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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.
An hour (symbol: h; [1] also abbreviated hr) is a unit of time historically reckoned as 1 ⁄ 24 of a day and defined contemporarily as exactly 3,600 seconds . There are 60 minutes in an hour, and 24 hours in a day. The hour was initially established in the ancient Near East as a variable measure of 1 ⁄ 12 of the night or daytime.
Earth-based: the day is based on the time it takes for the Earth to rotate on its own axis, as observed on a sundial [citation needed]. Units originally derived from this base include the week (seven days), and the fortnight (14 days). Subdivisions of the day include the hour (1/24 of a day), which is further subdivided into minutes and seconds ...
24-hour digital clock in Miaoli HSR station.. A time of day is written in the 24-hour notation in the form hh:mm (for example 01:23) or hh:mm:ss (for example, 01:23:45), where hh (00 to 23) is the number of full hours that have passed since midnight, mm (00 to 59) is the number of full minutes that have passed since the last full hour, and ss (00 to 59) is the number of seconds since the last ...
Metric time is the measure of time intervals using the metric system. The modern SI system defines the second as the base unit of time, and forms multiples and submultiples with metric prefixes such as kiloseconds and milliseconds. Other units of time – minute, hour, and day – are accepted for use with SI, but are not part of it.
The natural day (dies naturalis) ran from sunrise to sunset. [6] The hours were numbered from one to twelve as hora prima, hora secunda, hora tertia, etc. To indicate that it is a day or night hour, Romans used expressions such as for example prima diei hora (first hour of the day), and prima noctis hora (first hour of the night). [7]
GHA, the Greenwich Hour Angle of the apparent (actual) Sun; GMHA = Universal Time − Offset, the Greenwich Mean Hour Angle of the mean (fictitious) Sun. Here time and angle are quantities that are related by factors such as: 2 π radians = 360° = 1 day = 24 hours.
On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day). More simply, 1→2 is a complete ...