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<h1 align="center"><em>With Skyclock, instantly recognize <font color="#288DFF">day,</font> night, <u><font color="#8C8C8B">and twilight</font></u>.</em></h1>
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Glossary
GPS
The Global Positioning System is the only fully-functional satellite navigation system. Over two dozen GPS satellites broadcast precise timing signals by radio, allowing any GPS receiver to accurately determine its location (longitude, latitude, and altitude) in any weather, day or night, anywhere on Earth. GPS is made available by the United States Government free of charge as a public good.
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Sunrise
The time at which the first part of the Sun appears above the eastern horizon.
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Sunset
The time at which the Sun disappears below the western horizon. Also called sundown.
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Time zone
A time zone is a region of the Earth that has adopted the same standard time. Most adjacent time zones are exactly one hour apart, and by convention compute their standard time as an offset from Greenwich Mean Time. Standard time zones can be defined by geometrically subdividing the Earth’s spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15° of longitude apart. The standard time in neighboring zones is then exactly one hour different.
However, political and geographical practicalities have resulted in irregularly-shaped zones that follow political boundaries as well as being subject to occasional redefinition as political conditions change.
A few time zones have been added which have offsets of one half hour, 15 minutes or 45 minutes from adjacent time zones.
In addition within many (but not all) time zones, there are locations that shift their time by one hour over part of the year. Such shifts are known as “Daylight Saving Time” or “Summer Time” depending on the location. The times when the shifts occur vary from location to location depending on local politics. Lord Howe Island near Australia currently shifts by one half hour instead of one hour.
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Latitude
The imaginary lines that encircle the globe, that match the equator, are also known as parallel. The equator is the largest or longest parallel.
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Longitude
The imaginary lines that extend from pole-to-pole, vertically -arranged on a globe or map, are also called meridians.
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Horology
The study of the science and art of timekeeping devices. Clocks, watches, and chronometers are examples of instruments used to measure time.
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Twilight
The time before the sun rises, and after the sun sets, during which the rays of the Sun are providing light having been scattered in the upper atmosphere, and then reflected down to the lower atmosphere.
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Civil twilight
The time between sunset and when the Sun is 6 degrees below the horizon. The time between when the Sun is 6 degrees below the horizon and sunrise.
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Nautical twilight
The time between sunset and when the Sun is 12 degrees below the horizon. The time between when the Sun is 12 degrees below the horizon and sunrise.
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Astronomical twilight
The time between sunset and when the Sun is 18 degrees below the horizon. The time between when the Sun is 18 degrees below the horizon and sunrise.
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Coordinates
When both longitude and latitude are known and stated together, they coordinate, or cooperate in creating a location.
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Blue Hour
An expression referring generally to twilight periods. Originally from the french 'l'heure bleue', the phrase is meant to evoke the special qualities of the natural lighting during twilight. There is a useful list of literary references where the phrase is used in songs, books, and film, as well as links to other sites at the Wikipedia entry here.
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Madrugada
"Dawn" in Spanish and Portuguese.
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