English

The time interval between the senso­ry perception of a PHENOMENON and the motor reaction thereto. Personal equation may be either positive or negative, as an observer may anticipate the occurrence of an event, or wait until he actually sees it occur before making a record. This is a SYSTE­MATIC ERROR, treated as of the constant type. It is a PERSONAL ERROR, for which the term personal equation is reser­ved. It is of special significance in OBSERVA­TI­ONS of TIME, made to determine CHRONOMETER CORREC­TION.

The point of origin or termination of bundles of PERSPEC­TIVE RAYS. The two such points usually associated with a survey photograph are the interior perspective centre and the exterior perspective centre. In a perfect lens-camera system, PERSPECTIVE RAYS from the interior perspective centre to the photographic IMAGES enclose the same angles as the corresponding RAYS from the exterior perspective centre to the objects photographed. In a LENS having DISTORTION this is true only for a particular zone of the PHOTOGRAPH. In a perfectly adjusted lens-camera system the exterior and interior perspective centres correspond, respectively, to the front and rear NODAL POINTS of the camera lens.

Any plane containing the PERSPECTI­VE CENTRE. The intersection of a perspective plane and the GROUND will always appear as a straight line on an AERIAL PHOTOGRAPH.

A false BOTTOM indicated by an ECHO SOUNDER, some distance above the actual BOTTOM. Such an indication, quite common in the deeper parts of the OCEAN, is due to large quantities of small organisms. Also called deep scattering layer (DSL) or false bottom.

The visible aspect of an object. The amount by which a CYCLE has progressed from a specified origin. In ASTRONOMY, any of the stages of variation in the illumination of the MOON or a PLANET. See PHASE(S) OF THE MOON. In physics, the state of aggregation of a substance, for example, solid (ICE), liquid (water), or gas (va­pour).

The phase difference of two periodical­ly recurring PHENOMENA of the same FREQUENCY, expressed in angular measure. The angle at a CELESTIAL BODY between the SUN and EARTH.

Angular retarda­tion of the maximum of a constituent of the observed TIDE behind the correspon­ding maximum of the same con­stituent of the hypotheti­cal EQUILIBRIUM TIDE. Also called tidal epoch.

A visually discrete part of a light signal. It is bounded by changes between darkness and light (e.g. an occultation or flash), or between different colours, or between distinctly different luminous intensities, and it may be further discriminated by its duration.

The various appearances of the MOON during different parts of the SYNODICAL MONTH. The CYCLE begins with new moon or change of the MOON at CONJUNCTION. The visible part of the waxing moon increases in size during the first half of the CYCLE until full moon appears at OPPOSITION, after which the visible part of the waning moon decreases for the remainder of the CYCLE. First quarter occurs when the waxing moon is at east QUADRATURE,last quarter when the waning moon is at west QUADRATURE. From last quarter to new and from new to first quarter the MOON is crescent; from first quarter to full and from full to last quarter it is gibbous.