aberration, chromatic, a colored
blurring of the image in refracting telescopes and cameras produced by
unequal refraction of light of different colors. See Optics.
aberration of light, a displacement
in the apparent position of a star, caused by the motion of the earth.
aberration, spherical, a blurring
of the image produced in a telescope or camera by a mirror or lens with
a spherical surface. See Optics.
albedo, the fraction of light
energy reflected back by a surface. See Solar System.
alt-azimuth mounting, a mounting
of an astronomical instrument that enables it to swing in two directions,
one following a circle of altitude parallel to the horizon, and the other
along a vertical circle of azimuth. See the Astronomical Instruments section
of this article.
altitude, second coordinate
in the alt-azimuth system: the angular distance of a celestial body above
the observer's horizon. See Celestial Sphere.
angstrom unit, a unit of length
used in measuring the wavelength of light, equal to 10-8 cm. Light.
apex, the point on the celestial
sphere towards which an object such as the sun or the earth is moving.aphelion.The
point in the orbit of a planet, or any member of the solar system, where
it is furthest from the sun. See Orbit.
apogee, the point in the orbit
of the moon, or an artificial satellite, at which it is furthest from the
earth. See Orbit; Moon.
apsides, line of, the two extremities
of an orbit, such as aphelion, perihelion, are each called an apsis. The
line of apsides joins them and contains the major axis of the orbit. See
Orbit; Solar System.
Aries, first point of, the vernal
equinox. When first defined, about 2,000 years ago, the point was located
in the constellation Aries. Precession has subsequently caused the point
to move about 20 degrees westward into the constellation Pisces. (See equinox.)
asteroids, the numerous small
planets and irregular fragments that revolve in almost circular orbits
between the orbits of Mars and Jupiter. A few asteroids have exceptional
orbits and pass close to the earth. See Solar System.
astronomical unit, abbreviated
a. u., the mean distance between the center of the earth and the center
of the sun. Also equal to the semimajor axis of the earth's orbit, 92.9
million miles, or 149.5 million kilometers. See Solar System.
azimuth, first coordinate in
the alt-azimuth system: the angle between a vertical circle drawn through
a celestial body and the observer's celestial meridian. Usually measured
west of South by astronomers and east of North by surveyors. See Celestial
Baily's beads, bright spots
of light seen just before the crescent sun is extinguished by the moon
during a total eclipse of the sun. The "beads" are produced by sunlight
shining through the valleys on the moon's limbs. See Eclipses.
BD, abbreviation for Bonner
Durchmusterung , a star catalog produced at Bonn Observatory under the
direction of F. W. A. Argelander in 1863. Subsequently replaced by the
Zweiter Katalog der Astronomischen Gesellschaft, abbreviated A.G.K.2.
binary star, two stars revolving
in orbits around a common center of mass. A "visual binary" is one whose
components can be seen separately; a "spectroscopic binary" is one identified
by a Doppler displacement in its spectral lines; when the orbits of the
components are edgewise to the earth, each star in turn obscures the light
from the other, and the result is an "eclipsing binary." See Star.
Bode's Law, a numerical series
that gives approximately the distances of the planets from the sun.
See Solar System.
Cassegrain focus, the point
on the axis of a reflecting telescope at which a star image is formed in
the Cassegrain arrangement, which employs a hyperbolic secondary mirror
to reflect the light back through a hole in the center of the primary mirror;
used in spectrographic work. See the Astronomical Instruments section of
celestial equator, a great circle
equidistant from the north and south celestial poles; the fundamental circle
of the equatorial system, located vertically above the equator of the earth.
See Celestial Sphere.
celestial meridian, a great
circle passing through an observer's zenith and through the north and south
celestial poles, cutting the horizon at the north and south cardinal points.
See Celestial Sphere.
celestial sphere, an imaginary
sphere around the earth on whose surface celestial objects appear to be
projected. See Celestial Sphere.
cepheid, a pulsating star with
a periodic variation of brightness, named after d Cephei, the type star.
chromosphere, the inner atmosphere
of the sun, extending from 300 to 6,000 miles above the photosphere. See
circumpolar star, a star that
always remains above the horizon. The angle between the star and the celestial
pole must be less than or equal to the observer's latitude. See Celestial
cluster, a group of stars or
galaxies that shows a tendency to hold together by mutual gravitational
forces between the members. See Galaxies; Milky Way; Star.
color excess, the difference
between the observed color index of a star and the color index corresponding
to its spectral type. A measure of the reddening of the starlight that
results from the scattering of blue wavelengths by interstellar dust grains.
See Interstellar Matter; Milky Way.
color index, the difference
between the photographic and visual magnitude of a star or any other luminous
object. Red stars of low surface temperature have a color index of +1.0
magnitude, blue-white stars of high surface temperature have a color index
of about -0.2 magnitude. See Galaxies; Star.
comet, a member of the solar
system, probably composed of ice and dust, which usually forms a long gaseous
tail as it comes close to the sun. See Comet; Solar System.
conjunction, a close alignment
of two or more members of the solar system when viewed from the earth;
two planets are in conjunction when they have the same ecliptic longitude.
Mercury and Venus both exhibit two conjunctions with the sun each synodic
period; at "inferior conjunction" the planet is between the earth and the
sun, at "superior conjunction" the sun is between the planet and the earth.
See Solar System.
Copernican system, system suggested
by Copernicus in which the earth and other planets are considered to move
in paths around the sun. This heliocentric model forms the basis of our
present-day concept of the solar system. See Solar System.
corona, outer atmosphere of
the sun extending millions of miles above the photosphere. The corona is
subdivided into an outer portion visible only during a total eclipse of
the sun, and an inner portion made visible by the coronagraph. See Eclipses;
Radio Astronomy; Sun.
coronagraph, an instrument used
to observe the sun's corona.See the Astronomical Instruments section of
coudé optical system, an arrangement of a reflecting
telescope in which the light is deflected along the polar axis so that
the image remains in a fixed position even while the telescope is rotated
to follow a star. See the Astronomical Instruments section of this article.
counter-glow, (See gegenschein.)
culmination, the coincidence
of an object with the celestial meridian; at upper culmination the star
or planet is at maximum altitude, at lower culmination it is at minimum
altitude and may be below the horizon. See Celestial Sphere.
dark cloud, a relatively dense,
cool cloud of interstellar matter whose dust particles obscure the light
from stars behind it, giving the cloud the appearance of a region devoid
date line, international, a
line, following approximately the meridian of longitude 180°, established
for regulating the calendar in circumnavigation and in global communications.
The calendar is moved one day forward when passing westward across the
day, the time that elapses between
two successive upper culminations of a chosen point on the celestial sphere.
The vernal equinox is the point used to define the sidereal day; the mean
sun is used to define the mean solar day used in calendar reckoning. See
Time; Celestial Sphere.
declination, the second coordinate
in the equatorial system, the angular distance of a celestial body north
or south of the celestial equator. See Celestial Sphere.
diffraction, the bending of
light in passing a sharp edge or through a tiny aperture or slit.
diurnal circle, the apparent
path of a celestial body as it is carried round by the daily rotation of
the celestial sphere; a parallel of declination. See Celestial Sphere.
double star, two stars that
appear close together in the sky. An "optical double" is a chance alignment
of unrelated stars, a "physical double" is a permanent association controlled
by gravitational forces. See Star.
earthlight (earthshine), the
faint illumination of the dark side of the moon by sunlight reflected from
the earth; it produces the effect called "the old moon in the new moon's
arms." See Moon; Solar System.
eclipse, an alignment of two
or more celestial bodies so that one of them is obscured from view. The
moon hides the sun in a solar eclipse, the earth's shadow produces an eclipse
of the moon. See Eclipses; Solar System.
ecliptic, the apparent path
of the sun around the celestial sphere once per tropical year; a great
circle in the plane of the earth's orbit. See Celestial Sphere; Solar System.
elongation, the angular separation
of two points on the celestial sphere; for a planet, the angle from the
sun to the meridian of celestial longitude through the planet. See Solar
ephemeris, a table of the computed
position of the sun, moon, planets, satellites, etc., for successive instants
equatorial mounting, a mounting
of an astronomical instrument that enables it to swing in two directions,
one following a circle of declination parallel to the celestial equator,
and the other toward or away from the celestial pole along a meridian of
right ascension. See the Astronomical Instruments section of this article;
equinox, one of the two points
on the celestial sphere at which the ecliptic cuts the equator. The center
of the sun coincides with the vernal equinox on March 20 or 21, and with
the autumnal equinox on Sept. 22 or 23. At these dates the duration between
sunset and sunrise is approximately 12 hours for all locations on the earth.
The vernal equinox (U) marks the zero meridian for the equatorial and ecliptic
systems of coordinates. (See Aries, first point of.) See Celestial Sphere;
faculae, bright streaks of hot
gas located near the top of the photosphere of the sun. See Sun.
flare, solar, a sudden and short-lived
brightening of a region of the chromosphere near a sunspot or a group of
sunspots, caused by the sudden release of large amounts of magnetic energy
in a relatively small volume above the photosphere. See also Sun.
flash spectrum, a series of
crescent-shaped emission lines from gases in the solar chromosphere produced
by pointing a slitless spectrograph at the narrow crescent of the sun that
appears for the instant just before the total phase of a solar eclipse.See
the Astronomical Instruments section of this article; Eclipses.
Fraunhofer lines, dark absorption
lines that appear in the otherwise continuous spectrum of the sun and stars.
See the Astronomical Instruments section of this article. and Spectra;
galactic cluster, a group of
stars in the plane of a spiral galaxy. See Milky Way.
galaxy, a gigantic system of
stars, nebulas, dust, and gas. See Galaxies; Milky Way.
the faint hazy patch of light visible at night in that portion of sky opposite
to the sun, produced by the scattering of sunlight by meteoric particles
in space. See Solar System.
geocentric system, (See Ptolemaic
gibbous, the phase of the moon,
Venus, or Mercury when the illumination is less than full but greater than
the quarter phase. See Moon; Solar System.
globular cluster, many stars
forming a compact, almost spherical group. Located outside the plane of
a spiral galaxy. See Milky Way.
granulation, the mottled pattern
of the sun's photosphere.
green flash, the green rim sometimes
seen as the sun sets or rises on a clear horizon; produced by the greater
bending of green and blue rays as sunlight is refracted in the earth's
harvest moon, in northern latitudes,
the full moon occurring nearest to Sept. 22 or 23 when the sun is at the
autumnal equinox and the moon at the vernal equinox. See Moon.
heliosphere, the region around
the sun where the solar wind dominates the interstellar medium. The heliosphere
has been observed out to the orbit of Saturn, but it probably extends much
a diagram showing the relation between color, or spectral class, and luminosity
for the different types of stars. See Star.
horizon, colloquially, the circle
around an observer where the earth seems to meet the sky. The astronomical
horizon is a great circle equidistant from the zenith and nadir of an observer,
the fundamental circle of the alt-azimuth systems. See Celestial Sphere.
hour angle, the angular distance
measured around the celestial equator westward from the celestial meridian
to the hour circle passing through a chosen point on the celestial sphere.
The hour angle of a star equals sidereal time minus the right ascension
of the star. See Celestial Sphere.
hour circle, a great circle
on the celestial sphere passing through the north and south celestial poles.
Similar to a line of longitude on the earth. See Celestial Sphere.
IC, abbreviation for the First
and Second Index Catalogues of celestial objects published by Johan
L. E. Dreyer in 1894 and 1908.
inclination, the angle between
the plane of an orbit and a reference plane; for example, the angle between
a planet's orbit and the plane of the ecliptic. See Orbit; Solar System.
Kepler's laws, three laws governing
the motion of the planets around the sun. See Celestial Mechanics; Solar
latitude, celestial, the second
coordinate in the ecliptic system, the angular distance of a celestial
body north or south of the ecliptic. See Celestial Sphere.
latitude, galactic, the angular
distance of a celestial body north or south of a great circle chosen to
represent the plane of the Milky Way. See Celestial Sphere; Milky Way.
latitude, terrestrial, the angle,
measured at the center of the earth, from the equatorial plane to any chosen
libration, an apparent oscillation
of a secondary body when viewed from the primary. The side-to-side swing
of the face of the moon results from the ellipticity of the lunar orbit;
the nodding results from the obliquity of the lunar axis. See Moon; Solar
light-year, the distance traveled
by light in a vacuum during one tropical year: 5,878 billion miles.
longitude, celestial, first
coordinate in the ecliptic system, the angular distance measured eastward
along the ecliptic from the vernal equinox to the meridian of longitude
passing through the poles of the ecliptic and a celestial body. See Celestial
longitude, galactic, the angular
distance measured eastward along the galactic equator from a point designated
as the galactic center to the meridian of longitude passing through the
galactic poles and a celestial body. See Celestial Sphere; Milky Way.
longitude, terrestrial, the
angle measured at the center of the earth, between the points where the
meridian of Greenwich and the meridian through any chosen location cut
M, abbreviation for Charles
Messier's catalog of nebulas and star clusters, published in 1782.
magnitude, apparent magnitude
is a measure of the brightness of a celestial body. Absolute magnitude
is the apparent magnitude the body would have if its distance were ten
parsecs. See Star.
mass-luminosity relation, the
relationship between mass and absolute magnitude to which most stars conform.
mean sun, an imaginary body
that moves at a uniform rate around the celestial equator once per tropical
year, introduced for the purpose of obtaining a uniform time scale. See
meridian, a great circle on
the celestial sphere that passes through the north and south celestial
poles. See Celestial Sphere.
meteor, the glowing trail produced
when a fragment of solid material is destroyed as it strikes the earth's
upper atmosphere. See Meteor; Solar System.
meteorite, a solid fragment
that falls to earth from outer space. See Meteorite; Solar System.
midnight sun, the sun when it
is seen above the horizon at lower culmination in the summer months of
the arctic and antarctic regions. See Celestial Sphere.
Milky Way, our galaxy. Also,
the hazy irregular band across the night sky, produced by the light of
millions of unresolved stars in our galaxy. See Milky Way.
month, an arbitrary division
in the calendar year (calendar month); the time taken for the moon to repeat
its phases (synodic month); the time taken for the moon to make one revolution
around the earth and return to a fixed point on the celestial sphere (sidereal
month). See Moon; Calendar.
multiple star, three or more
stars in close proximity. See Star.
nadir, the point on the celestial
sphere vertically below an observer. See Celestial Sphere.
nebula, a cloud of dust and/or
gas in space made visible by emission, reflection, or absorption of light.
Also, the unresolved glow from a cluster of stars or a galaxy. See Galaxies;
Interstellar Matter; Nebula.
nebular hypothesis, a theory
that suggests that the planets and the sun condensed from a rotating cloud
of gas. See Solar System.
Newtonian focus, the point at
the side of a reflecting telescope at which a star image is formed after
reflection from the optical axis by a plane secondary mirror. See the Optical
Telescopes section of this article.
NGC, abbreviation for the New
General Catalogue of Johan L. E. Dreyer, published in 1887.
noctilucent cloud, a glowing
cloud seen at night when the rays of the sun illuminate meteor debris in
the upper atmosphere. See Earth; Meteor.
node, the two points at which
an orbit cuts a reference plane. The reference plane for members of the
solar system is the ecliptic, and the nodes of the earth's orbit are the
vernal and autumnal equinoxes. See Orbit.
nova, a star that increases
more than a thousandfold in brightness during the course of a few hours,
and remains for several weeks as a "new" star in the sky. See Nova.
nutation, a slight wobble in
the precessional motion of the earth's axis.
objective prism, a large prism
placed in front of a camera lens to produce a spectrum of each star within
the field of view. See the Astronomical Instruments section of this article.
oblateness, a measure of the
equatorial bulge of a rotating planet; it is equal to the equatorial diameter
minus the polar diameter divided by the equatorial diameter. See Solar
obliquity, the angular distance
between the pole of a planet's axis and the pole of the ecliptic. See Celestial
Sphere; Solar System.
occultation, the obscuration
of one celestial body by another. See Moon; Solar System.
opposition, the situation of
a planet when its celestial longitude differs by 180° from that of
the sun. At opposition a planet is on the celestial meridian at midnight,
reaches maximum brightness, and is closest to the earth. See Solar System.
optical axis, the line that
passes through the center of a lens or mirror perpendicular to the surface.
orbit, the path of a body in
space. See Orbit.
parallax, the apparent displacement
of an object against a background when viewed from two ends of a base line.
If the angle of parallax, p, is small, then the distance of the object
D, is related to the length of the base line, B, by the expression D =
B/p, provided that the base line is perpendicular to the line of sight
and p is measured in radians. For a given base line, the angle itself can
be used as a measure of distance.
parsec, the distance at which
an object would have a heliocentric parallax of 1 second of arc; equals
19.2 trillion miles, or 3.26 light-years. See Star.
penumbra, the region of partial
shadow surrounding the dark shadow cone in an eclipse. Also the lighter
fringe surrounding a dark sunspot. See Eclipses.
perigee, the point in the orbit
of the moon of an artificial satellite at which it is closest to the earth.
perihelion, the point in the
orbit of a planet, or any member of the solar system, at which it is closest
to the sun. See Orbit; Solar System.
the relation between absolute magnitude and period for Cepheid variable
stars. See Star.
period, sidereal, the time taken
for a planet to make one revolution in its orbit, starting and finishing
at a line from the sun to a fixed point on the celestial sphere. See Orbit;
period, synodic, the time taken
for a planet or the moon to make one revolution in its orbit starting and
finishing at a line from the center of the earth to the center of the sun.
See Orbit; SolarSystem.
perturbation, a disturbance
that causes a celestial body to deviate from its regular orbit. See Orbit;
phase, any stage in the cyclic
variation of visibility of the illuminated hemisphere of the moon or a
planet; crescent, gibbous, full. See Moon; Solar System.
phase angle, the angle between
a ray of light from the sun to the moon (or planet) and the reflected ray
from the moon (or planet) to the observer.
photosphere, the opaque layer
of the sun or a star that appears as an incandescent surface. See Star;
plage, a bright area around
a sunspot, located in the chromosphere. See Sun.
planetesimal theory, a rejected
theory according to which the planets condensed from a stream of fragments
torn out of the sun by a passing star. See Solar System.
polar axis, a rotation axis
in the equatorial mounting for a telescope, pointing toward the celestial
poles. See the Optical Telescopes section of this article.
pole, a point at which a diameter
cuts a sphere. The earth's axis cuts the earth's surface at the north and
south poles and the celestial sphere at the north and south celestial poles.
A plane cuts the celestial sphere in a great circle and any line perpendicular
to the plane cuts at two poles equidistant from the great circle.
precession, the conical motion
of the earth's axis around the pole of the ecliptic every 26,000 years,
caused by the gravitational pull of the moon and sun on the earth's equatorial
bulge. It produces a movement in the position of the vernal equinox and
changes in the coordinates of all celestial bodies. See Earth.
prominence, a hot filamentary
cloud of gas in the sun's corona which appears bright red when seen at
the limb of the sun.
proper motion, the change in
the apparent position of a star after allowance has been made for parallax,
aberration, and precession. See Star.
protoplanet, the original agglomeration
of material from which a planet formed. See Solar System.
Ptolemaic system, a system of
planetary motion described by Ptolemy in which the sun, moon, and planets
revolved around the stationary earth. Replaced by the Copernican system.
See Solar System.
quadrature, the situation of
the moon or planet when its celestial longitude differs by 90° from
that of the sun. See Moon; Solar System.
radial velocity, the speed of
a celestial body along the line of sight toward or away from the observer.
radiant, for a single meteor,
the point where the trail, when extended backward, intersects the celestial
sphere; for a stream of meteors traveling in parallel paths, the perspective
vanishing point of the trails from which the meteors appear to come. See
radio star, a small region in
the sky that emits radio waves. See Radio Astronomy.
red-shift, See Doppler Effect.
reflector, a telescope using
a mirror to form the primary image. See the Optical Telescopes section
of this article.
refractor, a telescope using
a lens to form the primary image. See the Optical Telescopes section of
regression, a clockwise revolution
of the nodes of an orbit when viewed from the north pole of the ecliptic.
resolving power, a measure of
the detail that can be seen with an instrument. If two stars q seconds
of arc apart can just be separated then the resolving power is 1/q. See
the Optical Telescopes section of this article.
right ascension, first coordinate
in the equatorial system. The angular distance measured eastward along
the celestial equator from the vernal equinox to the meridian of right
ascension passing through the celestial poles and a celestial body. See
saros, the interval of time
that elapses before the cycle of solar and lunar eclipses repeats; approximately
18 years, 11& days. See Eclipses.
satellite, a body that revolves
in an orbit around a more massive primary. See Moon; Orbit; Solar System.
scintillation, the irregular
variation of light from a star, colloquially called twinkling, produced
by refraction and diffraction in turbulent layers of the earth's atmosphere.
seasons, the four divisions
of the year, spring, summer, fall, and winter, that commence when the center
of the sun reaches critical points on the ecliptic, namely, the vernal
equinox, summer solstice, autumnal equinox, and winter solstice. See Seasons.
semimajor axis, one half of
the longest diameter of an ellipse. See Orbit; Solar System.
solar constant, a measure of
the power received from the sun; the number of calories passing normally
in 1 minute through 1 square centimeter of surface outside the atmosphere
of the earth at a distance of 1 astronomical unit from the sun; 2.00 cals/cm2/min.
solstices, the two points on
the ecliptic where the sun is at maximum declination north, +23¹°
(summer solstice for northern hemisphere) and maximum declination south,
-23¹° (winter solstice for northern hemisphere). See Celestial
Sphere; Solar System.
spectrum, the array of colors
into which a beam of light is decomposed by means of a prism or diffraction
grating. See Spectra.
spicule, a narrow jet of incandescent
gas that persists in the chromosphere of the sun for several minutes. See
sunspot, a cool region in the
photosphere of the sun, which is seen as a dark spot. See Sun.
telluric bands or lines, deficiencies
in the spectrum of the sun, moon, or planet, produced by absorption in
the atmosphere of the earth. See Spectra; Sun.
terminator, the line that separates
the illuminated and unilluminated hemispheres of a moon or planet. See
time, apparent solar, the hour
angle of the sun measured westward from the celestial meridian, 15°
corresponding to 1 hour. The moment when the sun is on the meridian is
apparent noon. Apparent time is given by a simple sundial. See Celestial
Sphere; Sundial; Time.
time, ephemeris, time defined
by the orbital motion of celestial bodies, principally the moon; used for
astronomical predictions. See Time.
time, equation of, the difference
at any instant between apparent and mean solar time; right ascension of
the mean sun minus right ascension of the real sun. See Time.
time, mean solar, the westward
hour angle of the mean sun. When the mean sun is on the meridian, the mean
solar time is 12 noon. See Celestial Sphere; Time.
time, sidereal, the westward
hour angle of the vernal equinox. See Celestial Sphere; Time.
time, standard, the time adopted
legally by a city or country for the major portion of the year. Standard
meridians at longitudes 15°, 30°, 45°, etc., west of Greenwich
have a mean solar time that is 1, 2, 3, etc., hours earlier than Greenwich
mean solar time. Usually, a city follows the time of the nearest standard
meridian. In summer months, a city may introduce daylight saving time,
when it follows the time of the next standard meridian to the west. See
time, universal, the mean solar
time of the Greenwich meridian. See Time.
transit, the passasge of a celestial
body across a line or region in the sky. A star is in transit when on the
celestial meridian; Mercury and Venus appear as dark spots when they transit
across the disc of the sun; a moon is in transit when it crosses the disc
of the primary planet such as Jupiter. See Solar System.
twilight, the faint sunlight
scattered by the earth's upper atmosphere after sunset and before sunrise.
Civil twilight ends when the sun is 6° below the horizon; astronomical
twilight ends and night falls when the sun is 18° below the horizon.
Twilight occurs on any planet or moon that has an atmosphere.
variable star, a star that varies
in brightness. An "eclipsing variable" is produced when stars of an eclipsing
binary system periodically obscure each other. An "intrinsic variable"
is a star that changes its luminosity, such as a Cepheid or a nova. See
vernal equinox, (See equinox;
Aries, first point of.)
year, anomalistic, the time
taken for the earth to make one revolution around the sun starting and
finishing at perihelion; 365.2596 days. See Orbit
year, eclipse, the interval
between two successive arrivals of the sun at the ascending node of the
moon's orbit; 346.620 days. See Eclipses.
year, leap, a year with 366
mean solar days, effected by inserting February 29th when the year number
is divisible by 4, such as 1964, or by 400 if the year marks a new century,
such as 2000. See Calendar; Time.
year, sidereal, the time taken
for the earth to make one revolution around the sun starting and finishing
at a line drawn from the center of the sun to a fixed point, on the celestial
sphere; 365.2564 days. See Time.
year, tropical, the interval
between two successive returns of the sun to the vernal equinox; 365.2422
days. This is the year on which the calendar is based. See Calendar.
zenith, the point on the celestial
sphere vertically above an observer. See Celestial Sphere.
zodiac, a zone that extends
about 9° on either side of the ecliptic containing the paths of the
sun, moon, and principal planets. It is marked by twelve constellations
and is divided into the twelve signs of the zodiac. See Constellation;
zodiacal light, a faint glow
extending along the ecliptic most easily seen in the direction of the sun
at the end of astronomical twilight; caused by the scattering of sunlight
by meteoric dust in the plane of the solar system. See Solar System.