A
absolute magnitude -- a
scale for measuring the actual brightness of a celestial object without accounting for the
distance of the object. Absolute magnitude measures how bright an object would appear if
it were exactly 10 parsecs (about 33 light years) away from Earth. On this scale, the Sun
has an absolute magnitude of +4.8 while it has an apparent magnitude of -26.7 because it
is so close.
accretion -- process of
particles sticking together to form larger bodies; for example, solar nebular dust
accreted to form chondrules, and planetesimals accreted to form planets.
achondrite -- a stony
meteorite representing differentiated planetary material.
albedo feature -- a dark
or light marking on the surface of an object that may or may not be a geological or
topographical feature. Albedo is the measure of the reflectivity of a planet, measured on
a scale from zero to one. An albedo of zero describes a planet that absorbs all the light
it receives. A planet with an albedo of one reflects all the light that shines on it.
Alpha Centauri -- the
closest bright star to our solar system.
altitude -- angle in
degrees above the horizon.
Andromeda --
the closest galaxy to the Milky Way. It is approximately 2 million light years away.
Angstrom -- abbreviated
Å. A unit of length equal to 10-8 cm (one-hundredth of a millionth of a centimeter). An
Angstrom is on the order of the size of an atom.
antimatter -- matter
consisting of particles with charges opposite that of ordinary matter. In antimatter,
protons have a negative charge while electrons have a positive charge.
antipodal point -- the
point that is directly on the opposite side of the planet; e.g., the Earth's north pole is
antipodal to its south pole.
apastron -- the point of
greatest separation of two stars, such as in a binary star system.
aperture -- the size of
the opening through which light passes in an optical instrument such as a camera or
telescope. A higher number represents a smaller opening while a lower number represents a
larger opening.
aphelion -- the point in
its orbit where a planet is farthest from the Sun.
apoapsis -- the point in
orbit farthest from the planet.
apogee -- the point in
orbit farthest from the Earth.
apparent magnitude -- the
apparent brightness of an object in the sky as it appears to an observer on Earth. Bright
objects have a low apparent magnitude while dim objects will have a higher apparent
magnitude.
arc degree -- a unit of
angular measure in which there are 360 arc degrees in a full circle.
arc minute -- one 1/60 of
a degree.
arc second -- abbreviated
arcsec. A unit of angular measure in which there are 60 arc seconds in 1 arc minute and
therefore 3600 arc seconds in 1 arc degree. There are 206,265 arcseconds per radian. One
arc second is equal to about 725 km on the Sun.
asteroid -- a small
planetary body in orbit around the Sun, larger than a meteoroid but smaller than a planet.
astrochemistry -- the
branch of science that explores the chemical interactions between dust and gas
interspersed between the stars.
Astronomical unit (AU) -- the
average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometers (92,960,116
miles).
atmosphere -- one
atmosphere is 14.7 pounds per square inch (105
aurora -- a glow in a
planet's ionosphere caused by the interaction between the planet's magnetic field and
charged particles from the Sun.
aurora borealis -- the Northern Lights caused by the interaction between the solar
wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the
southern hemisphere where it is known as the aurora australis.
aurora australis -- also
known as the southern lights, this is an atmospheric phenomenon that displays a diffuse
glow in the sky in the southern hemisphere. It is caused by charged particles from the Sun
as they interact with the Earth's magnetic field. Known as the Aurora Borealis in the
northern hemisphere.
autumnal equinox -- the
point on the celestial sphere where the sun crosses the celestial equator from north to
south. The time when the sun is at the autumnal equinox defines the first day of autumn.
This happens on about September 22 each year.
average orbital -- speed
around the Sun: This is a measure of how fast a planet moves through space, in kilometers
per hour.
axial inclination -- axial
inclination is the angle at which a planet's axis of rotation is tilted, with respect to
that planet's own orbit. On Earth, as well as other planets, this tilt is responsible for
the seasons.
axis -- also known as the
poles, this is an imaginary line through the center of rotation of an object.
azimuth -- the angular
distance of an object around or parallel to the horizon from a predefined zero point.
B
bar -- a unit of measure
of atmospheric pressure. One bar is equal to 0.987 atmospheres, 1.02 kg/cm2, 100
kilopascal, and 14.5 lbs/square inch.
Big Bang Theory -- the
theory that suggests that the universe was formed from a single point in space during a
cataclysmic explosion about 18 billion years ago. The force of the explosion accounts for
the current expansion of the universe.
binary stars -- a system
of two stars that revolve around a common center of gravity.
black hole -- the
collapsed core of a massive star. The remains of an extremely massive star pulled into a
small volume by the force of gravity.
blueshift -- a shift in
the lines of an object's spectrum toward the blue end. Blueshift indicates that an object
is moving toward the observer. The larger the blueshift, the faster the object is moving.
bolide -- an exploding
meteorite.
C
carbonaceous chondrite -- a
type of primitive chondrite with evidence of nebular processes.
celestial equator -- the
intersection of the earth's equatorial plane with the celestial sphere.
celestial pole -- the north and south celestial poles are points on the celestial
sphere where earths axis of rotation intersects the celestial sphere.
celestial sphere -- an
imaginary sphere centered on the earth on which all of the stars are imagined to be
projected.
cepheid -- a pulsating
variable star. This type of star undergoes a rhythmic pulsation as indicated by its
regular pattern of changing brightness as a function of time. The period of pulsation has
been demonstrated to be directly related to a Cepheid's intrinsic brightness making
observations of these stars one of the most powerful tools for determining distance known
to modern day astronomy.
Chicxulub Crater -- an ancient impact crater buried underneath the Yucatán Peninsula in Mexico generally associated with contributing to the extinction of the dinosaurs.
chondrite -- a meteorite
containing chondrules and other components produced in the solar nebula.
chondrule -- small,
glassy spheres commonly found in meteorites.
chromatic aberration -- an
optical lens defect causing color fringes, because the lens material brings different
colors of light to focus at different points.
chromosphere -- the layer
of the solar atmosphere that is located above the photosphere and beneath the transition
region and the corona. The chromosphere is hotter than the photosphere but not as hot as
the corona.
circumpolar star -- a
star that never sets but always stays above the horizon. This depends on the location of
the observer. The further South you go the fewer stars will be circumpolar. Polaris, the
North Star, is circumpolar in most of the northern hemisphere.
coma -- a spherical cloud
of material surrounding the head of a comet. This material is mostly gas that the Sun has
caused to boil off the comet's icy nucleus. This gas shines both by reflected sunlight and
light emitted by excited molecules. A cometary coma can extend up to a million miles from
the nucleus.
comet -- a chunk of frozen gasses, ice, and rocky debris that orbits the Sun in a
long, narrow ellipse. A comet nucleus is about the size of a mountain on earth. When a
comet nears the Sun, heat vaporizes the icy material producing a cloud of gaseous material
surrounding the nucleus, called a coma. As the nucleus begins to disintegrate, it also
produces a trail of dust or dust tail in its orbital path and a gas or ion tail pointing
away from the Sun. Comet comas can extend up to a million miles from the nucleus and comet
tails can be millions of miles long.
conjunction -- an event
that occurs when two or more celestial objects appear close close together in the sky.
constellation
-- a group of stars that appear to make a pattern in the sky. The Big Dipper is one of 88
constellations.
convection -- the
physical upwelling of hot matter, thus transporting energy from a lower, hotter region to
a higher, cooler region. A bubble of gas that is hotter than its surroundings expands and
rises. When it has cooled by passing on its extra heat to its surroundings, the bubble
sinks again. Convection can occur when there is a substantial decrease in temperature with
height, such as in the Sun's convection zone.
convection zone -- a
layer in a star in which convection currents are the main mechanism by which energy is
transported outward. In the Sun, a convection zone extends from just below the photosphere
to about seventy percent of the solar radius.
Copernicus, Nicholas
– scientist who proposed the the Sun was the center of the Solar System
(heliocentrism) and not the Earth.
corona -- the outermost
layer of the solar atmosphere. The corona consists of a highly rarefied gas with a low
density and a temperature greater than one million degrees Kelvin. It is visible to the
naked eye during a solar eclipse.
coronal mass ejections -- are
huge bubbles of gas threaded with magnetic field lines that are ejected over the course of
several hours. They are often associated with solar flares and prominence eruptions.
cosmic ray -- atomic
nuclei (mostly protons) that are observed to strike the Earth's atmosphere with extremely
high amounts of energy.
cosmic string -- a tube
like configuration of energy that is believed to have existed in the early universe. A
cosmic string would have a thickness smaller than a trillionth of an inch but its length
would extend from one end of the visible universe to the other.
cosmology -- a branch of
science that deals with studying the origin, structure, and nature of the universe.
crystalline -- indicates
a rock is composed of mineral crystals rather than glass. In general, when igneous melts
cool very fast they form glass (like obsidian), but when they cool slower, mineral
crystals have an opportunity to grow.
D
dark matter -- a term
used to describe matter in the universe that cannot be seen, but can be detected by its
gravitational effects on other bodies.
declination -- the
angular distance of an object in the sky from the celestial equator.
density -- the amount of
matter contained within a given volume. Density is measured in grams per cubic centimeter
(or kilograms per liter). The density of water is 1.0, iron is 7.9, and lead is 11.3.
differentiated -- when a
(partially) molten body has been divided into two or more fractions of dissimilar
compositions. In the case of the Earth, iron-nickel metal was differentiated from silicate
material to form the planet's core.
disk -- the surface of
the Sun or other celestial body projected against the sky.
Doppler effect -- the
apparent change in wavelength of sound or light emitted by an object in relation to an
observer's position. An object approaching the observer will have a shorter wavelength
(blue) while an object moving away will have a longer (red) wavelength. The Doppler effect
can be used to estimate an object's speed and direction.
double asteroid -- two
asteroids that revolve around each other and are held together by the gravity between
them.
double star -- a grouping
of two stars.
E
Earth -- (Latin- Terra,
Greek- Gaia) Earth is named after the ancient Germanic Goddess of the soil. It is
the third planet from the Sun.
eclipse -- the
partial or complete blocking from view of one object by another.
ejecta -- material from
beneath the surface of a body such as a moon or planet that is ejected by an impact such
as a meteor and distributed around the surface. Ejecta usually appears as a lighter color
than the surrounding surface.
electromagnetic spectrum -- the
entire range of all the various kinds or wavelengths of electromagnetic radiation,
including (from short to long wavelengths) gamma rays, x-rays, ultraviolet, optical
(visible), infrared, and radio waves.
electron flux -- the rate
of flow of electrons through a reference surface. In cgs units, measured in electrons s-1,
or simply s-1.
electron volt -- Abbreviated
ev. A unit of energy used to describe the total energy carried by a particle or photon.
The energy acquired by an electron when it accelerates through a potential difference of 1
volt in a vacuum. 1 ev = 1.6 x 10-12 erg.
ellipse -- an ellipse is
an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical
in shape rather than circular.
elliptical galaxy -- a
galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures
such as spiral arms.
elongation -- the angular
distance of a planetary body from the Sun as seen from Earth. A planet at greatest eastern
elongation is seen in the evening sky and a planet at greatest western elongation will be
seen in the morning sky.
emission nebula -- a type
of nebula that shines by emitting light when electrons recombine with protons to form
hydrogen atoms. The electron frequently approaches the proton in steps emitting energy as
light as it gets pulled in. In one of the most common "steps," the recombining
electron emits a photon of red light. Since many atoms in the nebula do this all at once,
the nebula appears red in color. This type of nebula is created when energetic ultraviolet
light from a hot star shines on a cloud of hydrogen gas, stripping away electrons from the
atoms (ionization). The free electrons can then begin the process of recombination.
energy flux -- the rate
of flow of energy through a reference surface. In cgs units, measured in erg s-1. Also
measured in watts, where 1 watt = 1 x 107 erg s-1. Flux density, the flux measured per
unit area, is also often referred to as "flux".
enstatite -- a type of
primitive chondrite. That chondrite is dominated by the silicate mineral enstatite.
ephemeris -- a table of
data arranged by date. Ephemeris tables are typically to list the positions of the Sun,
Moon, planets and other Solar System objects.
equinox -- the two days of the year on which
neither hemisphere is tilted toward or away from the sun. The equinoxes occur on or near March 21 and September 22. The
equinoxes signal the start of the Spring and Autumn seasons.
erg -- a cgs unit of
energy equal to work done by a force of 1 dyne acting over a distance of 1 cm. 107 (ten
million) erg s-1 (ergs per second) = 1 watt. Also, 1 Calorie = 4.2 x 1010 (42 billion)
ergs.
escape velocity -- the
speed required for an object to escape the gravitational pull of a planet or other body.
event horizon -- the
invisible boundary around a black hole past which nothing can escape the gravitational
pull - not even light.
evolved star -- a star
that is near the end of its life cycle where most of its fuel has been used up. At this
point the star begins to loose mass in the form of stellar wind.
extragalactic -- a term
that means outside of or beyond our own galaxy.
extraterrestrial -- a
term used to describe anything that does not originate on Earth.
eyepiece -- the lens at
the viewing end of a telescope.
F
faculae -- bright patches
that are visible on the Sun's surface, or photosphere.
filament --a strand of
cool gas suspended over the photosphere by magnetic fields, which appears dark as seen
against the disk of the Sun.
finder -- small,
wide-field telescope attached to a larger telescope. The finder is used to help point the
larger telescope to the desired viewing location.
fireball -- an extremely
bright meteor. Also known as bolides, fireballs can be several times brighter than the
full Moon. Some can even be accompanied by a sonic boom.
flare (solar) -- rapid
release of energy from a localized region on the Sun in the form of electromagnetic
radiation, energetic particles, and mass motions.
flare star -- a member of
a class of stars that show occasional, sudden, unpredicted increases in light. The total
energy released in a flare on a flare star can be much greater that the energy released in
a solar flare.
footpoint -- the
intersection of magnetic loops with the photosphere.
free electron -- an
electron that has broken free of it's atomic bond and is therefore not bound to an atom.
frequency -- the number
of repetitions per unit time of the oscillations of an electromagnetic wave (or other
wave). The higher the frequency, the greater the energy of the radiation and the smaller
the wavelength. Frequency is measured in Hertz.
fusion -- a process where
nuclei collide so fast they stick together and emit a great deal of energy. In the center
of most stars, hydrogen fuses together to form helium. Fusion is so powerful it supports
the star's enormous mass from collapsing in on itself, and heats the star so high it glows
as the bright object we see today.
G
galactic halo -- the name
given to the spherical region surrounding the center, or nucleus of a galaxy.
galactic nucleus --a
tight concentration of stars and gas found at the innermost regions of a galaxy.
Astronomers now believe that massive black holes may exist in the center of many galaxies.
galaxy -- a large system
of about 100 billion stars. Galaxies are classified in three main groups; spirals,
ellipticals and irregulars.
Galilean moons -- the
name given to Jupiter's four largest moons, Io, Europa, Callisto & Ganymede. They were
discovered independently by Galileo Galilei and Simon Marius.
Galileo Galilei
-– the first scientist to make systematic use of the telescope in looking at the
heavens.
gamma ray -- the highest
energy (shortest wavelength) photons in the electromagnetic spectrum. Gamma rays are often
defined to begin at 10 kev, although radiation from around 10 kev to several hundred kev
is also referred to as hard x-rays.
geomagnetic storm -- a
worldwide disturbance of the Earth's magnetic field, associated with solar activity.
geosynchronous orbit --
the orbit of a satellite that travels above the Earth's equator from west to east so that
it has a speed matching that of the Earth's rotation and remains stationary in relation to
the Earth (also called geostationary). Such an orbit has an altitude of about 35,900 km
(22,300 miles).
Giant Molecular Cloud (GMC)
-- massive clouds of gas in interstellar space composed primarily of hydrogen
molecules. These clouds have enough mass to produce thousands of stars and are frequently
the sites of new star formation.
globular cluster -- a
tight, spherical grouping of hundreds of thousands of stars. Globular clusters are
composed of older stars, and are usually found around the central regions of a galaxy.
granulation -- a pattern
of small cells that can be seen on the surface of the Sun. They are caused by the
convective motions of the hot gases inside the Sun.
gravitational lens -- a
concentration of matter such as a galaxy or cluster of galaxies that bends light rays from
a background object. Gravitational lensing results in duplicate images of distant objects.
gravity -- a mutual
physical force of nature that causes two bodies to attract each other with a magnitude
determined by their masses and distance between them.
Greenhouse Effect -- an
increase in temperature caused when incoming solar radiation is passed but outgoing
thermal radiation is blocked by the atmosphere. Carbon dioxide and water vapor are two of
the major gases responsible for this effect.
H
H II region -- a region
of hot gas surrounding a young star or stars that is mostly ionized. The energetic light
from these young stars ionizes the existing gas. This region typically appears red as it
glows with the photons emitted when elections recombine with hydrogen protons.
Hale Bopp -- a comet that passed Earth in 1997 and will not be visible again until approximately 4380. The passage of the comet inspired a mass suicide by the religious cult Heaven's Gate.
Halley's Comet -- one of the most famous comets that passed Earth in 1986 and will return in 2061. The comet was first recorded in the historical record by Chinese astronomers in 240 BC.
heliocentric -- sun
centered.
heliopause -- the point
at which the solar wind meets the interstellar medium or solar wind from other stars.
heliosphere -- the space within the boundary of the heliopause containing the Sun
and solar system.
helium -- the second
lightest and second most abundant element. The typical helium atom consists of a nucleus
of two protons and two neutrons surrounded by two electrons. Helium was first discovered
in our Sun. Roughly 25 percent of our Sun is helium.
hemisphere -- a half of
the celestial sphere that is divided into two halves by either the horizon, the celestial
equator, or the ecliptic.
hertz -- abbreviated Hz.
A unit of frequency equal to one cycle per second. One khz = 1000 Hz. One mhz = 106 (one
million) Hz. One ghz = 109 Hz.
hour angle -- the
telescope based coordinate specifying the angle, in the equatorial plane, from the
meridian to a plane containing the celestial object and the north and south celestial
poles.
H-R diagram -- The
Color-Magnitude Diagram is a graph upon which stars are plotted by spectral type and
actual luminosity. It is named for the two scientists Russell and Hertzsprung who first
used it in 1913.
Hubble's Law -- the law
of physics that states that the farther a galaxy is from us, the faster it is moving away
from us.
hydromagnetic wave -- a
wave in which both the plasma and magnetic field oscillate.
hypergalaxy -- a system
consisting of a spiral galaxy surrounded by several dwarf white galaxies, often
ellipticals. Our galaxy and the Andromeda galaxy are examples of hypergalaxies.
I
impact -- a collision
between two planetary bodies. In the case when one is much smaller than the other (like a
meteoroid colliding with the Earth), a crater may be produced on the larger body.
impact melt spherule -- spherules
of shock-melted rock ejected from an impact crater. Most of these objects cool rapidly in
the Earth's atmosphere and solidify to a glassy state. However, some may remain molten
until they splash onto the ground or into water where they are quenched to form solidified
particles of glass.
inclination -- a measure
of the tilt of a planet's orbital plane in relation to that of the Earth.
inertia -–
the tendency of a moving object to continue in a straight line or a stationary object to
remain in place.
inferior planet -- a
planet that orbits between the Earth and the Sun. Mercury and Venus are the only two
inferior planets in our Solar System.
infrared -- light that is
so red, humans cannot see it. A band of the electromagnetic spectrum between the visible
and the microwave. Photons of infrared light are less energetic than photons of visible
light.
interplanetary magnetic field
-- the magnetic field carried along with the solar wind.
interstellar medium -- the
gas and dust that exists in open space between the stars.
ion -- an atom that has lost or gained one or more electrons and
has become electrically charged as a result.
ionization -- the process
by which ions are produced, typically occurring by collisions with atoms or electrons
("collisional ionization"), or by interaction with electromagnetic radiation
("photoionization").
ionosphere -- the region
of the Earth's upper atmosphere containing a small percentage of free electrons and ions
produced by ultraviolet radiation. In Earth's atmosphere, the ionosphere begins at an
altitude of about 25 miles and extends outward about 250.
iron meteorite -- a
meteorite which is composed mainly of iron mixed with smaller amounts of nickel.
irregular galaxy -- a
galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually
filamentary or very clumpy in shape.
isotope -- one of two or
more atoms having the same number of protons in its nucleus, but a different number of
neutrons and, therefore, a different mass.
J
Jovian planet -- any of
the four outer, gaseous planets: Jupiter, Saturn, Uranus, and Neptune.
Julian Date (JD) -- the
interval of time in days and fraction of a day since 1 January 4713 BC,
Jupiter -–
Jupiter is named after the Roman king of the gods. It
is the fifth planet from the Sun.
K
Kepler's First Law -- planet
orbits the Sun in an ellipse with the Sun at one focus.
Kepler's Second Law -- a
ray directed from the Sun to a planet sweeps out equal areas in equal times.
Kepler's Third Law -- the
square of the period of a planet's orbit is proportional to the cube of that planet's
semimajor axis; the constant of proportionality is the same for all planets.
kev -- one thousand
electron volts.
kiloparsec -- a distance
equal to 1000 parsecs.
Kuiper belt -- a large
ring of icy, primitive objects beyond the orbit of
L
latitude -- a
measurement of distance from the equator, expressed in degrees north or south.
lenticular galaxy -- a
disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular
galaxies tend to look more like elliptical galaxies than spiral galaxies.
libration -- an effect
caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps
the same side toward the Earth, but due to libration, 59% of the Moon's surface can be
seen over a period of time.
light-year -- the
distance light travels through a vacuum in a year, at the rate of 300,000 kilometers per
second (671 million miles per hour); 1 light-year is equivalent to 9.46053e12 km,
5,880,000,000,000 miles or 63,240 AU.
local group -- a small
group of about two dozen galaxies of which our own Milky Way galaxy is a member.
longitude -- a
measurement of distance from the Prime
luminosity -- the amount
of light emitted by a star.
Luna – the
name of Earth’s only moon.
lunar eclipse -- a
phenomenon that occurs when the Moon passes into the shadow of the Earth. A partial lunar
eclipse occurs when the Moon passes into the penumbra, or partial shadow. In a total lunar
eclipse, the Moon passes into the Earth's umbra, or total shadow.
lunar month -- the
average time between successive new or full moons. A lunar month is equal to 29 days 12
hours 44 minutes. Also called a synodic month.
lunation -- the interval
of a complete lunar cycle, between one new Moon and the next. A lunation is equal to 29
days, 12 hours, and 44 minutes.
M
Magellanic clouds -- two
small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic
clouds are visible in the skies of the southern hemisphere.
magnetic field -- a field
of force that is generated by electric currents. The Sun's average large-scale magnetic
field, like that of the Earth, exhibits a north and a south pole linked by lines of
magnetic force.
magnetic field lines -- imaginary
lines that indicate the strength and direction of a magnetic field. The orientation of the
line and an arrow show the direction of the field. The lines are drawn closer together
where the field is stronger. Charged particles move freely along magnetic field lines, but
are inhibited by the magnetic force from moving across field lines.
magnetic pole -- either
of two limited regions in a magnet at which the magnet's field is most intense.
magnetosphere -- the area
around a planet most affected by its magnetic field. The boundary of this field is set by
the solar wind.
magnitude -- The degree
of brightness of a star or other object in the sky according to a scale on which the
brightest star has a magnitude -1.4 and the faintest visible star has magnitude 6.
Sometimes referred to as apparent magnitude. In this scale, each number is 2.5 times the
brightness of the previous number. Thus a star with a magnitude of 1 is 100 times brighter
than on with a visual magnitude of 6.
Main Belt -- the area
between Mars and Jupiter where most of the asteroids in our Solar System are found.
major planet -- a name
used to describe any planet that is considerably larger and more massive than the Earth,
and contains large quantities of hydrogen and helium. Jupiter and Neptune are examples of
major planets.
mare -- latin word for
"sea." Galileo thought the dark featureless areas on the Moon were bodies of
water, even though the Moon is essentially devoid of liquid water. The term is still
applied to the basalt-filled impact basins common on the face of the Moon visible from
Earth.
Mars –
(Greek- Ares) Mars is named after the Roman god of war.
It is the fourth planet from the Sun.
megaton -- an explosive
force equal to one million metric tons of TNT. The energy released in the explosion of one
megaton of TNT is equal to 4.2 x 1022 ergs.
Mercury –
(Greek- Hermes) Mercury is named after the Roman messenger of the gods. It is the closest to the Sun.
meridian -- an imaginary
circle drawn through the North and South poles of the celestial equator.
metamorphic rock -- a
rock that has been heated and compressed so that it recrystallizes, but does not melt.
meteor -- a small
particle of rock or dust that burns away in the Earth's atmosphere. Meteors are also
referred to as shooting stars.
Meteor Crater -- a 1200 ft. diameter, 570 ft. deep crater located in northern Arizona created by the impact of a large meteorite 50,000 years ago.
meteor shower -- an event
where a large number of meteors enter the Earth's atmosphere from the same direction in
space at nearly the same time. Most meteor showers take place when the Earth passes
through the debris left behind by a comet.
meteor –-
a meteoroid that has entered Earth’s atmosphere.
meteorite -- an object,
usually a chunk or metal or rock, that survives entry through the atmosphere to reach the
Earth's surface.
meteoroid
-– a chuck of rock or dust in space.
mev -- one million
electron volts.
Milky Way --
the galaxy where Earth is located.
millibar -- a measure of
atmospheric pressure equal to 1/1000 of a bar. Standard sea-level pressure on Earth is
about 1013 millibars.
minor planet -- another
name used to describe a large asteroid.
molecular cloud -- an
interstellar cloud of molecular hydrogen containing trace amounts of other molecules such
as carbon monoxide and ammonia.
moon -– a
planetary satellite.
N
nadir -- a term used to
describe a point directly underneath an object or body.
nebula -- a large amount
of gas and dust in space, spread out in an immense volume, usually illuminated by one or
more stars. Nebulae represent the raw material the stars are made of.
Neptune –
(Greek- Poseidon)
neutrino -- a fundamental
particle supposedly produced in massive numbers by the nuclear reactions in stars; they
are very hard to detect because the vast majority of them pass completely through the
Earth without interacting.
neutron star -- a compressed core of an exploded star made up almost entirely of
neutrons. Neutron stars have a strong gravitational field and some emit pulses of energy
along their axis. These are known as pulsars.
Newton's Second Law of Motion
-- for an unbalanced force acting on a body, the acceleration produced is proportional
to the force impressed; the constant of proportionality is the inertial mass of the body.
nova -–
the explosion of a dying giant or supergiant star.
nuclear fusion -- a
nuclear process whereby several small nuclei are combined to make a larger one whose mass
is slightly smaller than the sum of the small ones. Nuclear fusion is the reaction that
fuels the Sun, where hydrogen nuclei are fused to form helium.
O
oblateness -- a measure
of flattening at the poles of a planet or other celestial body.
obliquity -- the angle
between a body's equatorial plane and orbital plane.
occulation -- the blockage of light by the intervention of another object; a planet
can occult (block) the light from a distant star
observatory
-– a building that contains one or more telescopes.
Oort cloud -- a theoretical shell of comets that is believed to exist at the
outermost regions of our Solar System. The Oort cloud was named after the Dutch astronomer
who first proposed it.
open cluster --a
collection of young stars that formed together. They may or may not be still bound by
gravity.
opposition -- the position of a planet when it is exactly opposite the Sun as
seen from Earth. A planet at opposition is at its closest approach to the Earth and is
best suitable for observing.
orbit -- the path of an
object that is moving around a second object or point.
orbital period -- the
amount of time it takes a spacecraft or other object to travel once around it's orbit.
P
palimpsest -- a circular
feature on the surface of dark icy moons such as Ganymede and Callisto lacking the relief
associated with craters; Pamlimpsests are thought to be impact craters where the
topographic relief of the crater has been eliminated by slow adjustment of the icy
surface.
pallasite -- a stony-iron
meteorite in which nodules of olivine (a silicate mineral) are surrounded by a network of
iron-nickel metal.
parallax -- the apparent
change in position of two objects viewed from different locations.
parsec -- a large
distance often used in astronomy. A parsec is equal to 3.26 light years.
patera -- shallow crater; scalloped, complex edge.
peak ring -- a central uplift characterized by a ring of peaks rather than a single
peak; peak rings are typical of larger terrestrial craters above about 50 kilometers (30
miles) in diameter.
penumbra -- the area of partial illumination surrounding the darkest part of a
shadow caused by an eclipse.
periapsis -- the point in
the orbit closest to the planet.
perigee -- the point in
the orbit of the Moon or other satellite at which it is closest to the Earth.
perihelion -- the point
in the orbit of a planet or other body where it is closest to the Sun.
perturb -- to cause a
planet or satellite to deviate from a theoretically regular orbital motion.
phase -- the apparent
change in shape of the Moon and inferior planets as seen from Earth as they move in their
orbits.
photon -- a discrete
quantity of electromagnetic energy. Short wavelength (high frequency) photons carry more
energy than long wavelength (low frequency) photons.
photosphere -- The
visible surface of the Sun; the upper surface of a convecting layer of gases in the outer
portion of the sun whose temperature causes it to radiate light at visible wavelengths;
sunspots and faculae are observed in the photosphere.
planet -- a very large
body in orbit around a star. Planets can be composed mainly of rock or of dense gases.
planetary nebula
-- a shell of gas surrounding a small, white star.
planetesimal -- a rocky
and/or icy body, a few to several tens of kilometers in size, that was produced in the
solar nebula.
plasma -- plasma consists
of a gas heated to sufficiently high temperatures that the atoms ionize. The properties of
the gas are controlled by electromagnetic forces among constituent ions and electrons,
which results in a different type of behavior. Plasma is often considered the fourth state
of matter (besides solid, liquid, and gas). Most of the matter in the Universe is in the
plasma state.
Pluto –
(Greek- Hades) Pluto is named after the Roman god of the Underworld. It is the ninth planet from the Sun.
precession -- the
apparent shift of the celestial poles caused by a gradual wobble of the Earth's axis.
prominence -- an
explosion of hot gas that erupts from the Sun's surface. Solar prominences are usually
associated with sunspot activity and can cause interference with communications on Earth
due to their electromagnetic effects on the atmosphere.
proper motion -- the
apparent angular motion across the sky of an object relative to the Solar System.
proton -- a positively
charged elementary particle. A proton is 1836 times heavier than an electron.
protostar -- dense
regions of molecular clouds where stars are forming.
Proxima Centauri
-- the closest star to Earth besides the sun. It is approximately 4 light years away.
pulsar -- a spinning
neutron star (burnt-out star) that emits energy along its gravitational axis. This energy
is received as pulses as the star rotates.
Q
quadrature -- a point in
the orbit of a superior planet where it appears at right angles to the Sun as seem from
Earth.
quasar -- An unusually
bright object found in the remote areas of the universe. Quasars release incredible
amounts of energy and are among the oldest and farthest objects in the known universe.
They may be the nuclei of ancient, active galaxies.
R
radial velocity -- the
movement of an object either towards or away from a stationary observer.
radiant -- a point in the
sky from which meteors in a meteor shower seem to originate.
radiation -- energy
radiated through space in the form of waves or particles; photons.
radiation belt -- a
ring-shaped region around a planet in which electrically charged particles (usually
electrons and protons) are trapped.
radio galaxy -- a galaxy
that gives off large amounts of energy in the form of radio waves.
regolith -- the layer of rocky debris and dust made by metoritic impact that forms
the uppermost surface of planets, satellites and asteroids.
red giant -- a stage in the evolution of a star when the fuel begins to exhaust and
the star expands to about fifty times its normal size. The temperature cools, which gives
the star a reddish appearance.
redshift -- a shift in
the lines of an object's spectrum toward the red end. Redshift indicates that an object is
moving away from the observer. The larger the redshift, the faster the object is moving.
reflection nebula -- a
type of nebula that shines by reflected light. Bright stars near reflection nebulae emit
light into the region that is reflected by the large amount of dust there. The size of the
dust grains causes blue light to be reflected more efficiently than red light, so these
reflection nebulae frequently appear blue in color.
retrograde -- the
rotation or orbital motion of an object in a clockwise direction when viewed from the
north pole of the ecliptic; moving in the opposite sense from the great majority of solar
system bodies.
right ascension -- the amount of time that passes between the rising of Aries and
another celestial object. Right ascension is one unit of measure for locating an object in
the sky.
Roche limit -- the
smallest distance from a planet or other body at which purely gravitational forces can
hold together a satellite or secondary body of the same mean density as the primary. At a
lesser distance the tidal forces of the primary would break up the secondary.
rotation -- the spin of a
body about its axis.
S
satellite -- a natural or
artificial body in orbit around another object in space.
Saturn –-
(Greek- Kronos) Saturn is named after the Roman god of the harvest. It is the sixth planet from the Sun.
semimajor axis -- one-half
of the longest dimension of an ellipse.
Seyfert Galaxy -- a
main-sequence star which rotates rapidly, causing a loss of matter to an ever-expanding
shell.
Shoemaker-Levy 9 -- a comet that broke apart into several alrge pieces and collided with Jupiter in 1994. The scars from the impacts of the fragments were more visible than the the Great Red Spot and could be seen for months.
Sidereal -- relating to,
or concerned with the stars. Sidereal rotation is that measured with respect to the stars
rather than with respect to the Sun or the primary of a satellite.
Sidereal day -- the
interval of time between two consecutive transits of the vernal equinox. More intuitively,
it is the length of time required for Earth to make one full rotation with respect to the
celestial sphere -- approximately four minutes shorter than the solar day.
Sidereal month -- the
average period of revolution of the moon around the earth in reference to a fixed star,
equal to 27 days, 7 hours, 43 minutes in units of mean solar time.
Sidereal period -- the
period of revolution of a planet around the Sun or a satellite around its primary.
solar atmosphere -- the
atmosphere of the Sun. An atmosphere is generally the outermost gaseous layers of a
planet, natural satellite, or star. Only bodies with a strong gravitational pull can
retain an atmosphere. Atmosphere is used to describe the outer layer of the Sun because it
is relatively transparent at visible wavelengths. Parts of the solar atmosphere include
the photosphere, chromosphere, and the corona.
solar cycle -- the
approximately 11-year quasi-periodic variation in frequency or number of solar active
events.
solar eclipse -- a
phenomenon that occurs when the Earth passes into the shadow of the Moon. A total solar
eclipse occurs when the Moon is close enough to completely block the Sun's light. An
annular solar eclipse occurs when the Moon is farther away and is not able to completely
block the light. This results in a ring of light around the Moon.
solar flare -- a bright
eruption of hot gas in the Sun's photosphere. Solar prominences are usually only
detectable by specialized instruments but can be visible during a total solar eclipse.
Solar System --
the Sun with all the celestial bodies that revolve around it.
solar wind -- a flow of
charged particles that travels from the Sun out into the Solar System.
solstice -- the two days
of the year on which the noon sun is directly overhead at either 23.5° South or 23.5°
North. The solstices mark the beginning of the Summer and Winter seasons.
South Atlantic Anomaly -- the
region over the South Atlantic Ocean where the lower Van Allen belt of energetic,
electrically charged particles is particularly close to the Earth's surface. The excess
energy in the particles presents a problem for satellites in orbit around the Earth.
spectral line -- a line
in a spectrum due to the emission or absorption of electromagnetic radiation at a discrete
wavelength. Spectral lines result from discrete changes in the energy of an atom or
molecule. Different atoms or molecules can be identified by the unique sequence of
spectral lines associated with them.
spectrograph -- an
instrument that spreads light or other electromagnetic radiation into it's component
wavelengths (spectrum), recording the results photographically or electronically.
spectrometer -- the
instrument connected to a telescope that separates the light signals into different
frequencies, producing a spectrum.
spectroscopy -- the
technique of observing the spectra of visible light from an object to determine its
composition, temperature, density, and speed.
spectrum -- electromagnetic radiation arranged in order
of wavelength. A rainbow is a natural spectrum of visible light from the Sun.
spicules -- the range of
colors produced when visible light passes through a prism.
spiral galaxy -- a galaxy
that contains a prominent central bulge and luminous arms of gas , dust, and young stars
that wind out from the central nucleus in a spiral formation. Our galaxy, the Milky Way,
is a spiral galaxy.
star --
a giant ball of hot gas that creates and emits its own radiation through nuclear
fusion. Stars are arranged in various classes by their spectral characteristics. The chief
classes are identified by the letters O, B, A, F, G, K, M. Each class contains ten
subdivisions numbered from 0 to 9. The classes define a temperature or color sequence.
Stars of type O and B are Blue-white and have high temperatures (35,000°K / 20,000°K); A
stars are white, temperature 10,000°K; F and G stars are yellowish, temperature 7,000°K;
K stars are orange, and M stars are red, temperature 3,000°K. Wolf-Rayet stars are hot,
blue giants. They are extremely turbulent and have a temperature of 50,000 °K and higher
star cluster -- a large
grouping of stars, from a few dozen to a few hundred thousand, that are bound together by
their mutual gravitational attraction.
Steady State Theory -- the
theory that suggests the universe is expanding but exists in a constant, unchanging state
in the large scale. The theory states that new matter is being continually being created
to fill the gaps left by expansion. This theory has been abandoned by most astronomers in
favor of the big bang theory.
stellar wind -- the
ejection of gas from the surface of a star. Many different types of stars, including our
Sun, have stellar winds. The stellar wind of our Sun is also known as the Solar wind. A
star's stellar wind is strongest near the end of its life when it has consumed most of its
fuel.
stone meteorite -- a
meteorite which resembles a terrestrial rock and is composed of similar materials.
stony iron -- a meteorite
which contains regions resembling both a stone meteorite and an iron meteorite.
stratosphere -- the cold
region of a planetary atmosphere above the convecting regions (the troposphere), usually
without vertical motions but sometimes exhibiting strong horizontal jet streams.
Sun (Latin-
Sol, Greek- Helios) – the closest star to Earth. A giant ball of gas (70% H-28%He and
2% all other elements) without any solid surfaces. The Sun’s energy comes from the
nuclear fusion of Hydrogen into Helium.
sunspot -- a temporary disturbed area in the solar photosphere that appears dark
because it is cooler than the surrounding areas. Sunspots consist of concentrations of
strong magnetic flux. They usually occur in pairs or groups of opposite polarity that move
in unison across the face of the Sun as it rotates.
supergiant -- the stage
in a star's evolution where the core contracts and the star swells to about five hundreds
times its original size. The star's temperature drops, giving it a red color.
superior conjunction -- a
conjunction that occurs when a superior planet passes behind the Sun and is on the
opposite side of the Sun from the Earth.
supernova -- the death
explosion of a massive star, resulting in a sharp increase in brightness followed by a
gradual fading.
supernova remnant -- an
expanding shell of gas ejected at high speeds by a supernova explosion. Supernova remnants
are often visible as diffuse gaseous nebulae usually with a shell-like structure. Many
resemble "bubbles" in space.
synchronous rotation -- a
satellite's rotational period is equal to its orbital period; this causes the same side of
a satellite to always face the planet.
T
tektite -- Spherule of
molten rock ejected from an impact crater and then cooled rapidly to produce glass.
telescope -- an instrument used to collect large amounts of light from far away
objects and increase their visibility to the naked eye.
terrestrial -- a term
used to describe anything originating on the planet Earth.
terrestrial planet -- a
name given to a planet composed mainly of rock and iron, similar to that of Earth.
thermonuclear fusion -- the
combination of atomic nuclei at high temperatures to form more massive nuclei with the
simultaneous release of energy. Thermonuclear fusion is the power source at the core of
the Sun.
tidal forces -- the gravitational pull on planetary objects from nearby planets and
moons.
tide -- the
rise and fall of sea level caused by the gravitational pull of the Sun and Moon on the
Earth.
transit -- the passage of a celestial body across an observer's meridian; also the
passage of a celestial body across the disk of a larger one.
Trojan satellites -- satellites
which orbit at the Lagrangian points, 60° ahead of and 60° behind another satellite. For
example, Telesto and Calypso are trojans of Saturn's satellite Tethys.
troposphere -- the lower
regions of a planetary atmosphere where convection keeps the gas mixed and maintains a
steady increase of temperature with depth. Most clouds are in the troposphere.
U
ultraviolet -- electromagnetic
radiation at wavelengths shorter than the violet end of visible light. The atmosphere of
the Earth effectively blocks the transmission of most ultraviolet light, which can be
deadly to many forms of life. The light that is so blue humans cannot see it.
umbra -- the area of
total darkness in the shadow caused by an eclipse.
Universal Time (UT) -- also
known as Greenwich Mean Time, this is local time on the
universe -- all
of space and everything in it.
Uranus –
(Greek- Ouranos)Uranus is named after the Roman god of the sky. It is the seventh planet from the Sun.
V
Van Allen Belts -- radiation
zones of charged particles that surround the Earth. The shape of the Van Allen belts is
determined by the Earth's magnetic field.
variable star -- a star
that fluctuates in brightness. These include eclipsing binaries.
Venus –
(Greek- Aphrodite)named after the Roman goddess of love.
It is the planet closest in size to Earth and the second planted from the
Sun.
vernal equinox -- the
point on the celestial sphere where the sun crosses the celestial equator from south to
north. The time when the sun is at the vernal equinox defines the first day of spring.
This happens on about March 20 each year.
Virgo Cluster -- a
gigantic cluster of over 2000 galaxies that is located mainly within the constellation of
Virgo. This cluster is located about 60 million light years from Earth.
visible light -- wavelengths
of electromagnetic radiation that are visible to the human eye.
visual magnitude -- A
scale used by astronomers to measure the brightness of a star or other celestial object.
Visual magnitude measures only the visible light from the object. On this scale, bright
objects have a lower number than dim objects.
W
wavelength -- the
distance from crest to crest or trough to trough of an electromagnetic wave (see
electromagnetic radiation) or other wave.
white dwarf -- A very
small, white star that is the remnant core of a star that has completed fusion in its
core.
white light -- visible light that includes all colors and, therefore, all visible
wavelengths.
Wolf-Rayet stars -- are
hot, blue giants. They are extremely turbulent and have a temperature of 50,000 °K and
higher
X
x-ray -- electromagnetic
radiation of a very short wavelength and very high-energy. X-rays have shorter wavelengths
than ultraviolet light but longer wavelengths than cosmic rays. Because x-rays are
absorbed by the Earth's atmosphere, x-ray astronomy is performed in space.
x-ray star -- a bright
celestial object that gives off x-rays as a major portion of its radiation.
Z
zenith -- a point
directly overhead from an observer.
zodiac -- an imaginary
belt across the sky in which the Sun, Moon, and all of the planets can always be found.
zodiacal light -- a faint
cone of light that can sometimes be seen above the horizon after sunset or before sunrise.
Zodiacal light is caused by sunlight reflecting off small particles of material in the
plane of the Solar System.