What is Time ?

Time: From a Dictionary Point of View

Time is the sequential arrangement of all events, or the interval between two events in such a sequence.

Time is a nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.

Time is represented by a number, as in years, days, minutes, nanoseconds, etc.

Time is marked by a series of similar events, conditions and other phenomena that occur at regular intervals.

Time refers to a period when an activity occurs. The activity can be at regular intervals, such as harvest time, or arbitary, such as a time for a wedding, a time to go to sleep or a time to play golf.

Time can be used in the plural refering to an unspecified group of events occuring within a time frame, such as hard times or lonely times.

Time refers to the present with respect to prevailing conditions and trends as in, You must change with the times.

Time is a suitable or opportune moment or season as in a time for reflection.

Time is a period at one's disposal; something to be given as in, Do you have time for a chat?

Time can be an appointed or fated moment, such as He died before his time, or, Her time is near at hand, or, The time to do it is now.

Time refers to instances, recurring or random: He knocked several times. How many times have I told you? It was the last time I saw her.

Time is used to indicate the number of instances by which something is multiplied or divided: This tree is three times taller than the one next to it. The local library is many times smaller than the University library.

Time refers to ones life as in It happens only once in a lifetime.

Time is a persons experience during a specific period or on a certain occasion: He had a good time at the party.

Time marks a period of engagement, activity or pursuit as in a period of military service, a period of internship, school years, a prison sentence, or how long someone has been on the job.

Time designates periods of work as in full time, part time, a time to work and a time to play, getting paid double time on weekends.

Series from TV shows occur at the same time on the same day of each week on a regularly scheduled basis.

Time is the rate of speed of a measured activity such as marching in double time.

In music, keeping time means maintaining a rhythmic pattern occuring at regular intervals at a given tempo (speed). A song in 4/4 time means a there are four beats to a measure and each note is of equal duration. An 8th note is half of a note, a note is half of a note, and a note is half of a whole note.

In dance, choreography is timed to music. In poetry, metered verses are based on word accents and rhyme schemes. In film, movies run on the basis of the number of frames per second.

In sports, time marks the periods that occur during a game. A time-out is when the clock stops ticking. Half time is when the game is half over.

In astronomy, distances of stars are calculated in terms of light-years. Clocks and calanders are based on naturally occuring phenomena, like days and nights, seasonal changes, earth rotation and other celestial orbits. In the 20th century, atomic clocks are used to determine everyday use of time. In biology, there are biorhythms, circadian rhythms, migration patterns and other natural phenomena that occur in regular intervals.

Machines are calibrated by seconds, microseconds and nanoseconds.

Popular Phrases in Use of Time (English-based)

The passage of time.
Time waits for no man.
As time marches on.
As time goes by.
The time of our lives.
Time is of the essence.
Its all about timing.
How much time do you have?
What time is it?
Is it time yet?
How long will it take?
Hurry up! Were running out of time!
In due time.
If youre going to do the crime, be prepared to do the time.
Im waiting.
I cant wait. Youve got all the time in the world.
Not this time.
Next time.
The last time.
How many times do I have to tell you?
Youre out of time.
Take the Time.
When will I see you again?
Its all in the past.
The time is now.
Forget the past.
He timed it just right.
Wasted time.
It will only take a moment.
It took a very long time.
It wont take long at all.
In a New York Minute.
Time Management.
Good times and bad times.
Since the beginning of time.
Time goes on forever.
Until the end of time.

Ancient Observations of Time

Since ancient times, Mother Nature easily revealed time in naturally occuring events from the ceaseless rising and setting of the sun to the changes from summer to winter to the changing phases of the moon. From these natural observations humans were able to mark regularly occuring intervals in terms of years, months and days. It was simple mathwith the help of primitive clocksthat further divided the day into hours, minutes and seconds.

Through the centuries, discoveries and observations in astronomy led to more accurate measurement of counting cycles and fractions of cycles of regularly occuring events. Precise measurement of time is possible based on the earth rotating on its axis and around the sun at a fairly constant rate in juxtaposition to other celestial bodies. In the 21st century precise measurement is possible through advanced technology, such as radio telescopes (stationary and mobile), satellites, computers, atomic clocks and other devices.

Sidereal and Solar

A day is the period of time it takes for the earth to rotate once on its axis. The time it takes for the earth to rotate once in juxtaposition to fixed stars is called the sidereal day. All sidereal days are equal. The time it takes for the earth to rotate around the sun (from high noon to high noon) is the solar day.

Because of the earths motion in its orbit around the sun, the sun appears to move eastward against the fixed stars, and the earth must make slightly more than one complete rotation to bring the sun back to the observers meridian. The meridian is the great circle on the celestial sphere running through the north celestial pole and the observers zenith. The passage of the sun across the meridian marks high noon.

The earths orbital motion is not uniform, and the plane of the orbit is inclined to the celestial equator by a certain number of degrees. Because of the tilt of the earths axis of rotation, the times of sunrise and sunset vary from day to day. In the Northern Hemisphere there are long days and short nights in the summer and short days and long nights in the winter. So, the eastward motion of the sun against the stars is not uniform and the length of the true solar day varies seasonally. However, the solar day is on average four minutes longer than the sidereal day.

True solar time does not move at a constant rate. Consequently, the mean solar day has a length equal to the annual average of the actual solar day, forming the basis for mean solar time. Mean solar time is also not uniform, affected by tidal, weather and astronomical changes.

Solar Time

Solar time is defined by the position of the sun. The solar day is the time it takes for the sun to return to the same meridian in the sky. When the center of the sun is on an observers meridian, the observers local solar time is zero hours (noon).

Because the earth moves with varying speed in its orbit at different times of the year and because the plane of the earths equator is inclined to its orbital plane, the length of the solar day is different depending on the time of year.

Averaging out these differences in shifting angles and speed variations determines mean solar time. Mean solar time is the earths orbit measured relative to an imaginary sun (the mean sun) that lies in the earths equatorial plane. In mean solar time the earth orbits at a constant speed so each mean solar day is the same duration. The difference between the local solar time and the mean solar time at a given location is known as the equation of time.


A sundial measures time by the position of the sun. It is an instrument of ancient origin where an object called a gnomon, usually a carved stone, pin or metal plate, casts a shadow on a surface and as the sun moves the shadows are marked to show hours or fractions of hours. Some sundials were relatively small while others, such as pryamids and obelisks used in Egypt, were quite large.

Egypt, around the 15th century B.C., is credited with the early development of the sundial.

Around the 1st century A.D. the sundial was improved by setting the gnomon parallel to the earths axis of rotation for a more accurate measurement of the suns east to west motion.

Sundials were used to set clocks and watches well into the 18th century. The heliochronometer, using a fine wire as the gnomon, was later used in the 19th century.

Some parts of the world still use sundials but are now largely considered antiquated especially in relation to 21st century atomic clocks, radio telescopes and other advanced technology.

The largest sundial in the world was constructed around 1724 in Jaipur, India. It covers close to an acre. A number of museums and planetariums feature collections of sundials.


Equinox (pronounced kwinoks) is either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. The celestial sphere is an imaginary sphere of infinite radius surrounding the Earth where the sun, planets and stars are positioned in a 3-dimensional map of the universe (a Celestial Coordinate System).

It is used for describing the positions and motions of stars and other objects. For these purposes, any astronomical object can be thought of as being located at the point where the line of sight from the earth through the object intersects the surface of the celestial sphere. In astronomical coordinate systems, the coordinate axes are great circles on the celestial sphere. In most systems of this type, the reference points are fixed on the sphere, so the two coordinates needed to locate a body are relatively constant.

The vernal equinox is the point at which the sun appears to cross the celestial equator from south to north. This occurs around Mar. 21, the beginning of spring in the Northern Hemisphere. The autumnal equinox is the point at which the sun appears to cross the celestial equator north to south, marking the beginning of autumn, around Sept. 23.

On the date of either equinox, night and day are of equal length (12 hours each) in all parts of the world. The equinoxes are not fixed points on the celestial sphere but move westward along the ecliptic, passing through all the constellations of the zodiac in 26,000 years. This motion is called the precession of the equinoxes.

The vernal equinox is a reference point in the equatorial coordinate system. The Equatorial Coordinate System is the most commonly used astronomical coordinate system for indicating the positions of stars or other celestial objects on the celestial sphere.

To designate the position of a star, the astronomer considers an imaginary great circle passing through the celestial poles and through the star in question. This is the stars hour circle, analogous to a meridian of longitude on earth. The astronomer then measures the angle between the vernal equinox and the point where the hour circle intersects the celestial equator. This angle is called the stars right ascension and is measured in hours, minutes, and seconds rather than in the more familiar degrees, minutes, and seconds (There are 360 degrees or 24 hours in a full circle). The right ascension is always measured eastward from the vernal equinox.

Next the observer measures along the stars hour circle the angle between the celestial equator and the position of the star. This angle is called the declination of the star and is measured in degrees, minutes, and seconds north or south of the celestial equator, analogous to latitude on the earth.

Right ascension and declination together determine the location of a star on the celestial sphere. The ascensions and declinations of stars are listed in various reference tables published for astronomers and navigators. Because a stars position may change, tables are revised regularly.

Another reference point is the sigma point, where the observers celestial meridian intersects the celestial equator. The right ascension of the sigma point is equal to the observers local sidereal time. The angular distance from the sigma point to a stars hour circle is called its hour angle. It is equal to the stars right ascension minus the local sidereal time. Because the vernal equinox is not always visible in the night sky (especially in the spring), whereas the sigma point is always visible, the hour angle is used in actually locating a body in the sky.

Time Is Relative

Albert Einstein demonstrated in his theory of relativity, that when two observers are in relative motion, they will arrange events in a somewhat different time sequence. Consequently, events that are simultaneous in one observers time sequence will not be simultaneous in another observers sequence. In the theory of relativity, space and time are intertwined and inseparable aspects of a four-dimensional universe, now referred to as space-time. Time is no longer treated as an independent entity.

Not yet fully understood, in accordance with the theory of relativity, events appear to take place at a slower rate to an observer in a moving system compared to an observer in a stationary system. A moving clock will appear to run slower than a stationary clock of identical construction. This effect is called time dilation.

Time dilation has been confirmed by observing the decay of rapidly moving subatomic particles that spontaneously decay into other particles. Particles in motion decay more slowly than stationary particles.

Another area or theory relating physics and time is Time Reversal Invariance. Understanding this concept is complex and requires a strong understanding of physics. In the simplest of terms, Time Reversal Invariance is a quest to determine the forward direction of time by exploring the behavior of sub-atomic particles. The question is separate from the subjective view we have of moving from the past to the future.

According to classical physics, if all particles in a simple system are instantaneously reversed in their velocities, the system will proceed to retrace its entire past history. This property of the laws of classical physics is called time reversal invariance. It means that when all microscopic motions of individual particles are precisely defined, there is no fundamental distinction between forward and backward in time.

Biological Time

Animals and plants exhibit circadian (approximately 24 hours) cycles in temperature and metabolic rate. Circadian rhythms include such phenomena as the opening and closing of flowers, change in blood pressure and urine production. Migration patterns are fairly constant for many animals and birds. It remains unknown if other living organisms besides humans have a sense of time. Biological rhythm is a cyclic pattern of physiological changes or changes in activity in living organisms, most often synchronized with daily, monthly, or annual cyclical changes in the environment or is possibly genetic-based.

Diurnal animals are active during the day and nocturnal animals are active during the night. Marine organisms are affected by tide cycles. Monthly rhythms include weight changes in men and the menstrual period in women.

Annual cycles, or circannual rhythms, include bird migrations, reproductive activity, and mammalian hibernation. Daily cycles, or circadian rhythms, are in part a response to daylight or dark, and annual cycles in part responses to changes in the relative length of periods of daylight. Changes in the environment such as temperature or availability of food can determine and affect cyclical changes.

Research continues in determining if our sense of time is related to electrical rhythms in the brain. Alpha rhythms or waves are a pattern of smooth and regular electrical oscillations recorded by an electroencephalograph that occurs in the human brain when a person is awake and relaxed. Other physiological rhythms are apparent, as in the beating of a heart.

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