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The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin ante meridiem, translating to "before midday") and p.m. (from Latin post meridiem, translating to "after midday"). [1] [2] Each period consists of 12 hours numbered: 12 (acting as 0), [3] 1, 2, 3, 4
The time of day is typically expressed in English in terms of hours. Whole hours on a 12-hour clock are expressed using the contracted phrase o'clock, from the older of the clock. [6] (10 am and 10 pm are both read as "ten o'clock".) Hours on a 24-hour clock ("military time") are expressed as "hundred" or "hundred hours".
Because of the potential for confusion, xiǎoshí (小时; 小時, literally "small hour") is sometimes used for the hour as part of a 24-hour cycle, and shíchen (时辰; 時辰) is used for the hour as part of the old 12-hour cycle. Diǎn is also used interchangeably with shí for the hour.
German (Gallic) hours (half-dial): 2×12 hour system starting at midnight and restarted at noon. It is typical with the 12-hour dial with 12 at the top. The modern 24-hour system is a late-19th century adaptation of the German midnight-starting system, and then prevailed in the world with the exception of some Anglophone countries.
The two mutated plants were grown in both an environment that had a 20-hour light and dark cycle and in an environment that had a 28-hour light and dark cycle. [65] It was found that the variety of Arabidopsis with a 24-hour circadian rhythm cycle grew best in an environment that also had a 24-hour light and dark cycle. [65]
A solar calendar organizes dates based on the Sun's annual cycle, giving consistent start dates for the four seasons from year to year. A lunar calendar organizes dates based on the Moon's lunar phase. In common usage, a day starts at midnight, written as 00:00 or 12:00 am in 24-or 12-hour clocks, respectively.
In vertebrates, the master circadian clock is contained within the suprachiasmatic nucleus (SCN), a bilateral nerve cluster of about 20,000 neurons. [10] [11] The SCN itself is located in the hypothalamus, a small region of the brain situated directly above the optic chiasm, where it receives input from specialized photosensitive ganglion cells in the retina via the retinohypothalamic tract.
The hour hand of an analogue clock will typically cycle twice around the dial in this period as most analogue clocks are 12-hour, less common are analogue 24-hour clocks in which it cycles around once. 86.401 ks (24 h 0 min 1 s): One day with an added leap second on UTC time scale.