, Dominion Astrophysical Observatory,
Dominion Radio Astrophysical Observatory, DRAO, Dwarf Planet, Earth, Edmond Halley, Edwin Hubble, Einstein, Ejnar Hertzsprung,
ESA, ESIS, ESO, European Southern Observatory, European Space Information System, Events, Fajata Butte, Fred Hoyle, Fritz Zwicky,
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Hypernova, Institute Of Astronomy And Royal Greenwich Observatory, Isaac Newton, James Van Allen, James Webb,
Jet Propulsion Laboratory, Johannes Kepler, Joseph Von Fraunhofer, Joseph-Louis Lagrange, JPL, Jupiter, Karl Schwarzchild,
Keplers Laws, Kitt Peak National Observatory, KPNO, Kuiper Belt, Las Campanas And Magellan, Las Campanas Observatory,
Light Year, Machu Picchu, Magellan Project, Mars, Mauna Kea, Mcdonald Observatory, Mercury, Meteor, Meteorite,
Meteoroid, Milky Way, MIT, Moon, Mount Stromlo And Siding Springs Observatories, Mount Wilson Observatory, MSO & SSO,
NASA, National Aeronautics And Space Administration, National Astronomy And Ionosphere Center,
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National Solar Observatory, Nebula, Nebulae, Neptune, Neutron Star, Newgrange, Newton, Nicolaus Copernicus, Night Sky, NOAO,
Nova, NRAO, NSO, Observation, Observatories Of The Carnegie Institution Of Washington, Observatory, Oort Cloud, Parsec,
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Timmins, Tycho Brahe, U.Mass Amherst, UKIRT, United Kingdom Infra-Red Telescope, University Of Massachusetts Astronomy,
University Of Texas Department Of Astronomy, Uranus, Ut-Austin, Venus, Virtual Observatory Conference, VLT,
Wallace Astrophysical Observatory, William Herschel, Woodhenge
Measurements, Laws Constants & DefinitionsCompiled by Brian Timmins
MeasurementsSome Basic Statistics
Astronomical Units of Distance
LawsBode's Law:
Sometimes known as The Titius-Bode Law. Empirical rule whose progression of numbers represents the spacing of the
planets from the Sun. This mathematical relationship gives the distances in astronomical units of the planets (plus the
asteroids) out to Uranus, though there are inconsistancies. At one time thought to have physical significance, it is now
regarded as a mnemonic only. The math of the law is (0.3n + 0.4), with n = 0, 1, 2,4,8, etc.
Hubbles's Law:
A statement that the distances between galaxies or clusters of galaxies are
continuously increasing and that therefore the universe is expanding. Hubble calculated this as a result of measuring
the Doppler Shift. Work by other astronomers confirms the general features of Hubble's law, but one specific part -
"Hubble's constant" - has had to be drastically corrected. This suggests the relative rate at which the scale of the
universe changes with time. The present value lies between 57 and 73 km, per second, per megaparsec. There is still a
significant factor of uncertainty in this value, although the difference is just half of what is was in 1990. Hubble's
original value was between five and ten times too large because he underestimated the distances to the galaxies. The
Hubble constant has received much attention because its reciprocal can be thought of as representing the timescale of
the age of the universe.
Inverse Square Law:
The brightness of an object falls away in proportion to the reciprocal of the distance squared.
So, a light source is one sixteenth as bright at four unit of distance as it is at one unit of distance.
Keplers Three Laws of planetary motion
"The Law of Orbits": All planets move in elliptical orbits, with the sun at one focus.
 "The Law of Areas": A line that connects a planet to the sun sweeps out equal areas in equal times. "The Law of Harmonics": The square of the period of any planet is proportional to the cube of the semimajor axis of its orbit.
Newton's Law of Gravitation
Any two bodies attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Newton's Laws of Motion:
Three laws which form the foundation of classical mechanics, i.e. of the theory of ordinary motions (not motions on an atomic scale, covered by quantum
mechanics, and not at velocities close to that of light, covered by relativity). The laws introduce the concepts of force, mass and state (in modern terms).
Planck's Law
This is a formula giving the relationship between the temperature of a black body and the energy it emits at any wavelength.
Stefan's Law
Sometimes know as The Stefan-Boltzmann Law. A law stating that the energy emitted by a black body is a function of its temperature.
Wien's Law
In full, Wien's Displacement Law. This is a law of physics that states that there is an inverse relationship between the wavelength
of the peak of the emission of a black body and its temperature.
ConstantsAtomic Mass Unit
The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic and molecular masses.
It is defined to be one twelfth of the mass of an unbound atom of unbound carbon-12 at rest and in its ground state.
Avogadro's Constant
The Avogadro constant, also called the Avogadro number, is the number of "entities" (usually, atoms or molecules) in one
mole. That is, the number of carbon-12 atoms in 12 grams of unbound carbon-12 at rest and in its ground state. It is based
on Avogadros Principle that equal volumes of all gases at the same temperature and pressure contain the same number of molecules.
Nowadays the most accurate calulation is from the density of a crystal, the relative atomic mass, and the unit cell length,
determined from x-ray analysis.
Boltzmann constant
The Boltzmann constant is the physical constant relating energy and temperature at the particle level.
It is the gas constant divided by the Avogadro constant.
Fine Structure Constant
The Fine Structure Constant has been described as a 20th century mystery because of the difficulty in its calculation.
It measures the strength of the electromagnetic force that controls how charged elementary particles (such as electrons
and photons) interact. Because the constant is nearly equal to 1/137, and because it is a dimensionless constant,
some scientists have been led to wonder whether it has mathematical significance of its own, such as pi.
The fine structure constant can be derived from other constants as follows:
α = e2 (2ε0hc)-1
It may also be approximated thus: α ≈ α∞ = cos(π/137)/137Gravitational Constant "G"
The universal gravitational constant relates force to mass and distance in Newton's law of gravitation.
Hubble Constant
A ratio expressing the rate of apparent expansion of the universe, equal to the velocity at which a typical galaxy is receding
from Earth divided by its distance from Earth. The reciprocal of the Hubble Constant is the age of the universe. There is a
great deal of contention between two schools of physicists as to its value. See also: Olber's Paradox.
Mean angular velocity of Earth
The earth's orbit around the sun is elliptical rather than perfectly circular, so the earth's angular velocity around
the sun is actually variable. Since the ellipse is not extremely eccentric the mean value for angular velocity may be
used in most circumstances.
Obliquity of ecliptic
The angle made between a plane through the earth's equator and the plane of the earths orbit around Sol
Oort's Constants:
Two numbers which appear in the formulae which describe the rotational motions of the stars around the Milky Way galaxy in the
region of the Sun. The first Oort constant A, is a measure of the shear of the galaxy, analogous to the tendency of a
solid body to deform if part of it were to try to go more quickly around than another part. The second Oort constant B, is a
measure of the vorticity of the stars in the disk of the galaxy, or rather their tendency to orbit about a particular point.
DerivationA = ½ (v_o / R_o - dv/dR)B = - ½ ( v_o / R_o + dv/dR)
Planck constant
A physical constant that is used extensively in quantum mechanics and fixes the scale of quantization of
many phenomena, such as the relation between the energy of a photon (a quantum of light) and its wavelength.
Radiation density constantRydberg constant
A wave number characteristic of the wave spectrum of every element.
Solar Mass
The Solar Mass is defined for use in other calculations as being 1
Stefan-Boltzmann constant
The Stefan-Boltzmann constant, symbolized by the lowercase Greek letter sigma - σ, is a physical constant involving
black body radiation. A black body, also called an ideal radiator, is an object that radiates or absorbs energy
with perfect efficiency at all electromagnetic wavelengths. The constant defines the power per unit area emitted
by a black body as a function of its thermodynamic temperature.
DefinitionsColour Images It should be noted by those of you not "in the know" - so to speak - that coloured
astronomical images are rarely in true colour. More often they are produced by creating RGB coloured images taken with differing filters.
A common combination of filters is Iron(Fe), Calcium(Ca) and Oxygen(O) line in the spectrum of a star. Another combination in use by X-ray
astronomers is to use low energy X-rays (0.5-2 keV)-red, medium energy (2-4 keV)-green, and high energy (4-8 keV)-blue, as in the image
of Supernova 1181CE by the Chandra X-ray orbiting telescope, on your right.The Roche Limit
If a planet and its moon have the same densities, the minimum limit of separation is 2.423 of the planet's radius, measuring from the center of the planet to the center of the moon. If the
densities are not the same, this number changes. A moon that is denser than the planet can be closer, a moon that is less dense than the planet needs to be farther away. Inside this limit, tidal
forces will destroy the satellite. (Rings can exist well inside this limit)
Schwarzchild Radius
That distance, from a black hole, inside of which nothing, not even light, can escape.
The formula for calculating the Schwarzschild radius is as follows:
rs = 2GM ÷ c2Where:
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Owner & Webmaster:
Brian Timmins