# Second (physics)/Related Articles

From Citizendium, the Citizens' Compendium

*See also changes related to Second (physics), or pages that link to Second (physics) or to this page or whose text contains "Second (physics)".*

## Parent topics

- Time [r]: A fundamental quantity in physics - that what differs always between two inspections of the same system by the same observer.
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- International System of Units [r]: Metric unit system based on the metre, kilogram, second, ampere, kelvin, mole and candela.
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- Speed of light [r]: A physical constant
*c*describing the speed of electromagnetic radiation in vacuum. In the International System of Units the metre is the distance light travels in classical vacuum in 1/*c*seconds, using the defined value*c = c*≡ 299 792 458 m/s (exact)._{0}^{[e]}

## Subtopics

- Millisecond [r]:
*Add brief definition or description* - Microsecond [r]:
*Add brief definition or description* - Nanosecond [r]:
*Add brief definition or description* - Femtosecond [r]:
*Add brief definition or description* - Picosecond [r]:
*Add brief definition or description*

- Year [r]: A unit of time measurement that corresponds to one revolution of the earth around the sun, approximately 365¼ days.
^{[e]} - Month [r]: Unit of time originally corresponding approximately to one cycle of the moon's phases, or about 30 days or 4 weeks.
^{[e]} - Week [r]: Time unit equal to a number of days, now usually seven calendar days, previously weeks of between 4 and 20 days have been used historically in various places.
^{[e]} - Day [r]: Defined as 86,400 seconds, using the SI definition of second.
^{[e]} - Hour [r]:
*Add brief definition or description* - Minute [r]:
*Add brief definition or description*

- Metric units [r]: Obsolete term for a system of units in which the unit of length is the meter, the unit of mass the kilogram, and the unit of time the second.
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- Hertz [r]: The international (SI) unit of frequency, with units of inverse seconds, commonly called cycles per seocond.
^{[e]} - Frequency [r]: For a periodic (i.e., repeating) phenomena, the number of repetitions per unit of time, usually one second; measured in Hertz
^{[e]} - Electromagnetic wave [r]: A change, periodic in space and time, of an electric field
**E**(**r**,t) and a magnetic field**B**(**r**,t); a stream of electromagnetic waves, referred to as electromagnetic radiation, can be seen as a stream of massless elementary particles, named photons.^{[e]}

- Light second [r]: Distance that light travels in vacuum in one second; 2.99792458 * 10
^{8}m.^{[e]}

- Hyperfine level [r]:
*Add brief definition or description* - Caesium [r]: A chemical element, having the chemical symbol Cs, and atomic number (the number of protons) 55.
^{[e]} - Isotope [r]: An atom of a chemical element with a specific number of neutrons and hence a specific nuclear mass, such as carbon-14 (
^{14}C).^{[e]}