GLOSSARY 629occultation (534) The passage of a larger body in
front of a smaller body.
Oort cloud (571) The cloud of icy bodies—
extending from the outer part of our solar system
out to roughly 100,000 AU from the sun—that
acts as the source of most comets.
open orbit (85) An orbit that does not return to its
starting point; an escape orbit. (See closed orbit.)
outfl ow channel (483) Geological feature on
Mars that appears to have been caused by sudden
fl ooding.
outgassing (411) The release of gases from a
planet’s interior.
ovoid (537) Geological feature on Uranus’s moon
Miranda thought to be produced by circulation in
the solid icy mantle and crust.
ozone layer (436) In Earth’s atmosphere, a layer of
oxygen ions (O 3 ) lying 15 to 30 km high that pro-
tects the surface by absorbing ultraviolet radiation.
paradigm (61) A commonly accepted set of
scientifi c ideas and assumptions.
parallax (56) The apparent change in the posi-
tion of an object due to a change in the location
of the observer. Astronomical parallax is measured
in seconds of arc.
partial lunar eclipse (36) A lunar eclipse in
which the moon does not completely enter Earth’s
shadow.
partial solar eclipse (38) A solar eclipse in which
the moon does not completely cover the sun.
Paschen series (133) Spectral lines in the infrared
spectrum of hydrogen produced by transitions
whose lowest orbit is the third.
passing star hypothesis (398) The proposal that
our solar system formed when two stars passed
near each other and material was pulled out of
one to form the planets.
path of totality (39) The track of the moon’s um-
bral shadow over Earth’s surface. The sun is totally
eclipsed as seen from within this path.
penumbra (33) The portion of a shadow that is
only partially shaded.
penumbral lunar eclipse (37) A lunar eclipse in
which the moon enters the penumbra of Earth’s
shadow but does not reach the umbra.
perigee (39) The orbital point of closest approach
to Earth.
perihelion (23) The orbital point of closest
approach to the sun.
permitted orbit (124) One of the energy levels in
an atom that an electron may occupy.
photon (100) A quantum of electromagnetic en-
ergy; carries an amount of energy that depends
inversely on its wavelength.
photosphere (41) The bright visible surface of
the sun.
planet (3) A nonluminous object, larger than a
comet or asteroid, that orbits a star.
planetary nebula (172, 257) An expanding shell
of gas ejected from a star during the latter stages
of its evolution.planetesimal (409) One of the small bodies that
formed from the solar nebula and eventually grew
into protoplanets.
plastic (431) A material with the properties of a
solid but capable of fl owing under pressure.
plate tectonics (434) The constant destruction
and renewal of Earth’s surface by the motion of
sections of crust.
plutino (548) One of the icy Kuiper belt objects
that, like Pluto, are caught in a 3:2 orbital reso-
nance with Neptune.
polar axis (111) In an equatorial telescope
mounting, the axis which is parallel to Earth’s axis
of rotation.
positron (151) The antiparticle of the electron.
potential energy (86) The energy a body has by
virtue of its position. A weight on a high shelf has
more potential energy than a weight on a low shelf.
precession (17) The slow change in the direction
of Earth’s axis of rotation; one cycle takes nearly
26,000 years.
pressure (P) wave (429) In geophysics, a
mechanical wave of compression and rarefaction
that travels through Earth’s interior.
primary lens or mirror (101-102) The main
optical element in an astronomical telescope. The
large lens at the top of the telescope tube or the
large mirror at the bottom.
prime focus (110) The point at which the objective
mirror forms an image in a refl ecting telescope.
primeval atmosphere (433) Earth’s fi rst air, com-
posed of gases from the solar nebula.
primordial soup (588) The rich solution of or-
ganic molecules in Earth’s fi rst oceans.
prominence (41, 162) Eruption on the solar sur-
face; visible during total solar eclipses.
protein (584) Complex molecule composed of
amino acid units.
proton (123) A positively charged atomic particle
contained in the nucleus of an atom; the nucleus
of a hydrogen atom.
proton–proton chain (151) A series of three
nuclear reactions that build a helium atom by
adding together protons; the main energy source
in the sun.
protoplanet (409) Massive object resulting from
the coalescence of planetesimals in the solar
nebula and destined to become a planet.
protostar (169, 214) A collapsing cloud of gas
and dust destined to become a star.
pseudoscience (25) A subject that claims to obey
the rules of scientifi c reasoning but does not.
Examples include astrology, crystal power, and
pyramid power.
quantum mechanics (124) The study of the
behavior of atoms and atomic particles.
radial velocity (Vr) (139) That component of
an object’s velocity directed away from or toward
Earth.
radiant (556) The point in the sky from which
meteors in a shower seem to come.multiringed basin (447) Very large impact basin
in which there are concentric rings of mountains.
mutant (586) Offspring born with altered DNA.
nadir (18) The point on the bottom of the sky
directly under your feet.
nanometer (nm) (100) A unit of length equal to
10-9 m.
natural law (66) A conjecture about how nature
works in which scientists have overwhelming
confi dence.
natural motion (77) In Aristotelian physics, the
motion of objects toward their natural places—fi re
and air upward and earth and water downward.
natural selection (586) The process by which the
best traits are passed on, allowing the most able
to survive.
neap tide (87) Ocean tide of low amplitude
occurring at fi rst- and third-quarter moon.
Near-Earth Object (NEO) (562) An asteroid or
comet in an orbit that passes near or intersects
Earth’s orbit, that could potentially collide with
Earth.
nebula (168, 193) A cloud of gas and dust in
space.
nebular hypothesis (398) The proposal that the
solar system formed from a rotating cloud of gas.
neutrino (151) A neutral, massless atomic particle
that travels at or nearly at the speed of light.
neutron (123) An atomic particle with no charge
and about the same mass as a proton.
neutron stars (173) A small, highly dense star
composed almost entirely of tightly packed
neutrons; radius about 10 km.
Newtonian focus (110) The focal arrangement of
a refl ecting telescope in which a diagonal mirror
refl ects light out the side of the telescope tube for
easier access.
Noachian period (487) On Mars, the era that
extends from the formation of the crust to the end
of heavy cratering and includes the formation of
the valley networks.
node (43) A point where an object’s orbit passes
through the plane of Earth’s orbit.
north celestial pole (18) The point on the celes-
tial sphere directly above Earth’s North Pole.
north point (18) The point on the horizon directly
below the north celestial pole; exactly north.
nuclear fi ssion (150) Reaction that splits nuclei
into less massive fragments.
nuclear fusion (150) Reaction that joins the
nuclei of atoms to form more massive nuclei.
nucleus (of an atom) (123) The central core of
an atom containing protons and neutrons; carries
a net positive charge.
objective lens or mirror (101-102) The main
optical element in an astronomical telescope.
The large lens at the top of the telescope or large
mirror at the bottom.
oblateness (497) The fl attening of a spherical
body, usually caused by rotation.