ACCRETION DISK:
a disk surrounding a black hole or star in
which matter gravitationally falls onto
the central object.
ALTAZIMUTH MOUNT:
a mount that enables a telescope to move
freely both in altitude and in azimuth.
ALTITUDE:
the angle between an object's position on the
celestial sphere and the horizon
APERTURE:
the diameter of a telescopes primary lens
or mirror, the larger the aperture, the
greater the light-gathering power
APHELION:
the point farthest from the sun in an objects
orbit
ARCMINUTE:
a unit of angular size equal to 1/60
of a degree
ARCSECOND:
a unit of angular size equal to 1/3,600
of a degree(or l/60 of an arcminute).
ASTRONOMICAL UNIT(AU):
the average distance from Earth to the sun,
equal to about 93,000,000 miles(150,000,000km).
AVERTED VISION:
a technique that uses the more light-sensitive
rods in the eye to better see a faint object
AZIMUTH:
the angle measured eastward form due north to
the point on the horizon directly below
an object
BIG BANG(theory):
what scientists believe was a giant explosion
that created the universe 10 to 20 billion
years ago.
BINARY STAR:
a systemof two stars that orbit a common center
of gravity; also known as a double star
BLACK HOLE:
a region of space where gravity is so powerful
that not even light can escape
BROWN DWARF:
a gaseous object that forms like a star but lacks
the necessary mass to sustain nuclear fusion in
its core; a body intermediate in mass between a
star and planet.
CCD(charged-coupled device:
a silicon chip used ot detect light; CCDs are
far
more efficient at collecting light than
conventional film
CLOCK DRIVE:
a motor attached to an equatorial mount that
compensates for Earth's rotation and thus keeps
the telescope pointing at the same area of sky
CONJUNCTION:
a time when two or more bodies appear close
together in the sky
COSMIC BACKGROUND RADIATION:
microwave radiation that permeates the universe
and represents the still-cooling glow from
the Big Bang
COSMIC RAY:
an atomic nucleus(most are protons) moving
at a speed approaching that of light
DARK MATTER:
matter that exerts gravitiational force but
does not emit any detectable light or radiation;
dark matter comprises most of the mass of the
universe but its exact nature remains unknown
DECLINATION:
the angular distanc of an object above or
below the celestial equator; the celestial
sphere equivalent of latitude
DEEP-SKY OBJECTS:
objects located beyoond the solar system;
consists of stars, nebulae, star clusters,
and galaxies
DEGREE:
a unit of angular size equal to 1/360 the
circumference of the celestial sphere; the
sun and the moon both apear to be about a
half a degree wide
DOPPLER EFFECT:
the change in wavelength coming from a source
that's moving toward of away from an observer;
produces either a blueshift of redshift
ECLIPSE:
an event in which one body passes in front
of another, blocking it partially or
completely from view.
Annular:
a solar eclipse in which the moon does not
fully cover the sun's disk, allowing observers
to see a thin ring of sunlight
Lunar:
an eclipse of the moon caused by the Earth passing between the sun and moon
Solar:
an eclipse of the sun caused by the moon
passing between Earth and the sun
ECLIPTIC:
the plane of the Earth's orbit around the
sun; all the planets except Mercury and Pluto
have orbits in nearly the same plane
ELECTROMAGNETIC RADIATION:
the various forms of light;includes radio
waves, infrared light, visible light,
ultraviolet light, x rays and gamma rays.
ELONGATION:
the apparent angular separation of an object
from the sun
EQUATORIAL MOUNT:
a mount in which one axis lies parallel to
Earth's rotational axis; the motion of the
telescoope about this axis can compensate for
Earth's rotation
EVENT HORIZON:
the boundary of a black hole from inside which
light cannot escape
EYEPIECE:
a magnifying lens used to view the image produced
by a telescope's objective
FOCAL LENGTH:
the distance from a lens or mirror to the
point where it brings light to a focus
FOCAL RATIO (F/RATIO):
the ratio of the focal length of a lens or
mirror to its diameter
FOCUS:
the point at which rays of light passing through
a lens (or reflection off a mirror) converge
GAMMA RAYS:
the form of light with the shortest wavelength
and the most energy
GAS GIANT:
a large planet made primarily of gas such as
Jupiter, Saturn, Uranus and Neptune
GIBBOUS:
the phase of the moon between first quarter
and last quarter, when the moon appears more
than half illuminated
GLOBULAR CLUSTER:
a roughly spherical congregation of hundreds
of thousands of stars; most globular clusters
consist of old stars and exist in a galaxy's halo
GRAVITATIONAL LENSING:
the distortion of amplification of an object's
light due to the presence of a massive object
in the light path
H - Z
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