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The flow of sand in an hourglass can be used to measure the passage of time. It also concretely represents the present as being between the past and the future.

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. Time is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as a fourth dimension, along with three spatial dimensions.

Time has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars. Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems.

Time in physics is unambiguously operationally defined as "what a clock reads". See Units of Time. Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities. Time is used to define other quantities – such as velocity – so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.

Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms (see below). Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans.

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Time Topics

Time (book) touches upon nearly every topic in some way. Some of the most relevant are below:

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A chronometer watch is a watch tested and certified to meet certain precision standards. In Switzerland, only timepieces certified by the COSC may use the word 'Chronometer' on them. However, numerous prominent Swiss watch manufacturers do not submit their movements for COSC certification, although such movements would probably easily qualify as chronometers under the COSC certification rules.

The term chronometer is also used to describe a marine chronometer used for celestial navigation. The marine chronometer was invented by John Harrison in 1730. This was the first iteration of a series of chronometers which enabled accurate marine navigation. For the next 250 years, an accurate chronometer was essential to any kind of marine or air navigation until the implementation of global satellite navigation at the end of the 20th century. The marine chronometer is no longer used for navigation.


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Timewheel in Budapest, Hungary
The Timewheel in Budapest, initially unveiled in 2004 in commemoration of Hungary's accession to the European Union, is a 60-ton hourglass composed of granite, steel, and glass.

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The Venerable Bede

Bede (also Saint Bede, the Venerable Bede, or (from Latin) Beda) (c. 672 or 673 – May 25, 735), was a Benedictine monk... He is well known as an author and scholar, and his most famous work, Historia ecclesiastica gentis Anglorum (The Ecclesiastical History of the English People) gained him the title "The father of English history"... His work On the Reckoning of Time (De temporum ratione) included an introduction to the traditional ancient and medieval view of the cosmos, including an explanation of how the spherical earth influenced the changing length of daylight, of how the seasonal motion of the Sun and Moon influenced the changing appearance of the New Moon at evening twilight, and a quantitative relation between the changes of the Tides at a given place and the daily motion of the moon.

Did you know...

...that the second was known as a "second minute", the second small division of an hour?

...that the second is defined as the duration of 9,192,631,770 oscillations between the two hyperfine levels of the ground state in the Cesium-133 atom?

...that the smallest unit of time that could ever be measured is the Planck time (~ 5.4 × 10−44 seconds)?

..that despite Herodotus's claim that the sundial was invented in Babylon, the oldest known example is from Egypt?

... that merkhets were Ancient Egyptian timekeeping devices that tracked the movement of certain stars over the meridian in order to ascertain the time during the night, when sundials could not function?

Things you can do - Carpe diem because Tempus fugit

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Tu ne quaesieris, scire nefas, quem mihi, quem tibi Leuconoe, don't ask — it's dangerous to know —
finem di dederint, Leuconoe, nec Babylonios what end the gods will give me or you. Don't play with Babylonian
temptaris numeros. ut melius, quidquid erit, pati. fortune-telling either. Better just deal with whatever comes your way.
seu pluris hiemes seu tribuit Iuppiter ultimam, Whether you'll see several more winters or whether the last one
quae nunc oppositis debilitat pumicibus mare Jupiter gives you is the one even now pelting the rocks on the shore with the waves
Tyrrhenum: sapias, vina liques et spatio brevi of the Tyrrhenian sea — be smart, drink your wine. Scale back your long hopes
spem longam reseces. dum loquimur, fugerit invida to a short period. Even as we speak, envious time
aetas: carpe diem quam minimum credula postero. is running away from us. Seize the day, trusting little in the future.

Horace, Odes 1.11