DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Newtonian invariance
where ρ is the mass density of the object at the
point r ′ and dV ′ is the volume element at that
position. From this latter expression one also
has the partial differential equation
∇ 2 φ= 4πρ
where ∇ 2 is the Laplacian. See density, gradient.
Newtonian invariance See Galilean invariance.
Newtonian relativity See Galilean relativity.
Newtonian simultaneity In Newtonian Mechanics,
since time is absolute, the simultaneity
of two events is independent of the reference
system. That is if one observer finds that two
given events are simultaneous, then all other observers
regardless of their respective reference
systems will also observe that the two events are
simultaneous. This does not in general hold in
Relativistic Mechanics.
Newton (N) Unit of force equal to 1mkg
s −2 .
Newton’s laws of motion 1. A body at rest
remains at rest, and a body in uniform motion
remains in uniform motion, unless acted on by
an external force.
2. The acceleration of a body is equal to the
net applied force, divided by the mass of the
body:
a = F/m.
3. For every force there is an equal and opposite
reaction force.
The third law is exemplified by a person attracted
by gravity toward a planet. There is simultaneously
an equal opposite force attracting
the planet toward the person.
NGC 5195 Irregular Galaxy, Canes Venatici
at RA 13 h 30 m .0, dec +47 ◦ 16 ′ , mV = 9.6.
Companion of M51, lying about 4 ′ 35 ′′ North
of M101.
Nielsen–Olesen vortex (1973) In condensed
matter physics, and in cosmology, a defect (a
string) that arises from a breaking of a high temperature
symmetry, in a model with a complex
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(i.e., 2-component) scalar (Higgs) field φ, and
a massless gauge vector field Aµ. The phase
transition requires a potential forφ which has a
minimum at a nonzero valueσ of |φ| and which
is indifferent to the complex phase of φ. One
finds a physical “magnetic” flux proportional to
the circulation of the gauge field A·dℓ (Aµ
is the gauge field associated with the localU(1)
symmetry). Here, dℓ is taken along an arbitrary
closed loop encircling the defect in physical
space. Moreover, this flux is quantized, since
solutions of the Higgs field φ can have a complex
phase that increases by multiples of 2π as
one moves around the vortex.
Kinetic terms of the Higgs field contribute
to the overall energy of the configuration in the
form of a logarithmically divergent energy per
unit length. The presence of the gauge fieldAµ
cancels this divergence and hence the energy of
theconfigurationisconfinedtoacylindricalcore
of radius roughly given by the Compton wavelengths
of the Higgs and gauge bosons. Such
vortices exist in superconductors, and represent
the typical model for the description of cosmic
strings. See Abelian Higgs model, cosmic
string, local topological defect, winding number.
nimbus See cloud classification.
nitrogen Colorless, odorless gas, N, atomic
number 7. Natural atomic weight 14.0067.
Melting point 63.05 K, boiling point 77.36 K.
Naturally occurring isotopes 14 N (99.6%) and
15 N(0.4%). Relatively inert at low temperatures,
but its compounds are vital for biological
processes. Nitrogen constitutes about 78% (volume)
of the Earth’s atmosphere.
Noachian Geophysical epoch on the planet
Mars, more than 3.5 Gy BP, exemplified by the
oldest and heavily cratered regions lying in the
southern hemisphere of Mars.
nocturnal Pertaining to occurrences during
the night, as opposed to diurnal occurrences.
nodal month See month.