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International Atomic Time is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. It is a continuous scale of time, without leap seconds, and it is the principal realisation of Terrestrial Time. It is the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface and which has leap seconds.
In precise timekeeping, ΔT is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of International Atomic Time. Formally, ΔT is the time difference ΔT = TT − UT between Universal Time and Terrestrial Time. The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time. As well as this long-term drift in the length of the day there are short-term fluctuations in the length of day which are dealt with separately.
Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted 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 particular time unless a context is given. The term GMT is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. The leap second facility exists to provide this adjustment. The leap second was introduced in 1972 and since then 27 leap seconds have been added to UTC.
A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.
Universal Time is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with respect to the International Celestial Reference Frame (ICRF), called the Earth Rotation Angle. UT1 is the same everywhere on Earth. UT1 is required to follow the relationship
The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups.
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, based on the synodic rotation period. Traditionally, there are three types of time reckoning based on astronomical observations: apparent solar time and mean solar time, and sidereal time, which is based on the apparent motions of stars other than the Sun.
Earth's outer core is a fluid layer about 2,260 km (1,400 mi) thick, composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. The outer core begins approximately 2,889 km (1,795 mi) beneath Earth's surface at the core-mantle boundary and ends 5,150 km (3,200 mi) beneath Earth's surface at the inner core boundary.
Delta Sculptoris is a triple star system in the constellation Sculptor. It is approximately 137.4 light years from Earth.
Earth's inner core is the innermost geologic layer of the planet Earth. It is primarily a solid ball with a radius of about 1,220 km (760 mi), which is about 20% of Earth radius or 70% of the Moon's radius.
(Stanley) Keith Runcorn was a British physicist whose paleomagnetic reconstruction of the relative motions of Europe and America revived the theory of continental drift and was a major contribution to plate tectonics.
A tropical year or solar year is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice. It is the type of year used by tropical solar calendars. The solar year is one type of astronomical year and particular orbital period. Another type is the sidereal year, which is the time it takes Earth to complete one full orbit around the Sun as measured with respect to the fixed stars, resulting in a duration of 20 minutes longer than the tropical year, because of the precession of the equinoxes.
Coordinated Universal Time or UTC is the primary time standard by which the world regulates clocks and time. It is within about one 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).
The preliminary reference Earth model (PREM) plots the average of Earth's properties by depth. It includes a table of Earth properties, including elastic properties, attenuation, density, pressure, and gravity.
Shamita Das is an emeritus professor at the University of Oxford and an emeritus fellow at Exeter College. She is known for her research on earthquakes, in particular the speed that earthquakes can propagate through the earth.
HD 89571 is a binary star located in the northern circumpolar constellation Camelopardalis. It is faintly visible to the naked eye with a combined apparent magnitude of 5.51 and is estimated to be 142 light years away from the Solar System. However, it is receding with a heliocentric radial velocity of 3.5 km/s.
42 Leonis Minoris is a solitary, bluish-white hued star located in the northern constellation Leo Minor. It has a visual apparent magnitude of 5.35, allowing it to be faintly seen with the naked eye. Parallax measurements place it at a distance of 412 light years. The object has a heliocentric radial velocity of 12 km/s, indicating that it is drifting away from the Solar System.
HD 22764, also known as HR 1112, is an orange hued star located in the northern circumpolar constellation Camelopardalis. It has an apparent magnitude of 5.78, allowing it to be faintly visible to the naked eye. The object is located relatively far at a distance of approximately 1,770 light years based on Gaia DR3 parallax measurements but is approaching the Solar System with a heliocentric radial velocity of −12.5 km/s. At its current distance, HD 22764's brightness is diminished by 0.66 magnitudes due to interstellar dust.
John Matthew Wahr was an American geophysicist and geodesist, known for his research on Earth's rotation, Earth tides, ocean tides, post-glacial rebound, and other topics in the geosciences.
References
- ↑ "Meet the world's director of time". BBC News . 2 December 2008. Retrieved 6 December 2008.
- ↑ "Superintendents, Scientific Directors, and Department Directors Of the U.S. Naval Observatory". U.S. Naval Observatory . Archived from the original on 22 December 2008. Retrieved 6 December 2008.
- ↑ Holloway, Jonathan B. (10 April 2023). "GPS Pioneer Inducted into Naval Oceanography Hall of Fame". U.S. Naval Meteorology and Oceanography Command. U.S. Navy. Retrieved 12 April 2023.
- ↑ Jefferts, Steven (2010). "Review of Time: From Earth Rotation to Atomic Physics by Dennis D. McCarthy and P. Kenneth Seidelmann". Physics Today. AIP Publishing. 63 (12): 63–64. doi: 10.1063/1.3529005 . ISSN 0031-9228.
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