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rotation

slower than the body itself, can grow by emitting gravitational waves and can therefore radiate angular momentum. In meteorology, westward propagating atmospheric waves. Weather systems in the atmosphere are often associated with these Rossby waves, and they are important for weather patterns, including the formation of cyclones. The planetary Rossby waves play a fundamental role in large-scale ocean and atmosphere dynamics and long-term ocean climate. These waves take only a few months to cross an ocean basin in the tropics, but years to decades at higher latitudes in the ocean.

rotation The mathematical quantity (an antisymmetric rank-2, 3-dimensional tensor) that determines the angular velocity with which neighboring particles of a continuous medium revolve around a given particle (see also acceleration, expansion, kinematical invariants, shear and see tidal forces). All astronomical objects for which rotation can be measured (like stars, planets, galaxies) do rotate. The origin of their rotation is easily understood: The net angular momentum of all matter that eventually entered the object, with respect to the current position of the axis of rotation, was nonzero. Only planetoids and moons with no fluid core rotate like rigid bodies. Gaseous bodies (like the sun or Jupiter) and planets with hot fluid cores (like the Earth) display complicated patterns of local rotation. See rotation of the universe.

rotational discontinuity See hydromagnetic wave.

rotation curve In astronomy, a curve that describes the rotational velocity as a function of radius, typically of gas or stars in a disk galaxy. The rotation curve is obtained from the radial velocity measured on absorption or emission lines detected along a galaxy’s apparent major axis, and corrected for the inclination of the galaxy. A rotation curve, in a diagram where velocity in km s1 is plotted against angular or linear distance, is usually made up of a linear segment, where velocity increases linearly from zero with radius; a turning point, where the curve flattens; and a long, nearly flat or slowly decreasing segment. The so-called Keplerian trace, or a part of it, in which the orbital velocity decreases with

distance according to Kepler’s third law, after all the matter of a galaxy is left behind, is almost never observed. The maximum rotational velocity depends on the galaxy’s morphological type; typical values are 200 to 300 km s1. See long slit spectroscopy.

rotation of the universe The rotation tensor (see rotation) in our actual universe. In principle, rotation could be detected and even measured on the basis of observations of distant galaxies, but in practice the transverse motions necessary to detect rotation in any reasonable rotating model of the universes are far too small to measure.

r process The process of rapid neutron capture that is responsible for production of the neutron-rich isotopes of elements beyond iron (roughly germanium to uranium, and probably on to at least a few of the transuranics that no longer exist in the solar system). The “r” means that successive neutron captures occur more rapidly than beta decays back to the most stable nuclide at a given mass number. Therefore, since the beta-decay process depends in part on the total mass of the nucleus, the r- process of nucleosynthesis produces elements different than the s-process where neutron capture is slow with sufficient time for beta-decay between successive bombardments. The process almost certainly occurs in Type II supernovae, where both neutrons and iron peak nuclides are copiously available, but some details of how the products are ejected remain to be worked out. See supernova.

RR Lyrae star Stars on the horizontal branch whose brightness varies due to radial pulsation of the star. They fall within the instability strip in the H-R diagram, have periods of about half a day, brightnesses of about 40 times that of the sun, and are found both in globular clusters and in the halos of our own and other galaxies. Because they are all about the same brightness, they are useful distance indicators.

RS Canum Venaticorum stars A class of close binary stars in which the orbital period is from 1 to 14 days, where both stars are of roughly solar type and mass, but slightly

© 2001 by CRC Press LLC

Rydberg constant (R)

evolved, the more massive component is an F, G, or K main-sequence or sub-giant star, and there is a significant amount of chromospheric activity. Chromospheric activity refers to the degree of magnetic field related phenomena near the stellar surface, e.g., star spots and stellar flares. This combination, especially the interaction between the two winds, heats material to 107 K or more, so that RS CVn’s are often observed to have powerful flares which have durations of a fraction of an hour up to a few days, radiate 1033 to 1038 ergs, and emit light from radio waves through X-rays. Thus, the energy for these flares is believed to arise from the reconnection of tangled magnetic fields into simpler geometries. This large-scale reorganization releases a fraction of the energy stored within the magnetic field.

runoff channels One channel type considered to indicate fluvial activity on Mars. Runoff channels are pervasive throughout the plateau material of the southern hemisphere old cratered terrain, where they form immature drainage networks around large craters and hilly mountainous regions. They are mainly located in low to middle latitudes. The apparent “loss” at high latitudes has been explained as an observational artifact owing to lower resolution imagery; a consequence of terrain softening; or as a consequence of polar wandering.

Runoff channels are also present on the flanks of Ceraunius and Hecates Tholus, and Alba

Patera, volcanoes in the northern hemisphere. On Alba Patera they are highly integrated, and have a greater drainage density than those in the plateau material. On Ceraunius and Hecates Tholus their morphology is intermediate between those found in the plateau material and those found on Alba Patera. The northern hemisphere runoff channels are younger than those in the plateau material.

runup The vertical excursion of water that results when a wave strikes a beach or structure.

RV Tauri stars Highly evolved, asymptotic giant branch (probably) stars of relatively low mass which are so extended and bright that successive cycles of their pulsation interfere with each other, so they exhibit brightness minimum of alternating large and small amplitudes.

Rydberg constant (R) The fundamental constant that appears in the equation for the energy levels of hydrogen-like atoms; i.e., En = hcRZ2µ/n2, where h is Planck’s constant, c is the speed of light, Z is the atomic number, µ is the reduced mass of nucleus and electron, m is the mass of the electron, and n is the principal quantum number (n = 1, 2, · · · ).

R= me4/4πch¯3

=109737.312 cm1 .

© 2001 by CRC Press LLC

Satellite Laser Ranging (SLR)

S

Sagittarius A* Strong radio source located in Sagittarius and apparently located at the center of the Galaxy at RA 17h45m.6 , dec 2856 (at galactic longitude 0, latitude 0). Measurements of stellar velocity profiles near the center of the object indicate the presence of an object of mass approximately 2.6×106M in a sphere of radius 0.15 pc. This implies the existence of a massive black hole at the center of our galaxy, at about 8 kpc.

saline contraction coefficient Coefficient β expressing the relative change of density due to the change in salinity at constant potential temperature and pressure p; i.e., β = ρ1(∂ρ/∂S) ,p. β does not vary strongly for fresh-water to marine saline to hypersaline water ( 0.7 to 0.85). For actual values see Algorithms for computation of fundamental properties of seawater, UNESCO Tech. Pap. Mar. Sci.,

44, 53 pp.

salinity The total fractional amount of dissolved material in seawater defined as the total amount of solid materials in grams contained in 1 kg of seawater when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine, and all organic matter completely oxidized. The average salinity of seawater varies from about 3.5 to 3.7% by weight. There is an observed constant ratio of chlorinity to salinity: salinity in parts per thousand = (1.80655) (chlorinity in parts per thousand). See chlorinity.

salinity inversion

See density inversion.

saltation Eolian or fluvial transport of small material by the bouncing of that material along a surface. Saltation is particularly effective for the transport of sand-sized material (1/16 to 2 mm in diameter).

salt fingering See double diffusion.

salt wedge A horizontal tongue of saline water located beneath a layer of fresh water within an estuary.

sand budget A description of all of the sand within a littoral system and a description of where it is going.

sand bypassing A process, either natural or as a result of man’s activities, whereby sand is transported past a tidal inlet or entrance channel and reaches the beach on the other side.

sand spit A long, thin deposit of sand, deposited by longshore sediment transport. Typically located at the downdrift end of an island.

sand trap A trench dredged to capture sand. Such a trap is often used to facilitate sand bypassing. The trap is dredged near the updrift side of a channel or harbor entrance, captures sand, and the sand is later dredged out and placed on the downdrift beach.

Sargasso Sea Area in the north Atlantic Ocean between the West Indies and the Azores, at about 30 N lat. This relatively still sea is bounded by the North Atlantic, Canary, North Equatorial, and Antilles Currents, and the Gulf Stream. Named for the large amount of free floating seaweed (Sargassum) accumulated there.

saros 223 synodic (lunar) months = 19 eclipse years (almost exactly), about 18 years 11 days, a period of time over which the relative orientation of the moon and the sun repeat, so that lunar eclipses recur in similar patterns.

satellite altimetry A space-based ranging technique, which usually analyzes laser or radar pulses reflected off of the surface of a planet. When combined with precise orbit determination of the satellite, these rangings can be used to determine ocean, ice, and land surface topography. See also TOPEX/POSEIDON.

Satellite Laser Ranging (SLR) Determination of the distance between an artificial satellite with reflecting prisms and an observation station on the Earth’s surface, by radiating laser beam

© 2001 by CRC Press LLC

saturation

Altimetric satellite

Satellite ephemeris, r h, Observed satellite altitude

h

DSSH

h

MSS

r , Reference ellipsoid height

el

Earth’s center of mass

Fundamental measurements of satellite altimetry.

from the station, reflecting it from the prisms, and receiving the reflected laser beam. Since the location of the satellite can be determined independently, geodetic location of the station can be determined within an accuracy of several centimeters if the two-point distances are measured for at least three different locations of the satellite. Such observations are currently carried out at different stations in the world; one application is to measure the relative motion of tectonic plates. Satellite laser ranging is also used to provide information on Earth rotation and Earth orientation.

saturation See current saturation.

saturation adiabat A type of adiabatic process in movement of saturation moist air. Under the saturation adiabatic condition, when a saturated air parcel ascends, part of the vapor will condense to water or ice, and the air parcel will be still saturated. The latent heat released partly compensates the ascending adiabatic cooling, and the condensed water or ice remains in the saturated air parcel.

saturation adiabatic lapse rate The adiabatic cooling rate of rising air parcel which is saturated, and in which condensation is taking place as it rises, so that the latent heat release moderates the adiabatic cooling, and causes the saturation adiabatic lapse rate to be less than the dry adiabatic lapse rate. The saturation adiabatic lapse rate is not a constant, and varies inversely with the temperature and somewhat with change of pressure. The difference between the saturation adiabatic lapse rate and dry adiabatic lapse rate is determined by the possibly released latent heat. Since the temperature at

higher level atmosphere is lower, the saturation adiabatic lapse rate has a smaller value at low level and larger value at high level which can approach or equal the dry adiabatic lapse rate.

saturation vapor pressure The pressure exerted by the water vapor which is equilibrium with its condensed phase, which may be water or ice, the moist surface (or ice surface) saturation vapor pressure. In this state the same number of water molecules leave the water (ice) into the air as move from the air into the water (ice).

Saturation vapor pressure is only a function of temperature. Its formula for water surface is

E = E0 107.45t/(t+235)

where E is saturation vapor pressure, E0 = 6.1078 hPa, is the saturation vapor pressure at 0C, t is vapor temperature(C).

The ice surface saturation vapor pressure is

E = E0 109.5t/(t+265) .

Saturn The sixth planet from the sun. Named

after the Roman elder god of agriculture, Saturn has a mass M = 5.6841 × 1029 g, and an

equatorial radius of R = 60,330 km, giving it a mean density of 0.71 g cm3 and a surface gravity of 1.08 that of Earth. Its rotational period is 10 h 14 min. The rotation axis has an obliquity of 26.7. The rapid rotation gives the planet an oblateness of 0.09. Saturn’s orbit around the sun is characterized by a mean distance of 9.54 AU, an eccentricity of e = 0.056, and an orbital inclination of i = 2.5. Its sidereal period is 29.46 years and its synodic period is 378 days. Its average albedo is 0.47 but its atmospheric temperature is determined by an internal heat source. At one bar the temperature is about 140 K and increases inward. Saturn is a “gas giant” being composed primarily of hydrogen and helium and is the least dense, most oblate planet in the solar system. Saturn has prominent rings composed of large particles and numerous moons including Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Mimas, Enceladus, Tethys, Telesto, Calypso, Dione, Helene, Rhea, Titan, Hyperion, Iapetus, and Phoebe.

scalar A function or physical property that depends only on the position and time at which it

© 2001 by CRC Press LLC

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