In Euclidean geometry, an angle is the figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles are also formed by the intersection of two planes. These are called dihedral angles. Two intersecting curves define also an angle, which is the angle of the tangents at the intersection point. For example, the spherical angle formed by two great circles on a sphere equals the dihedral angle between the planes containing the great circles.
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a precise time unless a context is given.
Right ascension is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the point in question above the earth. When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system.
A time zone is an area that observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.
Universal Time (UT) is a time standard based on Earth's rotation. There are several versions of Universal Time, which differ by up to a few seconds. The most commonly used are Coordinated Universal Time (UTC) and UT1. All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects, but with a scaling factor and other adjustments to make them closer to solar time. UTC is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day. Two types of solar times are apparent solar time and mean solar time.
Noon is 12 o'clock in the daytime. It is written as 12 noon, 12 p.m., 12 pm, or 12:00.
The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. and p.m.. Each period consists of 12 hours numbered: 12, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11. The 24-hour/day cycle starts at 12 midnight, runs through 12 noon, and continues just before midnight at the end of the day. The 12-hour clock was developed from the middle of the second millennium BC to the 16th century AD.
The 24-hour clock, popularly referred to in the United States and some other countries as militarytime, is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours passed since midnight, from 0 to 23. This system is the most commonly used time notation in the world today, and is used by the international standard ISO 8601.
The midnight sun is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the sun remains visible at the local midnight. When the midnight sun is seen in the Arctic, the sun appears to move from left to right, but in Antarctica the equivalent apparent motion is from right to left. This occurs at latitudes from 65°44' to 90° north or south, and this doesn't stop exactly at the Arctic Circle or the Antarctic Circle, due to refraction.
The International Date Line (IDL) is an imaginary line of demarcation on the surface of Earth that runs from the North Pole to the South Pole and demarcates the boundary between one calendar day and the next. It passes through the middle of the Pacific Ocean, roughly following the 180° line of longitude but deviating to pass around some territories and island groups.
Swatch Internet Time is a decimal time concept introduced in 1998 by the Swatch corporation as part of their marketing campaign for their line of "Beat" watches.
A (geographic) meridian is the half of an imaginary great circle on the Earth's surface, terminated by the North Pole and the South Pole, connecting points of equal longitude, as measured in angular degrees east or west of the Prime Meridian. The position of a point along the meridian is given by that longitude and its latitude, measured in angular degrees north or south of the Equator. Each meridian is perpendicular to all circles of latitude. Meridians are half of a great circle on the Earth's surface. The length of a meridian on a modern ellipsoid model of the earth has been estimated at 20,003.93 km.
A degree, usually denoted by °, is a measurement of a plane angle in which one full rotation is 360 degrees.
Decimal time is the representation of the time of day using units which are decimally related. This term is often used specifically to refer to the time system used in France for a few years beginning in 1792 during the French Revolution, which divided the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal seconds, as opposed to the more familiar UTC time standard, which divides the day into 24 hours, each hour into 60 minutes and each minute into 60 seconds.
The sunrise equation can be used to derive the time of sunrise and sunset for any solar declination and latitude in terms of local solar time when sunrise and sunset actually occur. It is:
Clocks and watches with a 24-hour analog dial have an hour hand that makes one complete revolution, 360°, in a day. The more familiar 12-hour analog dial has an hour hand that makes two complete revolutions in a day.
Clock angle problems are a type of mathematical problem which involve finding the angle between the hands of an analog clock.
Coordinated Universal Time or UTC is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude, and is not adjusted for daylight saving time. It is effectively a successor to Greenwich Mean Time (GMT).
A clock position, or clock bearing, is the direction of an object observed from a vehicle, typically a vessel or an aircraft, relative to the orientation of the vehicle to the observer. The vehicle must be considered to have a front, a back, a left side and a right side. These quarters may have specialized names, such as bow and stern for a vessel, or nose and tail for an aircraft. The observer then measures or observes the angle made by the intersection of the line of sight to the longitudinal axis, the dimension of length, of the vessel, using the clock analogy.
