The second.
I see the "second" as a measure of time, not light.
LUX, Lumen or Candela do ring a bell and something like watt per square meter
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Yes the S.I. unit measure of light is a limen (lm) .
lumen (lm)
The SI derived unit for measuring the luminous flux of light being produced by a light source or received by a surface. The luminous intensity of a light source is measured in candela. One lumen represents the total flux of light emitted, equal to the intensity in candela multiplied by the solid angle in steradians (1/(4pi) of a sphere) into which the light is emitted. Thus the total flux of a one-candela light, if the light is emitted uniformly in all directions, is 4pi lumen.
The most commonly used unit to measure distances beyond our solar system is the light-year. This unit is the distance that light travels in one year in a vacuum, equivalent to about 9.46 trillion kilometers.
Light year, kilometre, metre, centimetre, millimetre, micrometre, nanometre. And for the old fashioned, light year, mile, furlong, chain, yard, foot, inch, thou. Both systems contain many other units that are used less often.
While it is truly an unconventional (and not very useful) unit, yes it is. Speed (and velocity) are shown as a ratio of elapsed distance to time. Therefore, since a light-year is a measure of distance and a century is a unit of time, this is a unit of speed (or it could be used to measure velocity).
The base unit is the meter. Larger and smaller units based on the meter are also used, for example downward: millimeter, micrometer (a.k.a. "micron"), nanometer; and upward: the kilometer. Larger units like megameter and gigameter could be used, but they are seldom used in practice.
(LY) Light Years and (AU) Astronomical Unit.
light years
The metre.
The meter is used to measure length.
The base unit for time is the second.
MassHowever, if, by 'primary unit', you mean 'base unit', then this is the kilogram not the gram.
The most commonly used unit to measure distances beyond our solar system is the light-year. This unit is the distance that light travels in one year in a vacuum, equivalent to about 9.46 trillion kilometers.
Light year, kilometre, metre, centimetre, millimetre, micrometre, nanometre. And for the old fashioned, light year, mile, furlong, chain, yard, foot, inch, thou. Both systems contain many other units that are used less often.
Light years: the distance that light travels in a year.
While it is truly an unconventional (and not very useful) unit, yes it is. Speed (and velocity) are shown as a ratio of elapsed distance to time. Therefore, since a light-year is a measure of distance and a century is a unit of time, this is a unit of speed (or it could be used to measure velocity).
The base unit is the meter. Larger and smaller units based on the meter are also used, for example downward: millimeter, micrometer (a.k.a. "micron"), nanometer; and upward: the kilometer. Larger units like megameter and gigameter could be used, but they are seldom used in practice.
The unit that measures the greatest distance is a light-year, which represents the distance that light travels in one year. This unit is frequently used in astronomy to measure vast distances between celestial objects.
(LY) Light Years and (AU) Astronomical Unit.