630 GLOSSARY
radiative zone (152) The region inside a star
where energy is carried outward as photons.
radio interferometer (115) Two or more radio
telescopes that combine their signals to achieve
the resolving power of a larger telescope.
ray (446) Ejecta from a meteorite impact, form-
ing white streamers radiating from some lunar
craters.
reconnection (163) The process in the sun’s
atmosphere by which opposing magnetic fi elds
combine and release energy to power solar fl ares.
red dwarf (172) Cool, low-mass star on the lower
main sequence.
redshift (138) The lengthening of the wave-
lengths of light seen when the source and observer
are receding from each other.
refl ecting telescope (101) A telescope that uses a
concave mirror to focus light into an image.
refracting telescope (101) A telescope that forms
images by bending (refracting) light with a lens.
regolith (451) A soil made up of crushed rock
fragments.
regular satellite (495) A moon with an orbit
that has small eccentricity, low inclination to
the equator of its parent planet, and is prograde.
Regular moons are thought to have formed with
their respective planets rather than having been
captured.
relative age (445) The age of a geological feature
referred to other features. For example, relative
ages reveal that the lunar maria are younger than
the highlands.
resolving power (104) The ability of a telescope
to reveal fi ne detail; depends on the diameter of
the telescope objective.
resonance (457) The coincidental agreement
between two periodic phenomena; commonly
applied to agreements between orbital periods,
which can make orbits more stable or less stable.
retrograde motion (56) The apparent backward
(westward) motion of planets as seen against the
background of stars.
revolution (20) The motion of an object in a
closed path about a point outside its volume;
Earth revolves around the sun.
rift valley (434) A long, straight, deep valley pro-
duced by the separation of crustal plates.
RNA (ribonucleic acid) (585) A long carbon-
chain molecule that uses the information stored
in DNA to manufacture complex molecules nec-
essary to the organism.
Roche limit (499) The minimum distance be-
tween a planet and a satellite that holds itself to-
gether by its own gravity. If a satellite’s orbit brings
it within its planet’s Roche limit, tidal forces will
pull the satellite apart.
rotation (20) The turning of a body about an axis
that passes through its volume; Earth rotates on
its axis.
saros cycle (44) An 18-year 11__ 31 -day period af-
ter which the pattern of lunar and solar eclipses
repeats.
Schmidt-Cassegrain focus (110) The optical
design of a refl ecting telescope in which a thin
correcting lens is placed at the top of a Cassegrain
telescope.
scientifi c argument (27) An honest, logical dis-
cussion of observations and theories intended to
reach a valid conclusion.
scientifi c method (8) The reasoning style by
which scientists test theories against evidence to
understand how nature works.
scientifi c model (17) An intellectual concept de-
signed to help you think about a natural process
without necessarily being a conjecture of truth.
scientifi c notation (3) The system of recording
very large or very small numbers by using powers
of 10.
secondary atmosphere (433) The gases outgassed
from a planet’s interior; rich in carbon dioxide.
secondary crater (446) A crater formed by the
impact of debris ejected from a larger crater.
secondary mirror (110) In a refl ecting telescope,
the mirror that refl ects the light to a point of easy
observation.
seeing (104) Atmospheric conditions on a given
night. When the atmosphere is unsteady, produc-
ing blurred images, the seeing is said to be poor.
seismic wave (429) A mechanical vibration that
travels through Earth; usually caused by an earth-
quake.
seismograph (429) An instrument that records
seismic waves.
selection effect (552, 555) An infl uence on the
probability that certain phenomena will be detected
or selected, which can alter the outcome of a survey.
semimajor axis (a) (64) Half of the longest axis
of an ellipse.
SETI (595) Search for Extra-Terrestrial Intelli-
gence.
shear (S) wave (429) A mechanical wave that
travels through Earth’s interior by the vibration
of particles perpendicular to the direction of wave
travel.
shepherd satellite (515) A satellite that, by its
gravitational fi eld, confi nes particles to a plan-
etary ring.
shield volcano (472) Wide, low-profi le volcanic
cone produced by highly liquid lava.
shock wave, or shock (205, 212) A sudden
change in pressure that travels as an intense sound
wave.
sidereal drive (111) The motor and gears on a
telescope that turn it westward to keep it pointed
at a star.
sidereal period (35) The period of rotation or
revolution of an astronomical body relative to the
stars.
sinuous rille (444) A narrow, winding valley on
the moon caused by ancient lava fl ows along nar-
row channels.
small-angle formula (39) The mathematical for-
mula that relates an object’s linear diameter and
distance to its angular diameter.smooth plain (459) Apparently young plain on
Mercury formed by lava fl ows at or soon after the
formation of the Caloris Basin.
solar constant (161) A measure of the energy
output of the sun; the total solar energy striking
1 m^2 just above Earth’s atmosphere in 1 second.
solar eclipse (38) The event that occurs when the
moon passes directly between Earth and the sun,
blocking your view of the sun.
solar nebula theory (399) The proposal that the
planets formed from the same cloud of gas and
dust that formed the sun.
solar system (3) The sun and the nonluminous
objects that orbit it, including the planets, com-
ets, and asteroids.
solar wind (147) Rapidly moving atoms and ions
that escape from the solar corona and blow out-
ward through the solar system.
south celestial pole (18) The point of the celes-
tial sphere directly above Earth’s South Pole.
south point (18) The point on the horizon direct-
ly above the south celestial pole; exactly south.
special theory of relativity (91) The fi rst of Ein-
stein’s theories of relativity, which deals with uni-
form motion.
spectral class or type (131) A star’s position in
the temperature classifi cation system O, B, A F,
G, K, and M. Based on the appearance of the
star’s spectrum.
spectral line (113) A dark or bright line that
crosses a spectrum at a specifi c wavelength.
spectral sequence (131) The arrangement of
spectral classes (O, B, A, F, G, K, M) ranging
from hot to cool.
spectrograph (113) A device that separates light
by wavelength to produce a spectrum.
spectrum (100) An arrangment of electromagnet-
ic radiation in order of wavelength or frequency.
spicule (146) Small, fl amelike projection in the
chromosphere of the sun.
spiral arm (6) Long, spiral pattern of bright stars,
star clusters, gas, and dust that extends from the
center to the edge of the disk of spiral galaxies.
spiral galaxy (178) A galaxy with an obvious disk
component containing gas; dust; hot, bright stars;
and spiral arms.
sporadic meteor (557) A meteor not part of a
meteor shower.
spring tide (87) Ocean tide of high amplitude
that occurs at full and new moon.
star (3) A celestial object composed of gas held
together by its own gravity and supported by nu-
clear fusion occurring in its interior.
stony-iron meteorite (552) A meteorite that is a
mixture of stone and iron.
stony meteorite (552) A meteorite composed of
silicate (rocky) material.
stromatolite (587) A layered fossil formation
caused by ancient mats of algae or bacteria that
build up mineral deposits season after season.