20 relations: Adoption of the Gregorian calendar, Astronomy, Calendar date, Day, Earth, Greenwich Mean Time, Gregorian calendar, International Astronomical Union, International System of Units, Julian calendar, Light-year, Newcomb's Tables of the Sun, Second, Sidereal year, Simon Newcomb, Terrestrial Time, Time, Tropical year, Unit of measurement, Year.

## Adoption of the Gregorian calendar

The adoption of the Gregorian Calendar was an event in the modern history of most nations and societies, marking a change from their traditional (or old style) dating system to the modern (or new style) dating system that is widely used around the world today.

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## Astronomy

Astronomy (from ἀστρονομία) is a natural science that studies celestial objects and phenomena.

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## Calendar date

A calendar date is a reference to a particular day represented within a calendar system.

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## Day

A day, a unit of time, is approximately the period of time during which the Earth completes one rotation with respect to the Sun (solar day).

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## Earth

Earth is the third planet from the Sun and the only astronomical object known to harbor life.

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## Greenwich Mean Time

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London.

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## Gregorian calendar

The Gregorian calendar is the most widely used civil calendar in the world.

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## International Astronomical Union

The International Astronomical Union (IAU; Union astronomique internationale, UAI) is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy.

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## International System of Units

The International System of Units (SI, abbreviated from the French Système international (d'unités)) is the modern form of the metric system, and is the most widely used system of measurement.

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## Julian calendar

The Julian calendar, proposed by Julius Caesar in 46 BC (708 AUC), was a reform of the Roman calendar.

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## Light-year

The light-year is a unit of length used to express astronomical distances and measures about 9.5 trillion kilometres or 5.9 trillion miles.

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## Newcomb's Tables of the Sun

Newcomb's Tables of the Sun is the short title and running head of a work by the American astronomer and mathematician Simon Newcomb entitled "Tables of the Motion of the Earth on its Axis and Around the Sun" on pages 1–169 of "Tables of the Four Inner Planets" (1895), volume VI of the serial publication Astronomical Papers Prepared for the Use of the American Ephemeris and Nautical Almanac.

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## Second

The second is the SI base unit of time, commonly understood and historically defined as 1/86,400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.

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## Sidereal year

A sidereal year (from Latin sidus "asterism, star") is the time taken by the Earth to orbit the Sun once with respect to the fixed stars.

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## Simon Newcomb

Simon Newcomb (March 12, 1835 – July 11, 1909) was a Canadian–American astronomer, applied mathematician and autodidactic polymath, who was Professor of Mathematics in the U.S. Navy and at Johns Hopkins.

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## Terrestrial Time

Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth.

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## Time

Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future.

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## Tropical year

A tropical year (also known as a solar year) is the time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice.

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## Unit of measurement

A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity.

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## Year

A year is the orbital period of the Earth moving in its orbit around the Sun.

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## Redirects here:

Julian centuries, Julian century, Julian millennia, Julian millennium.