GLOSSARY 631thermophile (591) A type of microorganism that
thrives in high-temperature environments.
tidal coupling (443) The locking of the rota-
tion of a body to its revolution around another
body.
tidal heating (505) The heating of a planet or
satellite because of friction caused by tides.
total lunar eclipse (36) A lunar eclipse in
which the moon completely enters Earth’s dark
shadow.
total solar eclipse (38) A solar eclipse in which
the moon completely covers the bright surface of
the sun.
totality (36) The period during a solar eclipse
when the sun’s photosphere is completely hidden
by the moon, or the period during a lunar eclipse
when the moon is completely inside the umbra of
Earth’s shadow.
transit (418) The passage of an extrasolar planet
across the disk of its parent star as observed from
Earth, partially blocking the light from the star
and allowing detection and study of the planet.
transition (133) The movement of an electron
from one atomic orbit to another.
Trojan asteroid (563) Small, rocky body caught
in Jupiter’s orbit at the Lagrangian points, 60°
ahead of and behind the planet.
ultraviolet radiation (100) Electromagnetic
radiation with wavelengths shorter than visible
light but longer than X-rays.
umbra (33) The region of a shadow that is totally
shaded.
uncompressed density (408) The density a plan-
et would have if its gravity did not compress it.
uniform circular motion (53) The classical belief
that the perfect heavens could move only by the
combination of constant motion along circular
orbits.
valley networks (483) Dry drainage channels re-
sembling streambeds found on Mars.
Van Allen belt (431) One of the radiation belts
of high-energy particles trapped in Earth’s mag-
netosphere.velocity (79) A rate of travel that specifi es both
speed and direction.
vernal equinox (22) The place on the celestial
sphere where the sun crosses the celestial equator
moving northward; also, the time of year when
the sun crosses this point, about March 21, and
spring begins in the Northern Hemisphere.
vesicular (450) A porous basalt rock formed by
solidifi ed lava with trapped bubbles.
violent motion (77) In Aristotelian physics, motion
other than natural motion. (See natural motion.)
water hole (595) The interval of the radio spec-
trum between the 21-cm hydrogen radiation and
the 18-cm OH radiation, likely wavelengths to
use in the search for extraterrestrial life.
wavelength (99) The distance between successive
peaks or troughs of a wave; usually represented by λ.
wavelength of maximum intensity (λmax) (128)
The wavelength at which a perfect radiator emits
the maximum amount of energy; depends only
on the object’s temperature.
weak force (150) One of the four forces of na-
ture; the weak force is responsible for some forms
of radioactive decay.
west point (18) The point on the western horizon
exactly halfway between the north point and the
south point; exactly west.
white dwarf (172) The remains of a dying star
that has collapsed to the size of Earth and is slowly
cooling off; at the lower left of the H–R diagram.
Widmanstätten pattern (552) Bands in iron me-
teorites due to large crystals of nickel–iron alloys.
winter solstice (22) The point on the celestial
sphere where the sun is farthest south; also, the
time of year when the sun passes this point, about
December 22, and winter begins in the Northern
Hemisphere.
Zeeman effect (155) The splitting of spectral
lines into multiple components when the atoms
are in a magnetic fi eld.
zenith (18) The point directly overhead on the sky.
zodiac (25) The band around the sky centered on
the ecliptic within which the planets move.strong force (150) One of the four forces of
nature; the strong force binds protons and neu-
trons together in atomic nuclei.
S-type asteroid (564) A type of asteroid common
in the inner asteroid belt, with relatively high
refl ectivity and reddish color, probably composed
of rocky material.
subduction zone (434) A region of a planetary
crust where a tectonic plate slides downward.
subsolar point (466) The point on a planet that
is directly below the sun.
summer solstice (22) The point on the celes-
tial sphere where the sun is at its most northerly
point; also, the time when the sun passes this
point, about June 22, and summer begins in the
Northern Hemisphere.
sunspot (143) Relatively dark spot on the sun
that contains intense magnetic fi elds.
supergiant (173) Exceptionally luminous star, 10
to 1000 times the sun’s diameter.
supergranule (145) A large granule on the sun’s
surface including many smaller granules.
supernova (173, 268) A “new” star appearing in
Earth’s sky and lasting for a year or so before fad-
ing. Caused by the violent explosion of a star.
synodic period (35) The period of rotation or
revolution of a celestial body with respect to the
sun.
T dwarf (135) A very-low-mass star at the bot-
tom end of the main sequence with a cool surface
and a low luminosity.
temperature (127) A measure of the velocity of
random motions among the atoms or molecules
in a material.
terminator (443) The dividing line between day-
light and darkness on a planet or moon.
Terrestrial planet (402) Earth-like planet—
small, dense, rocky.
theory (66) A system of assumptions and prin-
ciples applicable to a wide range of phenomena
that have been repeatedly verifi ed.
thermal energy (127) The energy stored in an ob-
ject as agitation among its atoms and molecules.