Source edition 1965. Please read the Introduction to find
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been updated since the 1965 edition.

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**Oberon**- A satellite of
Uranus orbiting at a mean distance of 587,000 kilometers.
**object glass**= objective. **objective**- The lens or combination of lenses which receives light rays from an object
and refracts them to form an image in the focal plane of the eyepiece of an
optical instrument, such as a telescope. Also called
*object glass.* **oblate spheroid**- An ellipsoid of revolution, the shorter axis of which is the axis of revolution.
*An ellipsoid of revolution, the longer axis of which is the axis of revolution, is called a prolate spheroid. The earth is approximately an oblate spheroid.***oblique**- Pertaining to, or measured on, an oblique projection, as
*oblique equator, oblique pole, oblique latitude.* **oblique coordinates**- Magnitudes
defining a point relative to two intersecting nonperpendicular lines, called
*axes.*See Cartesian coordinates. *The magnitude indicate the distance from each axis, measured along a parallel to the other side. The horizontal distance is called the abscissa and the other distance, the ordinate.***oblique projection**- A map projection with an axis inclined at an oblique angle to the plane of the equator.
**oblique shock**= oblique shock wave.**oblique shock wave**- A shock
wave inclined at an oblique angle to the direction of flow in a supersonic
flow field. Sometimes shortened to
*oblique shock.*Compare normal shock. **obliquity of the ecliptic (symbol ε)**- The angle between the plane of the ecliptic (the plane of the earth's orbit) and the plane of the celestial equator.
*The obliquity of the ecliptic is computed from the following formula: 23 degrees 27 minutes 08.26 seconds - 0.4684 (t - 1900) seconds, where t is the year for which the obliquity is desired.***observed**- In astronomy and navigation, pertaining to a value which has been measured in contrast to one which is computed.
**observed altitude**= true altitude.**occlusion**- Specifically, the trapping of undisolved gas in a solid during solidification.
**occultation**- The disappearance of a body behind another body of larger apparent size.
*When the moon passes between the observer and a star, the star is said to be occulted. The three associated terms, occultation, eclipse, and transit, are exemplified by the motions of the satellites of Jupiter. An eclipse occurs when a satellite passes into the shadow cast by the planet; an occultation occurs when a satellite passes directly behind the planet; so that it could not be seen even if it were illuminated; and a transit occurs when a satellite passes between the observer and the planet, showing against the disk of the planet.***Oct, Octn**- International Astronomical Union abbreviations for
*Octans.*See constellation. **octal notation**- Notation of numbers in the scale of eight. The octal digits can be represented by the eight possible combinations of three binary digits.
**octant**- See sextant.
**octave**- The interval between any two frequencies having the ratio of 1:2.
*The interval in octaves between any two frequencies is the logarithm to the base 2 (or 3.322 times the logarithm to the base 10) of the frequency ratio.***Octn**- International Astronomical Union abbreviations for
*Octans.*See constellation. **ocular**- Pertaining to or in relation with the eye.
**oculogravic illusion**= agravic illusion.**oculogyral illusion**- The apparent movement of an image in space in the same direction as that in which one seems to be turning when the semicircular canals are stimulated.
**oculogyric**- Referring to movements of the eyes.
**oersted**- The centimeter-gram-second electromagnetic unit of magnetic intensity. See gauss.
**off-center plan position indicator**- See plan position indicator.
**ogive**- A body of revolution formed by rotating a circular arc about an axis that intersects the arc; the shape of this body; also, a nose of a projectile or the like so shaped.
*Typically, an ogive has the outline of a Gothic arch, although by definition it may be rounded rather than pointed. See tangent ogive.***ohm (abbr Ω)**- The unit of electrical resistance; the resistance between two points of a conductor when a constant difference of potential of 1 volt, applied between these two points, produces in the conductor a current of 1 ampere (the conductor not being the source of any electromotive force).
**ohmic heating**- In plasma physics, the energy imparted to charged particles as they respond to an electric field and make collisions with other particles.
*The name was chosen for its similarity to the heat generated in an ohmic resistance due to the collisions of the charge carriers in their medium.***oilcan**- Of a sheet-metal skin or of other covering, to snap in and out between rows of rivets or between other places of support in a fashion like that of the bottom of an oilcan.
**omni**- 1. A prefix meaning
*all*, as in*omnidirectional.* - 2. Short for
*omnirange.* **omnidirectional range**= omnirange.**omnirange**- A radio navigation system providing a direct indication of the bearing of the
omnirange facility from the vehicle. Usually used in combination with distance
measuring equipment. Also called
*omnidirectional range.* **opacity**- Of an optical path, the reciprocal of transmission. See transmittance.
**opaque plasma**- A plasma through which an electromagnetic wave cannot propagate and is either absorbed or reflected.
*In general, a plasma is opaque for frequencies below the plasma frequency. The fact that a plasma is opaque over a certain frequency range will change the radiation properties within that frequency range. Any radiation emitted within the volume of the plasma is quickly absorbed. In this opaque region, therefore, the plasma can only radiate from its surface.***open-center plan position indicator**- See plan position indicator.
**open loop**- A system operating without feedback, or with only partial feedback. See closed loop system.
**open system**- A system that provides for the body's metabolism in an aircraft or spacecraft cabin by removal of respiratory products and of waste from the cabin and by use of stored food and oxygen. Compare closed system.
**operand**- In computer operations, a word on which an operation is to be performed.
**operating ratio = computing efficiency.****operative temperature**- In the study of human bioclimatology, one of several parameters devised to
measure the air's cooling effect upon a human body. It is equal to the
temperature at which a specified hypothetical environment would support the
same heat loss from an unclothed, reclining human body as the actual
environment. In the hypothetical environment, the wall and air temperatures
are equal and the air movement is 7.6 centimeters per second. From experiment
it has been found that the operative temperature
where t _{r}is the mean radiant temperature; t_{a}is the mean air temperature; t_{s}is the mean skin temperature (all in degrees C); and*v*is the airspeed in centimeters per second. **Oph, Ophi**- International Astronomical Union abbreviations for
*Ophiuchus.*See constellation. **Ophiuchus (abbr Oph, Ophi)**- See constellation.
**opposition**- 1. The situation of two celestial bodies having either celestial longitudes or sidereal hour angles differing by 180 degrees. The term is usually used only in relation to the position of a planet or the moon from the sun. Compare conjunction.
- 2. The situation of two periodic quantities differing by half a cycle.
**optical air mass (symbol m)**- A measure of the length of the path through the atmosphere to seal level
traversed by light rays from a celestial body, expressed as a multiple of the
path length for a light source at the zenith. Originally called, simply,
*air mass.*Also called*airpath.* **optical axis**- Of an antenna, a line parallel to, but offset from, the electrical axis of an antenna.
*This axis is offset by the distance necessary to have the optical sighting device removed from the electrical center of the antenna.***optical density**= photographic transmission density.**optical depth**= optical thickness.**optical double star**- Two stars in nearly the same line of sight but differing greatly in distance from the observer, as distinguished from a physical double star.
**optical haze**= terrestrial scintillation.**optical line of sight**- The generally curved path of visible light through the atmosphere.
*Often used erroneously for geometrical line of sight.***optically effective atmosphere**- That portion of the atmosphere
lying below the altitude from which scattered light at twilight still reaches
the observer with sufficient intensity to be discerned. Also called
*effective atmosphere.* *The top of this region lies between 50 and 60 kilometers.***optical mass**= laser.**optical path**- 1. = line of sight.
- 2. The path followed by a ray of light through an optical system.
**optical pyrometer**- A device for measuring the temperature of an incandescent radiating body by comparing its brightness for a selected wavelength interval within the visible spectrum with that of a standard source; a monochromatic radiation pyrometer.
*Temperatures measured by optical pyrometers are known as brightness temperatures and except for black bodies are less than the true temperatures.***optical slant range**- The horizontal distance in a homogeneous atmosphere for which the attenuation is the same as that actually encountered along the true oblique path.
**optical thickness**- Specifically, in calculations of the transfer of radiant
energy, the mass of a given absorbing or emitting material lying in a
vertical column of unit cross-sectional area and extending between two
specific levels. Also called
*optical depth.* *If z1 and z2 are the lower and upper limits, respectively, of a layer in which the variation of a density p of some absorbing or emitting substance is given as a function of height z, then the quantity*is called the optical thickness of that substance within that particular layer. **optical turbulence**- Irregular and fluctuating gradients of optical refractive index in the atmosphere.
*Optical turbulence is caused mainly by mixing of air of different temperatures, and particularly by thermal gradients which are sufficient to reverse the normal decrease in density with altitude, so that convection occurs.***optics**- See atmospheric optics.
**optimal**- Pertaining to a trajectory,
path, or control motion, one that minimizes or maximizes some quantity or
combination of quantities such as fuel, time, energy, distance, heat transfer,
etc. This optimum condition, or path, is commonly calculated by a type of
mathematics known as
*calculus of variations.* **OR**- 1. The logical operator which has the property that A or B is true if either A is true or B is true.
- 2. In Boolean algebra, the operation of union.
**orbit**- 1. The path of a body or particle under the influence of a gravitational or other force. For instance, the orbit of a celestial body is its path relative to another body around which it revolves.
*Orbit is commonly used to designate a closed path and trajectory to denote a path which is not closed. Thus, the trajectory of a sounding rocket, the orbit of a satellite.*- 2. To go around the earth or other body in an orbit, sense 1.
**orbital**- Taking place in orbit, as
*orbital refueling*,*orbital launch*, or pertaining to an orbit as*orbital plane.* **orbital elements**- A set of seven parameters defining the orbit of a body attracted by a central, inverse-square force.
*Several different set of parameters have been used. For artificial satellites the elements usually given are: longitude of the ascending node, Ω; inclination of the orbit plane, i; argument of perigee,w; eccentricity, e; semimajor axis, a; mean anomaly, M; and epoch, t*_{o}.**orbital glider**- See hypersonic glider.
**orbital motion**- Continuous motion in a closed path such as a circle or an ellipse.
**orbital period**- The interval between successive passages of a satellite
through the same point in its orbit. Often
called
*period.*See anomalistic period, nodical period, sidereal period. **orbital velocity**- 1. The average velocity at which an earth satellite or other orbiting body travels around its primary. Compare separation velocity.
- 2. The velocity of such a body at any given point in its orbit, as in
*its orbital velocity at the apogee is less than at the perigee.* - 3. = circular velocity.
**orbiting**- Of a spacecraft,
in orbit about the earth or other spatial body, as in
*an orbiting astronomical laboratory.* **OR-circuit**= OR-gate.**order of magnitude**- A factor of 10. Compare octave, magnitude.
*Two quantities of the same kind which differ by less than a factor of 10 are said to be of the same order of magnitude. Order of magnitude is used loosely by many writers to mean a pronounced difference in quantity but the difference may be much less or much more than a factor of 10.***order of reflection**- The number of hops, or trips, to the ionsphere and back to earth, that a radio wave makes in traveling from one point to another.
**ordinary ray**- That magnetoionic wave component deviating the least, in most of its
propagation characteristics, relative to those expected for a wave in the
absence of the earth's magnetic field. More exactly, if at fixed electron
density, the direction of the earth's magnetic field were rotated until its
direction is transverse to the direction of phase propagation, the wave
component whose propagation is then independent of the magnitude of the
earth's magnetic field. Also called
*ordinary-wave component.*See magnetic double refraction, magnetoionic theory. **ordinary wave component**= ordinary ray.**organ**- A portion or subassembly of a computer which constitutes the means of accomplishing some inclusive operation or function (e.g., arithmetic organ).
**OR-gate**- A gate
whose output is energized when any one or more of the inputs is in its
prescribed state. An OR-gate performs the function of the logical
*inclusive-OR*, of Boolean algebra. **Ori, Orio**- International Astronomical Union abbreviations for
*Orion.*See constellation. **origin**- The reference from which measurement begins.
**Orio**- International Astronomical Union abbreviations for
*Orion.*See constellation **Orion (abbr Ori, Orio)**- See constellation.
**orthochromatic**- See color sensitive, note.
**orthodrome**= great circle.**orthogonal**- Originally, at right angles; later generalized to mean the vanishing of a sum (or integral) of products.
*The cosine of the angle between two vectors V1 and V2 with the respective components x*_{1}, y_{1}, z_{1}, and x_{2}, y_{2}, z_{2}is proportional to the sum of products x_{1}x_{2}+ y_{1}y_{1}+ z_{1}z_{2}. Hence, if the vectors are perpendicular, the latter sum equals zero. For this reason any two series of numbers (x_{1}, x_{2}, ... x_{n}) and (y_{1}, y_{2}, ...y_{n}) is said to be orthogonal ifΣ x _{i}y_{i}= 0**orthogonal antennas**- In radar, a pair of transmitting and receiving antennas, or a single transmitting-receiving antenna, designed for the detection of a difference in polarization between the transmitted energy and the energy returned from the target.
**orthogonal curvilinear coordinates**- See curvilinear coordinates.
**orthogonal functions**- A set of functions, any two of which, by analogy to orthogonal vectors, vanish if their product is summed by integration over a specified interval.
*For example, f(x) and g(x) are orthogonal in the interval x = a to x = b if*The functions are also said to be normal if The most familiar examples of such functions, many of which have great importance in mathematical physics, are the sine and cosine functions between zero and 2pi. **orthogonic**- Pertaining to a state in which the spatial variations of a quantity have two planes of symmetry at right angles to each other.
**osciducer**- A transducer in which information pertaining to the stimulus is provided in the form of deviation from the center frequency of an oscillator.
**oscillation**- 1. Fluctuation or vibration on each side of a mean value or position.
- 2. Half an oscillatory cycle, consisting of a fluctuation or vibration in one direction; half a vibration.
- 3. The variation, usually with time, of the magnitude of a quantity with respect to a specified reference when the magnitude is alternately greater and smaller than the reference.
**oscillator**- A nonrotating device for producing alternating current.
**oscillatory wave**- A wave in which only the form advances, the individual particles moving in closed orbits, as ocean waves in deep water.
**oscilloscope**- 1. An instrument for producing a visual representation of oscillations or changes in an electric current.
- 2. Specifically, a
*cathode-ray oscilloscope.* *The face of the cathode-ray tube used for this representation is called a scope or screen.***osculating elements**- The orbital elements of an osculating orbit.
**osculating orbit**- The ellipse that a
satellite would follow after a specific time
*t*(the epoch of osculation) if all forces other than central inverse-square forces ceased to act from time*t*on. *An osculating orbit is tangent to the real, perturbed, orbit and has the same velocity at the point of tangency.***otitic barotrauma**= aero-otitis media.**otolith**- A small calcareous concretion located in the inner ear which plays a part in the mechanism or orientation.
**otolith organs**- Structures of the inner ear (utricle and saccule) which responds to linear acceleration and tilting.
**outer atmosphere**- Very generally, the atmosphere at a great distance from the earth's surface; an approximate synonym for exosphere.
**outer planets**- The planets with orbits larger than that of Mars: Jupiter, Saturn, Uranus, Neptune, and Pluto.
**outer product**= vector product.**outgassing**- The evolution of gas from a material in a vacuum.
**out of phase**- The condition of two or more cyclic motions
which are not at the same part of their cycles at the same instant. Also
called
*out of step.*Compare in phase. *Two or more cycles motions which are at the same part of their cycles at the same instant are said to be in phase or in step.***out of step**= out of phase.**output**- 1. The yield or product of an activity furnished by man, machine, or a system.
- 2. Power or energy delivered by an engine, generator, etc.
- 3. The electrical signal which emanates from a transducer and which is a function of the applied stimulus. Compare input.
*The quantity represented by the signal may be given in terms of electrical units, frequency, or time.***output unit**- In computer terminology, a unit which delivers information from the computer to an external device or from internal storage to external storage.
**overall heat-transfer coefficient**- (symbol
). The value**U***U*, in British thermal units per hour per square foot per degrees F in the equation*Q = UA(t$$*where_{1}- t_{2})*Q*is heat flow per unit time;*A*is area; and*t*is temperature. **overexpanding nozzle**- A nozzle in which the fluid is expanded to a lower pressure than the external pressure.
*An overexpanding nozzle has an exit area larger than the optimum.***oxidant**= oxidizer.**oxidizer**- (symbol o, used as subscript). Specifically, a substance (not necessarily containing oxygen) that supports the combustion of a fuel or propellant.
**oximeter**- An instrument for measuring the oxygen saturation of the blood.
**oxygen bottle**- A small container for pressurized oxygen used in life-support systems. See bailout bottle.
**oxygen mask**- A covering for the nose and lower face fitted with special attachments for breathing oxygen or a mixture of oxygen and other gases.
*The oxygen mask has provision for separating the expired breath from the incoming oxygen.***oxygen paradox**= posthypoxia paradox.**ozone layer**= ozonosphere.**ozonosphere**- The general stratum of the upper
atmosphere in which there is an appreciable ozone concentration and in
which ozone plays an important part in the radiation balance of the
atmosphere. This region lies roughly between 10 and 50 kilometers, with
maximum ozone concentration at about 20 to 25 kilometers. Also called
*ozone layer.*See atmospheric shell.