L-Z GRAPH 



loxodrome — See rhumb line. 



luciferase — An enzyme which is heat labile, has 

 protein characteristics, and catalyzes the oxida- 

 tion of luciferin in bioluminescence. 



lucif erin — A group of heat-stable compounds con- 

 taining amino acids and sliowing properties of 

 polypeptides. Bioluminescence is produced 

 when these compounds are oxidized by the cata- 

 lytic action of luciferase. See also oxylucif- 

 erin. 



lumachelle — Shell conglomerate formed of mol- 

 lusk shells which have been consolidated into 

 cemented aggregates. (2) 



luminance — (also called brightness). In pho- 

 tometry, a measure of the intrinsic luminous 

 intensity emitted by a source in a given direc- 

 tion. It may be defiiied as the illuminance pro- 

 duced by light from the source upon a unit 

 surface area oriented normal to the line of sight 

 at any distance from the source, divided by the 

 solid angle subtended by the source at the re- 

 ceiving surface. It is assumed that the medium 

 between source and receiver is perfectly trans- 

 parent; therefore, luminance is independent of 

 extinction between source and receiver. The 

 source may or may not be self-luminous. 



Luminance is a measure only of light; the 

 comparable term for electromagnetic radia- 

 tion in general is radiance. (5) 



luminescence — (or bioluminescence, phosphores- 

 cence). Any emission of light at temperatures 

 below that required for incandescence. (5) 



luminous intensity — (or candle poxoer, light in- 

 tensity). The intensity (flux per imit solid 

 angle) of visible radiation weighted to take into 

 account the variable response of the human eye 

 as a function of the wavelength of radiation. 

 The basic unit for luminous flux is the lumen. 



(5) 



lunar day — (or tidal day) . The interval between 

 two successive upper transits of the moon over a 

 local meridian. The period of the mean lunar 

 day, approximately 24.84 solar hours, is de- 

 rived from the rotation of the earth on its axis 

 relative to the movement of the moon about the 

 earth. 



lunar declination — The angular distance of the 

 moon expressed in degrees north or south of the 

 celestial equator ; it is indicated as positive when 

 north, and negative when south of the equator. 



Maximum declination is about 28i/^ degrees 

 and minimum declination about 181/^ degrees, 

 depending on the longitude of the moon's node. 

 Tides or tidal currents occurring near times of 

 maximum north or south declination are called 

 tropic tides or tidal currents. The moon's dec- 

 linational cycle of 27i/^ days is called a tropical 

 month. (50) 



lunar interval — The time difference between the 

 moon's transit of the Greenwich meridian and 

 a local meridian. (50) 



lunar tide — That part of the tide caused solely 

 by the tide-producing forces of the moon as 

 distinguished from that part caused by the forces 

 of the sun. (50) 



lunar transit — See transit. 



lunate bar — See bar. 



lunation — The period during which the moon 

 completes all its phases from one new moon to 

 the next new moon ; approximately 29.5 days. 



lunicurrent interval — 1. The interval between 

 the moon's transit (upper or lower) over a local 

 or the Greenwich meridian and a specified phase 

 of the tidal current following the transit, such 

 as strength of flood and ebb or slack water. 

 The interval is described as local or Greenwich 

 according to whether the reference is to the 

 moon's transit over the local or Greenwich me- 

 ridian. (50) 



2. See strength of flood interval, strength 

 of ebb interval. 



lunitidal interval — The interval between the 

 moon's transit (upper or lower) over the local 

 or Greenwich meridian and the following high 

 or low water. The average of all high water 

 intervals for all phases of the moon, the mean 

 high water lunitidal interval, is abbreviated to 

 high water interval. Similarly, the mean low 

 water lunitidal interval is abbreviated to low 

 water interval. The high water or low water 

 interval is described as local or Greenwich ac- 

 cording to whether the reference is to the transit 

 over the local or the Greenwich meridian. (50) 



lutaceous — See lutite. 



1 utite — ( sometimes spelled luty te ) . Sediments or 

 sedimentary rock composed of mud (silts and/ 

 or clays). (2) 



h-Zd graph — A graph used to determine in situ 

 depths of oceanographic observations by the 

 wire depth minus thermometric depth method. 



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