Dawn Dictionary

Achondrite - A meteorite consisting of igneous minerals and no chondrules, very similar to terrestrial igneous rocks

Albedo - The ratio of the amount of light reflected from a surface to the amount of incident light

Aperture - The effective diameter of the primary mirror or lens of a telescope

Aphelion - The point in a planetary orbit that is at the greatest distance from the Sun

Apparent Magnitude - The brightness of a star as it appears to the eye or to the telescope, as measured in units of magnitude—the brighter the star, the smaller the apparent magnitude.

Asteroid - (also called a “minor planet”) A small solar system object in orbit around the sun composed mostly of rock. Many of these objects orbit the sun between Mars and Jupiter. Their size can range anywhere from 100 meters in diameter to almost 1000 kilometers.
Principal asteroids – Ceres, Pallas, Juno and Vesta
C-type asteroids are carbon-rich, very dark, reflect on 3%-9% of sunlight.
S-type asteroids are comprised of metallic nickel-iron mixed with iron- and magnesium-silicates.
V-type asteroids contain more pyroxene than S-type asteroids.

Asteroid belt - A region of space lying between Mars (1.5 AU) and Jupiter (5.2 AU), where the great majority of the asteroids are found

Astronomical Unit (AU) - The mean distance between the Earth and the Sun: 149,598,500km; the preferred unit for distances within the Solar System

Axis of rotation - The theoretical straight line through a celestial body, around which it rotates

Bode’s Law - This law describes a mathematical method for calculating planetary distances which states that the distance to the nth planet is 0.4 + (0.3)n Astronomical Units. Bodes law works surprisingly well out to Uranus.

Brightness magnitude - The apparent brightness of a star is called the apparent magnitude and that is what is measured by a telescope: how much energy the star puts into the telescope's collecting area per second.

Brightness - Refers to the amount of light coming from an object

Canon law - The body of ecclesiastical law by which a Christian church is governed.

Celestial body - A solid object found in space; in the heavens

Celestial Police - A group of astronomers looking for Kepler’s missing planet between Jupiter and Mars

Ceres - Largest of the known asteroids, and the first to be discovered (by Piazzi in 1801)

Chondrite - A meteorite containing chondrules, which are small, spherical silicate objects on the order of a millimeter in size that formed in the solar nebula before the asteroids were formed

Chondrules -A crystallized sphere of rocky material found in chondrite meteorites.  Named after the Greek for seeds, they are essentially tiny igneous rocks that formed in the solar nebula.

Circumscribe - To draw a line around; to encircle

Classical planets - Mercury, Venus, Mars, Jupiter and Saturn are known as “the classical planets

Collodian - A viscous solution used in coating photographic films; the wet collodion process produced plates that had a much higher sensitivity than was previously available

Comet - A small solar system object orbiting the Sun consisting of ice, dust and gas that form a coma and sometimes a visible tail whenever they orbit close to the Sun

Comet debris - Clumps of dust, ice and rocky material that are released from comets as they approach the sun

Concave - Hollowed or rounded inward like the inside of a bowl

Concave lens - A lens whose interior surface in hollowed or curved inward, away from the viewpoint, making the interior of the lens thinner than the outer edges

Convex - Curved or rounded outward like the exterior of a sphere or circle

Convex lens - A lens whose interior surface is curved outward, toward viewpoint, making the interior of the lens thicker than the outer edges

Convex-profile inversion - A modeling technique for producing a three-dimensional shape from a two-dimension convex profile

Copernican system - A heliocentric model of the universe; the hypothesis that the earth and the other planets orbit the sun

Coulomb - A measure of the amount of electric current, being the quantity transferred in one second by a current of one ampere. 1 coulomb = 1 A s.

Crust - The outer part of a planet, moon, or asteroid composed essentially of crystalline rocks—generally the crustal composition is different from the bulk composition of the planet (adapted from Merriam-Webster.com).

Cyclic - Moving or recurring in cycles or periods

Dawn Discovery Mission - The ninth of NASA’s Discovery Program missions will investigate two of the largest protoplanets in the main asteroid belt, Vesta and Ceres.

Declination (DEC) - Declination is measured in degrees, and refers to how far above the imaginary"celestial equator" an object is (like latitude on the Earth).

Ecliptic - The mean plane of the Earth's orbit around the Sun

Elastic Collision - A collision between two particles which conserves the total kinetic energy and momentum of the system

Electromagnetic Radiation - "Waves" of electrical and magnetic "disturbance", radiated as visible light, radio waves, or any other manifestation of the electromagnetic spectrum

Electron Volt (eV) - A unit of energy used to indicate the energy of a charged particle. 1 eV is the energy gained when an electron is accelerated by a potential of one volt. 1 electron volt = 1.6 x 10-12 ergs

Elliptical - Shaped like an ellipse, a plane curve in which the sum of the distances of each point along its periphery from two points - its "foci" - are equal

Emission spectrum - A spectrum of energy released from a source, usually in the form of electromagnetic radiation

Emit (also see Light) - To send forth or to send out electromagnetic radiation

Emulsion - A suspension of sensitive silver salt in a viscous medium, used for coating plates or films

Ephemeris (Ephemerides; plural) - A table that gives the positions of objects in the sky at various times

Epicycle - Circular orbit of a body round a point that is itself in a circular orbit round a parent body—such a system was formulated to explain some planetary orbits in the Solar System before they were known to be elliptical

Epithermal Neutron - A neutron that has undergone energy loss due to scattering collisions, but is not in thermalequilibrium with the regolith (E = ~0.3 eV to ~600 keV)

Evolutionary path - In astronomy, the theory that more complex and varied bodies develop from simpler ones

Exposure - To lay open to the influences of climate or other external forces

Fast Neutron—A neutron ejected at high kinetic energy in a nuclear reaction (E = ~103 eV to 6 MeV)

Fusion Crust -Fusion crust is the outer covering that a meteorite has acquired as a result of the melting of its surface layer as it passes through the atmosphere. The compression of the Earth's atmosphere heats the surface of the meteoroid to its melting point. Most of the melted material is ablated (removed): what remains resolidifies as the fusion crust. The thickness of these crusts can range from less than one millimeter up to several millimeters. Typically, the leading side of the meteoroid will have a thinner crust than the trailing side. Fusion crusts can be light or dark in color depending on the amount of iron in the meteorite minerals.

Gamma rays - Very energetic EMR photons with energies between 1 MeV–¬10 GeV
Inelastic collision; a collision between two particles in which part of their kinetic energy is transformed into another form of energy (gamma rays)—the total amount of energy remains the same

Geocentric - Having the Earth at the center.

Heliocentric - Having the Sun at the center.

Hubble Space Telescope (HST) - NASA launched the Hubble Space Telescope in 1990. It orbits around the Earth every 97 minutes about 600 kilometers above the Earth, above the distorting effects of the atmosphere. Hubble’s instruments include cameras and spectrographs, using mirrors to focus and magnify light.

Igneous - In the case of meteorites the term “igneous” refers to a rock formed by solidification of a molten rock or metal. Examples of igneous meteorites include the iron meteorites, and basaltic meteorites (Howardites and Eucrites) thought to come from the asteroid Vesta.

Impact basin (also impact crater) - A circular depression on the surface of a planet, moon, asteroid, or other celestial body. Craters are typically caused by meteorite impacts. In the center of craters on Earth a crater lake often accumulates, and in craters formed by meteorites a central island (caused by rebounding crustal rock after the impact) is usually a prominent feature in the lake.

Inclination - In astronomy, the angle between one plane and another. The (equatorial) inclination of a planet is the angle between the plane of its equator and that of its orbit. The inclination of the orbit of a planet in the Solar System other than Earth is the angle between the plane of that orbit and the ecliptic.

Inelastic Collision - A reaction involving a change in the kinetic energy of the system, as in ionization, excitation, or capture; or a process which changes the energy level of the system.

Ion - A charged particle consisting of an atom, or group of atoms, that has either lost or gained electrons.

Ionization - Loss or gain by an atom of one or more electrons, by which process the atom becomes an ion and instead of being neutral, has a charge: positive if it has lost an electron, negative if it has gained one.

Ion jet propulsion - Any device in which an ionized propellant stream is accelerated by interactions of external and internal magnetic fields with electric currents driven through the stream, thereby imparting thrust to a spacecraft on which is it mounted.

IPS - Acronymn for Ion Propulsion System. (See ion jet propulsion above.)

Inscribe - To draw on figure within another so as to have as many incidences as possible.

Light - The radiant energy that enables organs of vision to perform the function of sight; more accurately called luminous energy. (see also Electromagnetic Radiation).
Visible - Electromagnetic radiation with wavelengths of or close to those detectable by the eye.
Emitted - An emanation of light from a light-giving body, such as the Sun
Infrared - Electromagnetic radiation with wavelengths too long to be detectable by the eye.
Reflected - Light that is bent back from an object.

Light Curve - A plot of the amount of light detected from an object (i.e. the apparent magnitude) as a function of time. Light curves provide evidence of eclipsing binaries, variable stars, and track the progress of nova and supernova explosions.

Magnification - The effect of an optical system on the apparent angular size of an object—an increase in angular size occurs if the magnification factor is greater than 1

Magma - Molten rock material within a melted planet or asteroid from which igneous rock results by cooling (adapted from Merriam-Webster.com)

Magnitude - An astronomical unit of brightness—originally corresponding to the eye's response to starlight—the magnitude system is logarithmic, with 5 magnitudes corresponding to a factor of 100 in brightness. To further confuse things larger magnitudes correspond to fainter objects.

Mantle - The part of a planet or asteroid between the crust and the core

Metamorphic - In the case of meteorites the term “metamorphic” refers to a rock containing minerals that have recrystallized due to heat in a parent body asteroid.

Meteor - The flash and trail of light that we see in the night sky caused by a meteoroid passing through the atmosphere

Meteorite - An object that passes through the Earth's atmosphere and is too large to be destroyed before it hits the surface—meteorites may in some way be connected with asteroids.

Meteoroid - An interplanetary piece of matter larger than dust, but smaller than an asteroid or dwarf planet

Minor planet - (see Asteroid)

Nebulosity - A hazy, cloudy, or misty characteristic of matter in a gaseous or finely divided state

Neutron capture - A process in which a neutron is absorbed by a nucleus to produce an excited nucleus that transitions to ground state by emission of one or more gamma rays

Observatory - A building equipped for observation of natural phenomena, as in meteorology or astronomy

Opposition - A configuration of the Sun, Earth and a planet or asteroid in which the apparent geocentric longitude of the planet or asteroid differs by 180 degrees from the apparent geocentric longitude of the Sun

Orbit - The path in space followed by a celestial body

Patent - A writing securing to an inventor the exclusive right to make, use, and vend his invention

Period of revolution - The interval of time required to complete one revolution in an orbit

Photochemical - Materials in which the absorption of light leads to a chemical reaction

Photographic plate - The flat surface on which the emulsion containing light-sensitive chemicals is placed; after exposing to light, the emulsion forms a negative of the object photographed.

Photometry - The measurement of light. Specifically refers to the procedure of highly accurate measuring of the apparent magnitudes of astronomical objects. In general, astronomers measure only a portion of the wavelength spectrum when they do photometry. Different types of photometry are defined by the portion of the wavelength that they examine. Differential photometry, or "UBV Photometry", measures the light within three standard regions defined by filters. These are Ultraviolet, Blue and Visual (hence UBV). There are many different photometry systems and standards.

Photomultiplier tube - A vacuum encapsulated photocathode from which electrons are ejected by the photoelectric effect followed by multiple cathodes from which many additional electrons are emitted in a cascade—when finally collected, the original single electron may have generated a pulse of over one million electrons; first used in astronomy in the early 1950s in a process known as differential photometry

Planetary orbit - The path in space followed by a planet

Planet - An object that formed in the disk surrounding a star—to be called a planet, an object must be more massive than Pluto (1/500 the Earth's mass) and less massive than ten times Jupiter's mass

Planetoids - (see Asteroids)

Power - The magnifying capacity of a lens, expressed as the number of times it multiplies the diameter of an object

Protoplanets - Early stage in the formation of planets according to the theory by which planetary systems evolve through the condensation of gas clouds surrounding a young star

Ptolemaic system or Ptolemaic Model of the Universe - A geocentric model in which the Earth remained stationary as the other planets the Sun, the Moon and the stars orbited it on their spheres— was eventually replaced by the Copernican model

Pyroxene - A common metasilicate, chiefly of calcium and magnesium, usually in short, thick, prismatic crystals; next to feldspar the most frequent constituent of igneous rocks

Reciprocity failure - The tendency for film emulsions to suffer a rapid fall-off in their ability to gather data after an initial surge of light

Reflection - The return of light or sound waves from a surface

Reflecting telescope - Telescope that uses mirrors to magnify and focus an image onto an eyepiece

Refraction - The bending of light as it passes from one medium to another

Refracting Telescope - Telescope that uses lenses to magnify and focus an image onto an eyepiece

Regmaglypts -Thumbprint-like indentations that are caused by parts of the meteorite melting and being sloughed off in its descent through Earth’s atmosphere

Regolith - The layer of rocky or icy debris and dust made by meteoritic impact that forms the uppermost surface of planets, moons and asteroids

Retina - The sensitive membrane of specialized cells in the eye which receives the image formed by the lens and is connected with the brain by the optic nerve

Right ascension (RA) - Right ascension is measured in hours of time. It is similar to longitude on the Earth. Astronomers have chosen the Vernal Equinox to define the starting point for the measurement of right ascension. The Vernal Equinox is the point where the Sun appears to cross the Celestial Equator at the beginning of spring. It is therefore one of the two points where the Ecliptic intersects the Celestial Equator.

Rotation - Of a single body in space, spinning on an axis; of a planetary system, rotation is generally planar in relation to the parent star

Rotation axis - (see Axis of rotation)

Scintillator - A phosphor-containing radiation detector that absorbs a photon or ionizing particle

Semiconductor - A substance or material, typically crystalline, that conducts electricity better than an insulator, but not as well as a conductor, because it allows current to flow only under certain condictions—common semiconductors are silicon, germanium, gallium, and arsenide

Semi-major axis - Half the length of the major axis of an ellipse; a standard element used to describe an elliptical orbit

Sensitivity - The capacity to respond to stimulation

Serendipity - Finding without seeking

Silver bromide - A yellowish, odorless powder that darkens on exposure to light; is used in photographic colloids

Silver iodide - A light yellow, odorless power that slowly darkens by light; is used in photographic colloids

Solar Nebula - The cloud of gas and dust that condensed and accumulated to form the Solar System.

Solar System - The complex of planets, asteroids, comets, debris gravitationally bound to the Sun and including the particles and fields emitted from the sun and interacting with its components

Spacecraft - A vehicle that can travel in outer space

Speckle interferometry - A process that uses ground-based telescopes and computer technology to make highly detailed or high-resolution images of asteroids by clustering together loads of tiny “specks” to form a clearer picture

Spectra - The energies of the photons emitted or absorbed by one atom are different from those of all other atoms. The photon energies are directly related to their frequencies, which set their colors in the spectrum, so by observing the colors of the photons, it is possible to determine which atoms are being observed. This can be done in a laboratory, and it can also be done with the light reaching us from stars, near or distant, which enables us to identify the atoms that stars are made of.

Spectroscope - An optical instrument used for producing and observing spectra of light or other radiation

Spectroscopy - Spectroscopy is the study of the detailed features of a star's spectrum, done by measuring the intensity of the star's light at as many different wavelengths as possible. The resulting spectrum of light allows one to locate the emission and absorption lines, determine the composition of the star, its doppler shift, its spectral type, and its luminosity class.

Telescope - An optical instrument for making distant objects appear closer or larger

Theory - In science a theory is a verified hypothesis applicable to many related phenomena.

Thermal Neutron - A neutron produced by fission and slowed by a moderator, so that it is in thermal equilibrium with its surrounding medium;  (E< ~0.3 eV)

Titius-Bode Law - The Titius-Bode Rule was first devised in 1772 and comprised the series 0 + 4/10, 3 + 4/10, 6 + 4/10, 12 + 4/10, 24 + 4/10 and so on. It was found to describe fairly accurately the distance in astronomical units of the then known planets from the Sun.

Vesta - The brightest of all minor planets, at times approaching naked-eye visibility, was discovered by Olbers in 1807.

Volatiles - Substances that change into a vapor at relatively low temperatures