# phase

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## phase

, in astronomy

phase, in astronomy, the measure of how much of the illuminated surface of a planet or satellite can be seen from a point at a distance from that body; the term is most often used to describe the moon as seen from the earth. When the moon is between the earth and the sun, we cannot see the lighted half at all, and the moon is said to be new. For a few days before and after the new moon we can see a small part of the lighted half, which appears as a crescent with the horns, or cusps, pointing away from the sun. When the moon has completed half its orbit from new moon to new moon, it is on the opposite side of the earth from the sun and we see the entire lighted half; this phase is called the full moon. When the moon is at quadrature with the sun, having completed either one quarter or three quarters of its orbit from new moon to new moon, half the lighted side is visible; this phase is called the half-moon. The half-moon between the new moon and the full moon is known as the first quarter and that between the full moon and new moon is known as the last quarter. Between the first quarter and the full moon and between the full moon and the last quarter we see more than half the lighted side; this phase is called gibbous. Of the planets, only Mercury and Venus, whose orbits pass between the earth and sun, show all the phases that the moon shows; the other planets are always either gibbous or full.

## Phase (periodic phenomena)

The fractional part of a period through which the time variable of a periodic quantity (alternating electric current, vibration) has moved, as measured at any point in time from an arbitrary time origin. In the case of a sinusoidally varying quantity, the time origin is usually assumed to be the last point at which the quantity passed through a zero position from a negative to a positive direction.

In comparing the phase relationships at a given instant between two time-varying quantities, the phase of one is usually assumed to be zero, and the phase of the other is described, with respect to the first, as the fractional part of a period through which the second quantity must vary to achieve a zero of its own (see illustration). In this case, the fractional part of the period is usually expressed in terms of angular measure, with one period being equal to 360° or 2π radians. See Phase-angle measurement, Sine wave

An illustration of the meaning of phase for a sinusoidal wave
McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.

## phase

(fayz)
1. The appearance of the illuminated surface of the Moon or a planet at a particular time during its orbit or of the Sun during an eclipse. Specifically it is the fraction of the object's apparent disk (taken to be circular) that is illuminated. Unlike the superior planets, Mercury and Venus can exhibit those phases in which half or less of the illuminated hemisphere is visible from Earth. See also phases, lunar.
2. The fraction of one complete cycle of a regularly recurring quantity that has elapsed with respect to a fixed datum point. The phase difference, ⊂, is the difference in phase between two electrical oscillations, wavetrains, etc., of the same frequency (i.e. coherent signals) and is usually expressed in terms of part of one complete cycle or wavelength. ‘Phase difference’ is often referred to simply as ‘phase’, as it is when considering the coherent signals in the two arms of a radio interferometer.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Phase

the argument of the function cos (ωt+ φ), describing a harmonic oscillatory process, where ω is the angular frequency, t is the time, and φ is the initial phase of oscillations, that is, the phase at the initial time t = 0. The phase can be determined with an accuracy up to the value of an arbitrary term that is a multiple of 2π. Usually, the only values of interest are the phase differences in various harmonic processes. For oscillations that have the same frequency, the phase difference is always equal to the difference between the initial phases φ1 – φ2 and does not depend on the time reference point. For oscillations that have differing frequencies ωl and ω2, the phase relationships are characterized by the reduced phase difference

which similarly is not dependent on the time reference point. The auditory perception of the direction from which a sound approaches is associated with the difference in the phase of acoustic vibrations arriving at each ear.

## Phase

in thermodynamics, a state of thermodynamic equilibrium of a substance, the physical properties of which differ from those of other equilibrium states—that is, other phases—of the same substance (seeEQUILIBRIUM, THERMODYNAMIC). A nonequilibrium metastable state of a substance is sometimes called a phase or, more explicitly, a metastable phase.

A transition of a substance from one phase to another, which is called a phase transition, is associated with a qualitative change in the properties of the substance. For example, the gaseous, liquid, and crystalline states, or phases, of a substance are substantially different with respect to the nature of the motion of particles (molecules) and the presence or absence of ordered structure (seeSTATES OF AGGREGATION). At high temperatures and pressures, a substance undergoes a transition to the plasma state. Various crystalline phases may differ from one another in, for example, electrical conductivity, the presence of an electric or magnetic moment, the type of crystal structure, and the existence of superconductivity. Various liquid phases may differ from one another in, for example, the concentration of components, the presence or absence of superfluidity (as in liquid 3He and 4He; seeQUANTUM FLUID), and the anisotropy of elastic and electrical properties (as in liquid crystals).

In most cases, phases are spatially homogeneous. However, there are some exceptions, such as ferromagnets in weak magnetic fields (seeDOMAINS) and the mixed state of type II superconductors (seeSUPERCONDUCTIVITY).

### REFERENCES

Landau, L. D., A. I. Akhiezer, and E. M. Lifshits. Kurs obshchei fiziki: Mekhanika i molekuliarnaia fizika, 2nd ed. Moscow, 1969.
Iavorskii, B. M., and A. A. Pinskii. Osnovy fiziki, vol. 1. Moscow, 1969.

V. L. POKROVSKII

## phase

[fāz]
(astronomy)
One of the cyclically repeating appearances of the moon or other orbiting body as seen from earth.
(chemistry)
Portion of a physical system (liquid, gas, solid) that is homogeneous throughout, has definable boundaries, and can be separated physically from other phases.
(mathematics)
An additive constant in the argument of a trigonometric function.
(metallurgy)
A constituent of an alloy that is physically distinct and is homogeneous in chemical composition.
(physics)
The fractional part of a period through which the time variable of a periodic quantity (alternating electric current, vibration) has moved, as measured at any point in time from an arbitrary time origin; usually expressed in terms of angular measure, with one period being equal to 360° or 2π radians.
For a sinusoidally varying quantity, the phase (first definition) with the time origin located at the last point at which the quantity passed through a zero position from a negative to a positive direction.
The argument of the trigonometric function describing the space and time variation of a sinusoidal disturbance, y = A cos [(2π/λ)(x-vt)], where x and t are the space and time coordinates, v is the velocity of propagation, and λ is the wavelength.
(thermodynamics)
The type of state of a system, such as solid, liquid, or gas.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

## phase

One of the basic services provided by the architect as part of the professional services agreement between the architect and owner; divided into the following phases: schematic design, design development, construction documents, bidding (negotiation), and construction contract administration.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

## phase

1. Astronomy one of the recurring shapes of the portion of the moon or an inferior planet illuminated by the sun
2. Physics
a. the fraction of a cycle of a periodic quantity that has been completed at a specific reference time, expressed as an angle
b. (as modifier): a phase shift
3. Physics a particular stage in a periodic process or phenomenon
4. in phase (of two waveforms) reaching corresponding phases at the same time
5. out of phase (of two waveforms) not in phase
6. Chem a distinct state of matter characterized by homogeneous composition and properties and the possession of a clearly defined boundary
7. Zoology a variation in the normal form of an animal, esp a colour variation, brought about by seasonal or geographical change
8. Biology a stage in mitosis or meiosis
9. Electrical engineering one of the circuits in a system in which there are two or more alternating voltages displaced by equal amounts in phase (sense 5)
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

## phase

(1)
The offset of one's waking-sleeping schedule with respect to the standard 24-hour cycle; a useful concept among people who often work at night and/or according to no fixed schedule. It is not uncommon to change one's phase by as much as 6 hours per day on a regular basis. "What's your phase?" "I've been getting in about 8 P.M. lately, but I'm going to wrap around to the day schedule by Friday." A person who is roughly 12 hours out of phase is sometimes said to be in "night mode". (The term "day mode" is also (but less frequently) used, meaning you're working 9 to 5 (or, more likely, 10 to 6).) The act of altering one's cycle is called "changing phase"; "phase shifting" has also been recently reported from Caltech.

## phase

(2)
"change phase the hard way": To stay awake for a very long time in order to get into a different phase.

## phase

(3)
"change phase the easy way": To stay asleep, etc. However, some claim that either staying awake longer or sleeping longer is easy, and that it is *shortening* your day or night that is really hard (see wrap around). The "jet lag" that afflicts travelers who cross many time-zone boundaries may be attributed to two distinct causes: the strain of travel per se, and the strain of changing phase. Hackers who suddenly find that they must change phase drastically in a short period of time, particularly the hard way, experience something very like jet lag without travelling.
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