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In physics, time is defined by its measurement: time is what a clock reads. [1] In classical, non-relativistic physics, it is a scalar quantity (often denoted by the symbol ) and, like length, mass, and charge, is usually described as a fundamental quantity.
Evening. Evening is the period of a day that begins at the end of daylight and overlaps with the beginning of night. [1] It generally indicates the period of time when the sun is close to the horizon and comprises the periods of civil, nautical and astronomical twilight. The exact times when evening begins and ends are subjective and depend on ...
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, and into the future. [1] [2] [3] It is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the ...
Time perception. In psychology and neuroscience, time perception or chronoception is the subjective experience, or sense, of time, which is measured by someone's own perception of the duration of the indefinite and unfolding of events. [1][2][3] The perceived time interval between two successive events is referred to as perceived duration.
Clock mechanism. An atomic clock is based on a system of atoms which may be in one of two possible energy states. A group of atoms in one state is prepared, then subjected to microwave radiation. If the radiation is of the correct frequency, a number of atoms will transition to the other energy state.
Day. A quarter-day cycle at Midtown Manhattan, from afternoon to dusk. A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, noon, afternoon, evening, and night.
The time 8:45 may be spoken as "eight forty-five" or "(a) quarter to nine". [20] In older English, it was common for the number 25 to be expressed as "five-and-twenty". [21] In this way the time 8:35 may be phrased as "five-and-twenty to 9", [22] although this styling fell out of fashion in the later part of the 1900s and is now rarely used. [23]
The meaning of the preceding definition is as follows. The caesium atom has a ground state electron state with configuration [Xe] 6s 1 and, consequently, atomic term symbol 2 S 1/2. This means that there is one unpaired electron and the total electron spin of the atom is 1/2. Moreover, the nucleus of caesium-133 has a nuclear spin equal to 7/2.