GLOSSARY 627eccentricity (e) (64) A measure of the fl attening of
an ellipse. An ellipse of e = 0 is circular. The closer
to 1 e becomes, the more fl attened the ellipse.
eclipse season (43) That period when the sun is
near a node of the moon’s orbit and eclipses are
possible.
eclipse year (44) The time the sun takes to circle
the sky and return to a node of the moon’s orbit;
346.62 days.
ecliptic (21) The apparent path of the sun around
the sky.
ejecta (446) Pulverized rock scattered by meteorite
impacts on a planetary surface.
electromagnetic radiation (99) Changing elec-
tric and magnetic fi elds that travel through space
and transfer energy from one place to another—
for example, light, radio waves, and the like.
electron (123) Low-mass atomic particle carrying
a negative charge.
ellipse (64) A closed curve enclosing two points
(foci) such that the total distance from one focus
to any point on the curve back to the other focus
equals a constant.
elliptical galaxy (178, 334) A galaxy that is round
or elliptical in outline; it contains little gas and
dust, no disk or spiral arms, and few hot, bright
stars.
emission line (132) A bright line in a spectrum
caused by the emission of photons from atoms.
emission (bright-line) spectrum (132) A
spectrum containing emission lines.
energy (86) The capacity of a natural system to
perform work—for example, thermal energy.
energy level (126) One of a number of states an
electron may occupy in an atom, depending on its
binding energy.
enzyme (584) Special protein that controls pro-
cesses in an organism.
epicycle (57) The small circle followed by a planet
in the Ptolemaic theory. The center of the epicycle
follows a larger circle (deferent) around Earth.
equant (57) The point off-center in the deferent
from which the center of the epicycle appears to
move uniformly.
equatorial mounting (111) A telescope mounting
that allows motion parallel to and perpendicular
to the celestial equator.
escape velocity (Ve) (85) The initial velocity an
object needs to escape from the surface of a ce-
lestial body.
evening star (24) Any planet visible in the sky
just after sunset.
evolutionary hypothesis (398) Explanation for
natural events that involves gradual changes as
opposed to sudden catastrophic changes—for
example, the formation of the planets in the gas
cloud around the forming sun.
excited atom (126) An atom in which an electron
has moved from a lower to a higher orbit.
extrasolar planet (417) A planet orbiting a star
other than the sun.
extremophile (591) An organism that can survive
in an extreme environment, for example, very low
or high temperatures, high acidity, extreme dry-
ness, etc.
eyepiece (102) A short-focal-length lens used to
enlarge the image in a telescope; the lens nearest
the eye.
fall (552) A meteorite seen to fall. (See fi nd.)
false-color image (113) A representation of
graphical data in which the colors are altered or
added to reveal details.
fi eld (81) A way of explaining action at a distance;
a particle produces a fi eld of infl uence (gravita-
tional, electric, or magnetic) to which another
particle in the fi eld responds.
fi eld of view (2) The area visible in an image;
usually given as the diameter of the region.
fi lament (162) (1) On the sun, a prominence seen
silhouetted against the solar surface. (2) A linear re-
gion containing many galaxies and galaxy clusters,
part of the large-scale structure of the universe.
fi ltergram (146) An image (usually of the sun)
taken in the light of a specifi c region of the spec-
trum—for example, an H-alpha fi ltergram.
fi nd (552) A meteorite that is found but was not
seen to fall. (See fall.)
fl are (163) A violent eruption on the sun’s surface.
fl ux (15) A measure of the fl ow of energy onto or
through a surface. Usually applied to light.
focal length (102) The distance from a lens to the
point where it focuses parallel rays of light.
folded mountain range (434) A long range of
mountains formed by the compression of a plan-
et’s crust—for example, the Andes on Earth.
forward scattering (499) The optical property
of fi nely divided particles to preferentially direct
light in the original direction of the light’s travel.
frequency (99) The number of times a given event
occurs in a given time; for a wave, the number of
cycles that pass the observer in 1 second.
galaxy (5) A very large collection of gas, dust, and
stars orbiting a common center of mass. The sun
and Earth are located in the Milky Way Galaxy.
Galilean moons, Galilean satellites (68, 504)
The four largest satellites of Jupiter, named after
their discoverer, Galileo.
gas (type I) tail (568) The tail of a comet
produced by gas blown outward by the solar
wind. (See dust tail.)
gene (585) A unit of DNA containing genetic
information that infl uences a particular inherited
trait.
general theory of relativity (92) Einstein’s more
sophisticated theory of space and time, which de-
scribes gravity as a curvature of space-time.
geocentric universe (54) A model universe with
Earth at the center, such as the Ptolemaic universe.
geosynchronous satellite (84) An Earth satellite
in an eastward orbit whose period is 24 hours. A
satellite in such an orbit remains above the same
spot on Earth’s surface.giant (172) Large, cool, highly luminous star in
the upper right of the H–R diagram, typically 10
to 100 times the diameter of the sun.
global warming (437) The gradual increase in
the surface temperature of Earth caused by hu-
man modifi cations to Earth’s atmosphere.
gossamer ring (502) The dimmest part of
Jupiter’s ring produced by dust particles orbiting
near small moons.
granulation (144) The fi ne structure visible on the
solar surface caused by rising currents of hot gas
and sinking currents of cool gas below the surface.
grating (113) A piece of material in which numerous
microscopic parallel lines are scribed; light encoun-
tering a grating is dispersed to form a spectrum.
gravitational collapse (410) The stage in the for-
mation of a massive planet when it grows massive
enough to begin capturing gas directly from the
nebula around it.
greenhouse effect (437) The process by which a
carbon dioxide atmosphere traps heat and raises
the temperature of a planetary surface.
grooved terrain (505) Region of the surface of
Ganymede consisting of bright, parallel grooves.
ground state (126) The lowest permitted electron
orbit in an atom.
habitable zone (594) The region around a star
within which an orbiting planet can have surface
temperatures allowing liquid water.
half-life (405) The time required for half of the
atoms in a radioactive sample to decay.
heat (127) Energy fl owing from a warm body to
a cool body by the agitation of particles such as
atoms or molecules.
heat of formation (410) In planetology, the
heat released by the infall of matter during the
formation of a planetary body.
heavy bombardment (413) The period of intense
meteorite impacts early in the formation of the plan-
ets, when the solar system was fi lled with debris.
heliocentric universe (58) A model of the uni-
verse with the sun at the center, such as the Co-
pernican universe.
helioseismology (148) The study of the interior of
the sun by the analysis of its modes of vibration.
Hesperian period (488) On Mars, the geological
era from the decline of heavy cratering and lava
fl ows and the melting of subsurface ice to form
the outfl ow channels.
Hirayama family (564) Family of asteroids with
orbits of similar size, shape, and orientation;
thought to be fragments of larger bodies.
horizon (18) The line that marks the apparent
intersection of Earth and the sky.
horoscope (25) A chart showing the positions of
the sun, moon, planets, and constellations at the
time of a person’s birth; used in astrology to at-
tempt to read character or foretell the future.
hot Jupiter (419) A massive and presumably Jo-
vian planet that orbits close to its star and conse-
quently has a high temperature.