DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Nasmyth spectrum F(k/k η)
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normalized wavenumber k/k
η
Universal spectrum of oceanic turbulence according
to Nasmyth. The circles are the measured points and
they are plotted against the cyclic wavenumber nor-
malized by the Kolmogorov wavenumberkη.
and has a Lorentzian shape given by:
φ(ν)=
γ/4π 2
γ/4π 2 +(ν) 2
whereγ is the radiation damping constant. In a
description in terms of classical radiation theory,
γ= 8π2e2 ,
3mecλ2 where e and me are the charge and mass of the
electron, and c is the speed of light.
natural remanent magnetism Many volcanic
rocks contain magnetic minerals. When
these rocks cool from the molten state, they are
permanently magnetized by the Earth’s magnetic
field at that time. This natural remanent
magnetism provides information on the time
evolution of the Earth’s magnetic field and continental
drift.
nautical mile A unit of length equal to exactly
1852 m.
nautical twilight See twilight.
Navier–Stokes equations The complicated
set of partial differential equations for the motion
of a viscous fluid subject to external forces.
neap tide The tide produced when the gravitational
pull of the sun is in quadrature, i.e., at
© 2001 by CRC Press LLC
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nearly diurnal free wobble
right angles, to that of the moon. They occur
twice a month at about the times of the lunar
first and last quarters. In these situations the
difference between high and low tides is unusually
small, with both the high tide lower and the
low tide higher than usual.
NEAR A Near Earth Asteroid Rendezvous
(NEAR) spacecraft that was the first Discovery
mission launched on February 17, 1996. NEAR
took photographs of Comet Hyakutake in March
1996, flew by C-asteroid 253 Mathilde on June
27, 1997, and S-asteroid 433 Eros on December
23, 1998, and also past the Main Belt asteroid
Illya. It began orbiting S-asteroid 433 Eros in
February 2000.
Its primary mission was to explore 433 Eros
in detail for approximately 1 year. The mission
will help answer basic questions about the nature
and origin of near-Earth objects, as well as provide
clues about how the Earth itself was formed.
Studies were made of the asteroid’s size, shape,
mass, magnetic field, composition, and surface
and internal structure. It achieved this using
the spacecraft’s X-ray/gamma ray spectrometer,
near infrared imaging spectrograph, multispectral
camera fitted with a CCD imaging detector,
laser altimeter, and magnetometer. A radio
science experiment was performed using the
NEAR tracking system to estimate the gravity
field of the asteroid. Periapsis of the orbit was as
low as 24 km above the surface of the asteroid.
Although NEAR has no landing legs, on February
12, 2001 it was landed on the surface of Eros
at a speed of about 1.9 m/s. It photographed on
the way down, the last image from 120 m altitude.
Remarkably it continued to function, after
impact sending back γ -ray data.
nearly diurnal free wobble The retrograde
motion of the Free Core Nutation (FCN) as
viewed from points fixed on the Earth. The wobble
appears from the Earth to have frequencies
of ωwobble = ωnutation − , where is the frequency
of the Earth’s rotation. Very long baseline
interferometry observations, which measure
the nutation in space relative to fixed stars,
place the FCN frequency ωnutation ≈ 433.2 cycles/sidereal
day.
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