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astronomical latitude

Some asteroids are probably dead comets which have lost most of their icy material due to their many passages around the sun. Some asteroids have been found to show comet-like characteristics, and the asteroid Chiron (for which the Centaur asteroids were named) has now been reclassiﬁed as a comet on this basis. The largest asteroid is Ceres, which has a diameter of about 950 km. The asteroids within the asteroid belt, however, are believed to be left-over debris from the formation of the solar system, which was never allowed to accrete into a planet due to the gravitational inﬂuence of nearby Jupiter. Images taken by spacecraft show that asteroids are generally irregular, heavily cratered objects. Some may be solid rock, although many are likely collections of small debris (“rubble piles”) held together only by their mutual gravity.

asteroid classiﬁcation A classiﬁcation of asteroids according to their spectra and albedo: C- type, apparently similar to carbonaceous chondrite meteorites; extremely dark (albedo approximately 0.03). More than 75% of known asteroids fall into this class. S-type, albedo

.10-.22; spectra indicating metallic nickel-iron mixed with ironand magnesium-silicates; approximately 17% of the total. M-type, albedo

.10-.18; pure nickel-iron.

asteroid orbital classiﬁcation Main Belt: asteroids orbiting between Mars and Jupiter roughly 2 to 4 AU from the sun; Near-Earth Asteroids (NEAs): asteroids that closely approach the Earth; Aten asteroids: asteroids with semimajor axes less than 1.0 AU and aphelion distances greater than 0.983 AU; Apollo asteroids: asteroids with semimajor axes greater than 1.0 AU and perihelion distances less than 1.017 AU; Amor asteroids: asteroids with perihelion distances between 1.017 and 1.3 AU; Trojans asteroids: asteroids located near Jupiter’s Lagrange points (60◦ ahead and behind Jupiter in its orbit).

Asterope Magnitude 5.8 type B9 star at RA 03h45m, dec +24◦33 ; one of the “seven sisters” of the Pleiades.

asthenosphere The inner region of a terrestrial planet which undergoes ductile ﬂow (also

called solid state convection). In the Earth, the asthenosphere is composed of the lower part of the mantle and is the region between 100 and 640 km depth. It is marked by low seismic velocities and high seismic-wave attenuation. The ability of the asthenosphere to ﬂow over long time periods (thousands to millions of years) helps to transport heat from the deep interior of a body and leads to plate tectonic activity on Earth as the rigid outer lithosphere rides atop the asthenosphere.

Astraea Fifth asteroid to be discovered, in 1845. Orbit: semimajor axis 2.574 AU, eccentricity 0.1923, inclination to the ecliptic 5◦.36772, period 4.13 years.

astrochemistry Chemistry occurring under extraterrestrial conditions including: reactions of atoms, ions, radicals, and neutral molecules in the gas phase, and reactions of such species in ices on metal or mineral surfaces and in/on ices on grains, comets, and satellites, especially induced by impinging atoms, ions, and photons.

astrometric binary A binary star system that reveals itself as a single point of light whose position or centroid shifts with the orbit period. A famous example is Sirius, recognized by Bessell in 1844 as having a very faint companion of roughly its own mass, accounting for the shift of its position with a 50-year period. Improved angular resolution or sensitivity can turn an astronometric binary into a visual binary. See binary star system, visual binary system.

astrometry The measurement of positions and motions of celestial objects.

astronomical latitude Deﬁned as the angle between the local vertical, as deﬁned by gravity, and the Earth’s equatorial plane, counted positive northward and negative southward. (See also latitude.) Astronomical latitude is generally within 10 arc of geodetic latitude in value. The local vertical, in this sense, is the normal to the geoid; in simple terms, it is the upwards line deﬁned by the plumb bob. The difference between astronomical latitude and geodetic latitude is due to small, local gravity variations. These are caused by mass concentrations,

astronomical refraction

such as mountains, lakes, and large ore deposits, which cause the plumb line to deviate slightly from the normal to the ellipsoid.

astronomical refraction The apparent angular displacement toward the zenith in the position of a celestial body, due to the fact that the atmosphere over any observer is apparently a planar slab with density decreasing upward. The effect vanishes overhead and is largest near the horizon, where it becomes as much as 30 . The fact that the sun is refracted to appear above its true angular position contributes measurably to the length of the apparent day. Also called atmospheric scintillation.

astronomical scintillation Any irregular scintillation such as motion, time dependent chromatic refraction, defocusing, etc. of an image of a celestial body, produced by irregularities in the Earth’s atmosphere. The effects have periods of 0.1 to 10 sec and are apparently caused by atmospheric irregularities in the centimeter to decimeter and meter ranges, within the ﬁrst 100 m of the telescope aperture.

astronomical tide Fluctuations in mean water level (averaged over a time scale of minutes) that arise due to the gravitational interaction of (primarily) the earth, moon, and sun. May also be used to refer to the resulting currents.

astronomical twilight	See twilight.
astronomical unit (AU)	The mean distance
8		×
between the sun and the Earth (1.4959787

10 km). This is the baseline used for trigonometric parallax observations of distances to other stars.

astronomy, infrared The observation of astronomical objects at infrared (IR) wavelengths, approximately in the range from 1 to 200 µm, that provide information on atomic motions that cause changes in charge distribution. The midinfrared spans approximately the range from 2.5 to 25 µm and includes fundamental transitions for bond stretching and bending of most interstellar molecules. Longer and shorter wavelengths, known as the far and near IR, respectively, correspond to low frequency motions of

groups of atoms and overtones of far and mid-IR features.

astronomy, infrared: interstellar	grains,
comets, satellites, and asteroids	Absorp-

tion, reﬂection, and emission at infrared (IR) wavelengths provide astronomers with unique molecular information for molecules not visible at other wavelengths, such as radio, because they lack a permanent dipole moment, or are solids, such as ices on interstellar grains or solar system bodies. IR spectroscopy of these solid materials, measured in absorption and reﬂection, respectively, have supplied most remotely measured information about the mineralogy and chemical composition of interstellar grains and solar system surfaces. Most spectra of outer solar system bodies have been measured in reﬂected sunlight in the near IR because solar radiation diminishes with increasing wavelength so they are dark in the mid-IR.

astronomy, ultraviolet: interstellar The observation of astronomical objects and phenomena at ultraviolet (UV) wavelengths, approximately in the range from 100 to 4000 Å, provide information on the electronic transitions of materials, molecules, and reactive species. UV absorption of interstellar materials have helped to put constraints on the form and distribution of most carbon bearing species in the galaxy. See diffuse interstellar bands (DIBs).

asymmetry factor In scattering, the mean cosine of the scattering angle.

asymmetry parameter Asymmetry factor.

asymptotic The (normalized) angular shape of the radiance distribution at depths far from the boundary of a homogeneous medium; the directional and depth dependencies of the asymptotic radiance distribution decouple and all radiometric variables (e.g., irradiances) vary spatially at the same rate as the radiance, as governed by the inherent optical properties only. See diffuse attenuation coefﬁcient.

asymptotically simple space-time A spacetime (M, g) is said to be asymptotically simple

atmosphere

if there exists a space-time (M˜ , g)˜ , such that M is a submanifold of M˜ with boundary I and

•g˜ab = I2gab, I > 0 M

•On I, I = 0 and aI = 0

•Any null geodetic curve in M has two endpoints in I

•In a neighborhood of I, the space-time is empty (or has only electromagnetic ﬁelds)

asymptotic diffuse attenuation coefﬁcient

The value of the diffuse attenuation coefﬁcient in the asymptotic regime; it depends on the inherent optical properties only.

asymptotic ﬂatness The assumption in theoretical/analytical descriptions of gravitational ﬁelds, that the gravitational potential goes to zero at spatial inﬁnity, i.e., far away from its sources. In general relativity, the gravitational ﬁeld is reﬂected in curvature of spacetime, so requiring ﬂatness has a direct connection to requiring vanishing gravitational effects. In situations with a nonvanishing central mass m, asymptotic ﬂatness requires the metric approach ﬂat +O(Gm/c2r). Thus, a space-time I with Lorentzian metric g is said to be asymptotically ﬂat (at spatial inﬁnity) if a set of spherical coordinates (t, r, θ, φ) can be introduced, such that g approaches the Minkowski tensor for large r:


r	lim	g = diag −1, 1, r2, r2 sin2	θ .
	→+∞
asymptotic giant branch (AGB) star			Star

of low or intermediate mass ( 0.8 to 5 solar masses) in the advanced evolutionary phase where the primary energy sources are fusion of hydrogen (by the CNO cycle) to helium and of helium (by the triple-alpha process) to carbon in thin shells surrounding an inert carbon-oxygen core. The phase is important for two reasons. First, the star develops several zones of convection which cross back and forth so as to mix to the surface products of the interior nuclear reactions, including nitrogen from the CNO cycle, carbon from the triple-alpha process, and the products of the s process, including barium and,

sometimes, technitium, thus conﬁrming the occurrence of these reactions. The longest-lived isotope of T c has a half life less than a million years, showing that the reactions must be occurring recently. Second, the star expels a wind of up to 10−6 to 10−4 solar masses per year, and this mass loss both terminates the interior nuclear reactions and determines that the core will become a white dwarf rather than ignitingcarbon fusion. The phase lasts only about 0.01% of the longest, main-sequence, phase. The name derives from the location of these stars on the HR diagram in a diagonal strip that approaches tangentially at high luminosity to the main red giant branch. AGB stars are much brighter and more extended, but cooler on the surface, than the same stars were on the main-sequence. See CNO cycle, convection, HR diagram, main se- quence star, red giant, s process, triple-alpha process, white dwarf.

asymptotic regime In oceanography, depths at which the rate of decay with depth of all radiometric variables, given by the asymptotic diffuse attenuation coefﬁcient, depends only on the inherent optical properties.

Aten asteroid A member of a class of asteroids with Venus-crossing orbits, in contrast to the majority of asteroids that orbit between Mars and Jupiter. There are 30 known members of the Aten class.

Atlas A moon of Saturn, also designated SXV. It was discovered by R. Terrile in 1980 in Voyager photos. Its orbit has an eccentricity of 0, an inclination of 0.3◦, and a semimajor axis of 1.38×105 km. Its size is roughly 20×10 km, and its mass has not yet been determined. It appears to be a shepherd satellite of Saturn’s A ring and orbits Saturn once every 0.602 Earth days. Also, magnitude 3.8 type B9 star at RA 03h49m, dec +24◦03 ; “Father” of the “seven sisters” of the Pleiades.

atmosphere The gaseous envelop surrounding the Earth and retained in the Earth’s gravitational ﬁeld, which contains the troposphere (up to about 10 to 17 km), stratosphere (up to about 55 km), mesosphere (up to about 80 km), and ionosphere (up to over 150 km). The total

atmosphere effect

mass of the atmosphere is about 5.3 × 1018 kg, which is about one-millionth of the total mass of Earth. At sea level, average pressure is 1013.25 hPa, temperature 288.15 K, and density is 1.225 kg/m3. The density of the atmosphere decreases rapidly with height, and about threequarters of the mass of the atmosphere is contained within the troposphere. The atmosphere has no precise upper limit. Formally one deﬁnes the top of the atmosphere at 1000 km altitude, which is also the highest observed altitude of aurora.

atmosphere effect Whenever a gas that is a weak absorber in the visible and a strong absorber in the infrared is a constituent of a planetary atmosphere, it contributes toward raising the surface temperature of the planet. The warming results from the fact that incoming radiation can penetrate to the ground with relatively little absorption, while much of the outgoing longwave radiation is “trapped” by the atmosphere and emitted back to the ground. This is called the atmosphere effect. This warming is commonly referred to as the “greenhouse effect”.

atmospheric angular momentum As wind ﬂows in the atmosphere, an air parcel rotates about the Earth’s axis, so the atmosphere contains angular momentum. In tropical easterlies, friction with the Earth’s surface transfers angular momentum to the atmosphere; in the midlatitiude westerlies in both hemispheres, angular momentum is transferred from the atmosphere to the surface. Over long periods of time, the angular momentum of the atmosphere is in a steady state. Thus, there must be angular momentum transport from the tropics to mid-latitude in the two hemispheres. In the tropics, the mean meridional circulation plays an important role in the meridional transport of atmospheric angular momentum; and at mid-latitudes transient eddies and stationary eddies play a major role. Short term variations in the total atmospheric angular momentum can be observed in the rotation rate of the soled Earth.

atmospheric conductivity Conductivity of the atmosphere, determined by ion concentration and ion mobility. The conductivity in-

creases roughly exponentially with height because ion mobility depends on the number of collisions between air particles and thus increases with increasing height. Since the mobility of small ions is much larger than that of large ones, aerosol particles form a sink for small ions, reducing the atmospheric conductivity.

atmospheric electric ﬁeld The atmospheric electric ﬁeld on the ground is about −100 V/m with strong variations depending on weather conditions and the availability of dust particles. With increasing height, the atmospheric electric ﬁeld decreases because the conductivity increases. The atmospheric electric ﬁeld is part of the global electric circuit which can be conceptualized as a spherical capacitor formed by the terrestrial surface and the bottom of the ionosphere ﬁlled with a slightly conductive medium, the atmosphere. Thunderstorms work as generators, driving a current from the surface to the bottom of the ionosphere. The circuit is closed through the fair weather atmosphere which acts as a resistor.

atmospheric noise Radio noise produced by natural electrical discharges below the ionosphere and reaching the receiving point, where it is observed, along normal propagation paths between the Earth’s surface and the ionosphere. Distant lightning has usually been thought to be the main source for this noise. See galactic noise.

atmospheric pressure The ambient air pressure at a particular time and location. Expressed as an absolute pressure (i.e., relative to a vacuum). See also gauge pressure. “Standard” atmospheric pressure is taken as 14.7 lb/in2 or 101.3 kPa.

atmospherics A lightning stroke transmits a wide range of electromagnetic radiation, the most familiar being visible light. The electromagnetic emissions are short-lived, like the optical emissions. Those that can be reﬂected by the Earth’s ionosphere can propagate to remote locations in the earth-ionosphere waveguide where they can be observed. At frequencies used for early high frequency radio communications ( 1 to 30 MHz) the propagated light-