Time

Discussion

What is time?

Nam Tempus, Spatium, Locum & Motum, ut omnibus notiſſima, non definio.		I do not define time, space, place, and motion, as being well known to all.
Isaac Newton (1642–1727)

Zeit ist das, was man an der Uhr abliest.		Time is what a clock tells you.
Albert Einstein (1879–1955)

More quotes…

What time is it?

There's time in the sense of "What time is it?" And then there's time in the sense of how long did it take for something to happen. Time in the former sense is determined with a calendar or a clock. Time in the latter sense is measured with a stopwatch or an interval timer. In day to day living, time is a cultural construct whereby an event can be associated with a series of numbers. In physics, time is a measure of the interval between two events.

Natural units of time

In the International System of Units the second is the base unit of time, but for most of human existence (for as long as there have been things that looked and acted like humans) the natural units of time have been the day, the year, and the month.

the day

From the view of someone stuck on the surface of the Earth, day is the time when the Sun is visible in the sky. A day begins when the Sun appears to rise above and ends when the Sun appears to settle below the apparent boundary between the Earth and the sky called the horizon. The middle of one of these days is the moment when the Sun it at its highest position above the horizon called noon (from an archaic term meaning "ninth hour") or midday (since it occurs in the middle of the day, halfway between sunrise and sunset).

Night is the time when the Sun isn't visible because it is below the horizon. The day can also be thought of as extending into the night. For most of us alive in the 21st century, the transition from one day to the next takes place when we sleep (or when we probably should be sleeping). When we wake up, a new day is already underway. For people who follow this convention, the end of one day and the beginning of the next occurs when the Sun it at its lowest position below the horizon called midnight (since it occurs in the middle of the night, halfway between the sunset of one day and the sunrise of the next). This solar midnight marks the boundary of what is called the solar day. In many places, a legally agreed upon time called midnight marks the boundary of what is called the civil day. Solar midnight and the start of a civil day are usually less than an hour apart at most locations.

Even when the Sun isn't visible, we know it's still there. That's because we now know that the Earth is a rotating sphere. Day occurs when we're on the side of the Earth facing the Sun. Noon or midday occurs when the side of the Earth we're on is closest to the Sun for that day. Similarly, night occurs when we're on the side of the Earth facing away from the Sun and midnight occurs when that side is farthest from the Sun for that night. We say the Sun is up in the day and down at night because these are the directions we'd have to head to reach it from a location on the surface of the Earth (or at least the words up and down give the general sense of the vertical component of that direction).

The year

The month

The moonth.

cultural units, part 1: units longer than a day

People be sayin.

multiples of a year

decade, century, millennium, eon

Victorial anniversary list

Quincenera

fractions of a year (multiples of a month)

seasons

semesters, trimesters, quarters

fractions of a month (multiples of a day)

1 fortnight ½(28) = 14 days

1 week ¼(28) = 7 days

Calendars

calendars

fractions of a day

hours, minutes, seconds

Clock Time

What time is it?

solar time

Solar time is the time of day measured using the position of the sun in the sky. Because where you see the sun in the sky depends on where you are on Earth, solar time is hyperlocal. It wouldn't be useful for syncing up activities over distances greater than a person could cover on horseback after a full day of riding. But then if you lived at a time when human and animal power were state of the art technology, how synchronized were activities expected to be?

The Earth is not the world's best timekeeper. If it was we'd be using sundials more often. The Earth rotates about its axis with a reasonably constant angular speed (more on why it's not perfectly constant later), but revolves around the Sun tracing out an elliptical path with a variable angular speed. This gives us solar days that are longest when then Earth is closest to the Sun and shortest when the Earth is farthest from the Sun.

Sundials and regular clocks match on only four days each year: April 15, June 12, September 1, and December 25.

International Atomic Time (TAI)

The SI unit of time is the second [s].

The hyperfine transition is the basis of International Atomic Time (TAI). By definition, the outermost electron in an ordinary cesium 133 atom cycles through this transition 9,192,631,770 times in one second.

International Atomic Time (abbreviated TAI after the French Temps Atomique International) began at midnight GMT on the first day of 1958 and has continued advancing forward at the rate of one second every 9,192,631,770 periods of the hyperfine transition in ¹³³Cs. TAI is maintained by the Bureau International des Poids et Mesures (BIPM) in Paris, which periodically averages the time kept by various atomic clocks around the world. The BIPM then disseminates correction factors needed to synchronize these clocks with the master clock in the Observatoire de Paris.

Universal Time (UT1)

The Earth is not an effective timekeeper. For most of the last two hundred years the mean solar day has been slightly longer than the 86,400 s currently defined by the International System.

Universal Time (UT), or more specifically the variant known as UT1, is in effect the mean solar time. It is continuous (i.e. there are no leap seconds) but has a variable rate because of the Earth's non-uniform rotation period. It is needed for computing sidereal time, which is an essential part of pointing telescopes in the right direction.

Coordinated Universal Time (UTC)

Coordinated Universal Time (abbreviated UTC) is the basis of legal time throughout the world. All local civil times differ from UTC by either a whole number of hours, an odd number of half hours, or (extremely raely) an odd number of quarter hours. but never by any other amount.

The unusual abbreviation UTC was chosen so as not to show an obvious language favoritism. In English, Coordinated Universal Time would be abbreviated CUT, while in French, Temps universel coordonné would be abbreviated TUC. UTC is neither of those.

One second of Coordinated Universal Time (UTC) is the same as one second of International Atomic Time, but UTC and TAI are slightly out of step. TAI marches forward uniformly, while UTC is adjusted from time to time to keep it synchronized with the Earth's rotation as measured by UT1.

When UT1 lags too far behind UTC, a leap second is inserted at the end of the day before January 1 or July 1 as appropriate. When this happens, 23:59:59 is followed by the unusual time of 23:59:60 before turning over to 00:00:00 and starting the next day. In the unlikely event that UT1 were to lead UTC (that is, if the Earth's rate of rotation were to increase) the provision exists for the insertion of a negative leap second. Were this to ever occur, 23:59:58 of one day would be followed by 00:00:00 of the next, skipping 23:59:59 altogether. In any case the absolute difference between UTC and UT1 must never exceed 0.9 s.

The decision on when to insert a leap second is made by the International Earth Rotation Service (IERS) in Frankfurt based on observations of the Earth's orientation in space.

Adding leap seconds to Coordinated Universal Time (UTC) keeps it in line with the rotation of the Earth as measured by Universal Time (UT1).

Coordinated Universal Time, CUT — , TUC

Are leap seconds even necessary?

• In seven or eight centuries the difference between TAI and UT1 will be about an hour.
• By the year 5000, day and night will have reversed; that is, 12 noon will occur in the middle of the night and 12 midnight will synch up with the midday sun.

What would be so wrong with that? Does it really matter what number we assign to a position of the Sun in the sky?

In 1582 Pope Gregory XIII managed to extract ten days from the calendar. On 4 October 1582 the Catholic world went to sleep. When they woke up it was 15 October 1582. By 1752 the protestant nation of England and her American colonies also accepted the change. (They needed to add 11 days to catch up.) In 1873 Japan made the switch. (They needed 12 days.) Then Russia in 1917 and China in 1949 (13 days). The Greek Orthodox Church is possibly the only European agency that has not accepted this change (although the nation of Greece made the switch in 1923).

I think the big thing is that everyone agrees what time (or day) it is. Not that the time is any particular number. Time is a social construct, remember.

cosmological timeline