Time Travel

Scientific Explanations

Is time travel possible? “Maybe someone will come back from the future and tell us the answers.” — Stephen Hawking from the Preface, Black Holes and Time Warps: Einstein’s Outrageous Legacy, by Kip S. Thorne.

“One reason that time travel is so fascinating is that we have such a great desire to do it.” — J. Richard Gott, in his book, Time Travel in Einstein’s Universe: The Physical Possibilities of Travel Through Time.

Also from J. Richard Gott, “Einstein proved we can travel forward by moving near light speed. Backward requires a wormhole, cosmic string and a lot of luck.”

Scientists believe they may one day be able to travel through time by looking more closely at the dust swirling with a comet as it hurdles through our galaxy. — Goddard Space Flight Center

Accordingly, whether its Einstein, Hawkings, Gott, Thorne or a host of other top scientific minds, the word is that nothing in Newtonian physics, Einstein’s Theory of Relativity, or the laws of quantum mechanics is sufficient to prove time travel is impossible.

Currently, we can travel through the three dimensions of space pretty much at will moving forward or back, left or right, up or down. But when it comes to the 4th dimension of time were stuck.

The speculations that follow are purposefully random and non-linear. This is designed to promote the argument between linear and non-linear views of time and space where things occur either all at once or time marches on in one direction towards the future.

The Time Tunnel - A popular 1960's Science Fiction TV Series on Time Travel

The Time Tunnel – A popular 1960′s Science Fiction TV Series on Time Travel

The possibility of time travel lies somewhere between phenomenon at cosmological scales and those at the quantum, subatomic level, while still conforming to the classical mechanics of motion, electromagnetism and relativity. Or, maybe it doesn’t. Traveling faster than the speed of light? Warping the time-space continuum? Parallel space-time continuums? Quarks, quasars, gamma ray bursts, positrons, black holes and wormholes? Is the universe expanding, contracting, infinite, warped? Inertia? Dark matter?

Some physicists allegedly have demonstrated that time travel is already taking place at the subatomic level, based on such theories at Time Reversal Invariance, String Theory, time dilation experiments and other theories.

It was in 1905 that Einstein introduced the Theory of Special Relativity, dramatically changing our understanding of time, space, and motion. Interestingly, H.G. Wells published his novel, The Time Machine, only 10 years before.

Special Relativity is based on two ideas. The first idea is that the laws of physics are the same for everyone as long as they are moving at a constant velocity. Einstein showed back in 1905 with his special theory of relativity that time slows down for objects moving close to the speed of light, that is, from the viewpoint of a stationary observer.

The second idea is that the speed of light is constant for everyone, whether its measured on earth or in a spaceship traveling at high speeds. In 1915 Einstein published his Theory of General Relativity, which permitted calculations based on changing speed.

The Theory of Special Relativity has undergone rigorous testing since Einstein first introduced it, so much so, that its no longer viewed as a theory.

Physics experiments, particularly in particle accelerators, continue to explore such areas as time dilation and time reversal invariance. Einstein concluded it would be impossible for a spaceship to reach the speed of light. However, scientists have used particle accelerators to make an electron attain 0.999 999 999 95% of light speed. Experiments continue to explore changing mass size of particles as they approach the speed of light. As mass size increases, the energy required to move mass also increases.

Physicists at the CPLEAR experiment have measured directly for the first time that, for the particles called kaons, time is different when it moves forwards or backwards. An international team is working on the experiment known as CPLEAR, at CERN, the European Laboratory for Particle Physics, in Geneva.

If fundamental particles can no longer be broken down into smaller particles, have we reached the end or the beginning, to be more exact? Does wave theory allow for more theoretical possibilities? Whats the difference between particles and waves and what bearing does such a difference have on time travel?

Some theories particularly of the unified field theory variety predict that the speed of light can change in time. Other theories predict particles can travel faster than the speed of light.

A positron (the antiparticle associated with the electron) can be considered to be an electron going backward in time. In an electron-positron pair where the positron is annihilated in a collision with another, different electron, a zigzag, N-shaped path takes shape: forward in time as an electron, then backward in time as a positron, then forward in time again as an electron.

The search for gravitons the particles of gravity holds promise in understanding the effects of gravity on time. Can time affect gravity? Is there a particle of time?

There seems to be a number of forces and/or factors that can warp time and space, from electromagnetism to black holes, from the shape of the universe to subatomic collisions at high energies, from crystalline forms of radiation to cosmic rays, from gravity to mass to velocity.

Some theorists claim travel into the past is not possible while others claim there are places where time doesn’t exist. Philosophizing about such theories is mind boggling enough yet alone fooling around with the mathematical equations that will ultimately prove or disprove one theory over another.

If time travel were possible, then where are the time travelers from the future? The latter question is similar to Fermis Paradox, which explores the notion that extraterrestrial life exists, but if so, then where are these so-called aliens?

Other influences on the possibility of time travel include chaos theory, quantum theory, string theory, M-Theory, theory of relativity, electromagnetic theory, gravity, space-time and even parallel universes.

How many worlds are there in the Many-Worlds theory?

Is what we imagine possible?

What does The Arrow of Time mean? Does time only travel one way into the future?

According to one interpretation of quantum theory (and there are many), each parallel world in a Parallel Universes theory is just as real as our own, with an alternative history for every possible outcome of every decision ever made.

The granny paradox or grandfather paradox, whichever gender frame of reference you choose, asks the question, what would happen if you went back into time and killed your grandmother, who gave birth to your mother, who gave birth to you? The rule seems to be you cant travel back or forward in the same universe you can’t undo what’s already been done. In turn, this means you cant undo what is going to be done, which means the future already exists.

By a process called “quantum tunneling,” one electron can disappear in one universe and appear in another. Can the same thing happen for things larger than an electron? What about the electrons in our own bodies? Does such tunneling occur externally or internally, such as in altered states of mind? If ghosts exist, then in what dimension do they exist? Is the body a time machine?

Wormholes seem to offer many possibilities for time travel. A wormhole is a thin tube of space-time connecting distant regions of the universe. Wormholes might also link to parallel or baby universes, providing the possibility of time travel. Wormholes are related to black holes, an effect allegedly created by sources of intense gravity where the fabric of space itself is bent, folded or distorted in various ways creating a short path between otherwise remote areas of space.

The wormhole was a fictional device used by Carl Sagan in his novel, Contact (1985). Kip S. Thorne and others at the California Institute of Technology set out to find whether wormholes were consistent with known physics. They speculated that a wormhole would resemble a black hole an object in space with tremendous gravity. Black holes are considered one-way journeys to nowhere. A wormhole has an exit and an entrance. Other scientists suggest a wormhole is a connection between two black holes.

And then theres anti-gravity.

Some scientists speculate a way to travel a 1000 years into the future is to travel to a star 500 light-years away and return, going both ways at 99.995% the speed of light. When returning, the earth will be 1,000 years older but the traveler will have aged only 10 years. Likewise, if an astronaut was sent to the planet Mercury and lived there for 30 years before returning, the astronaut would be about 22 seconds younger than if the astronaut had stayed on earth.

Clocks on Mercury tick more slowly than those on Earth because Mercury circles the sun at a faster speed. Gravity allegedly affects clocks much like velocity does and Mercury falls into a different gravitational field in relation to the sun than the earth does.

Cosmic strings, thin strands of energy millions of light-years long, offer other possibilities in terms of time travel.

Somewhere between quantum and chaos

Are schizophrenics receiving information from other worlds or perhaps time travelers? Could the voices prophets hear in their heads be the voices of time travelers?

What would happen if suddenly all our clocks stopped? Time wouldn’t stop, since time is a natural phenomenon. But clocks are man-made and subject to the forces of nature, like gravity, electromagnetism, space-time curvatures and other natural and cosmological forces. If clocks did stop, the world as we know it would be plunged into chaos.

Who knows, maybe time travel is a simple as a walk in the park. All it takes is a savvy 5th grader with a toy particle accelerator capable of producing wormholes.

Atomic Clocks

The oldest clock is the Earth. We know its morning when the Sun rises, noon when the Sun is overhead, and evening when the Sun sets. The Earths accuracy as a clock is about one thousandth of a second per day. Man-made atomic clocks, by measuring the resonant frequency of a given atom (currently Cesium, Hydrogen or Mercury), are accurate to more than a billionth of a second per day.

Time measured by the rotation of the Earth is not uniform when compared to the time kept by atomic clocks. Such irregularities in the earths rotation are determined by scientists using radio telescopes to observe quasars, the most distant objects in the universe.

In 1972, by international agreement, it was decided to let atomic clocks run independently of the Earth and then coordinate the two. To keep the difference between Earth time and atomic time within nine tenths of a second, as the two times get out of sync, leap seconds are added to the atomic time scale.

The International Earth Rotation Service is the organization that monitors the difference between the two time scales and calls for leap seconds to be inserted when necessary. Leap seconds are added because the Earths rotation tends to slow down. If the Earth were to speed up, a leap second would be removed.

The U. S. Naval Observatory provides the Master Clock for the Department of Defense and the entire nation. Modern electronic systems, such as electronic navigation or communication systems depend increasingly on precise time and time interval (PTTI). Examples are the ground-based LORAN-C navigation system and the satellite based Global Positioning System (GPS).

These systems are all based on the travel time of the electromagnetic signals: an accuracy of 10 nanoseconds (ten one-billionths of a second) corresponding to a positional accuracy of about 10 feet. In fast communications, time synchronization is equally important. All of these systems are referenced to the U. S. Naval Observatory Master Clock.

The present Master Clock is based on a system of 59 atomic clocks: 10 hydrogen masers and 49 HP-5071 cesiums. These clocks are distributed over 12 environmentally controlled clock vaults, to insure their stability.

The U. S. Naval Observatory is the largest single contributor to the international time scale (UTC), which is computed in Paris, France, at the International Bureau of Weights and Measures.

Events, like astronomical and weather phenomena, are often given in Universal Time (UT), sometimes colloquially referred to as Greenwich Mean Time (GMT). In 1884, under international agreement, the prime meridian was established as running through the Royal Observatory in Greenwich, England, setting the standard of Greenwich Mean Time (GMT).

The two termsUT and GMTare often used loosely to refer to time kept on the Greenwich meridian (longitude zero), five hours ahead of Eastern Standard Time. In keeping with tradition, the start of a solar day occurred at noon. In 1925 the numbering system for GMT was changed so that the day began at midnight to make it consistent with the civil day. Some confusion in terminology resulted, however, and in 1928 the International Astronomical Union (IAU) changed the designation of the standard time of the prime meridian to universal time. Greenwich Mean Time is a widely used historical term, but one that has been used in several ways. Because of ambiguity, it is no longer used in technical contexts.

Times given in UT are almost always given in terms of a 24-hour clock. Thus, 14:42 (often written simply 1442) is 2:42 p.m., and 21:17 (2117) is 9:17 p.m. Sometimes a Z is appended to a time to indicate UT, as in 0935Z.

In 1955 the IAU defined several kinds of UT. The initial values of universal time obtained at 75 observatories, denoted UT0, differ slightly because of polar motion. By adding a correction each observatory converts UT0 into UT1, which gives the Earths rotational position in space. An empirical correction to take account of annual changes in the speed of rotation is then added to convert UT1 to UT2. However, UT2 has since been superseded by atomic time Universal time is also called world time, Z time, and Zulu time.

In the most common civil usage, UT refers to a time scale called Coordinated Universal Time (UTC). UTC is the basis for the worldwide system of civil time and is determined by atomic clocks. The International Bureau of Weights and Measures makes use of data from the clocks to provide the international standard UTCaccurate to nearly one nanosecond (billionth of a second) per day. The length of a UTC second is defined in terms of an atomic transition of the element cesium under specific conditions, and is not directly related to any astronomical phenomena.

UTC is the time distributed by standard radio stations that broadcast time, such as WWV and WWVH. It is also obtained from the Global Positioning System (GPS) satellites. The difference between UTC and UT1 is made available electronically and broadcast so that navigators can obtain UT1.

Standard time within U.S. time zones is established by a certain number of hours offset from UTC. Since a day is 24 hours long, the world may be split into 15-degree wide longitudinal bands (360 degrees/24 hours). Each band represents one hour. As an example, Huntsville, Alabama is located at approximately 90 degrees west longitude; hence, local time lags UTC time by 6 hours (90/15, assuming Central Standard Time, 5 hours in Central Daylight Time). So, if the universal time is 14:30 UTC, United States Central Standard Time would be 8:30 am CST.

What Is An Atomic Clock?
An atomic clock is an electronic timekeeping device controlled by atomic or molecular oscillations. A timekeeping device must contain or be connected to some apparatus that oscillates at a uniform rate to control the rate of movement of its hands or the rate of change of its digits. Mechanical clocks and watches use oscillating balance wheels, pendulums, and tuning forks.

Because the frequency of such oscillations is so high, it is not possible to use them as a direct means of controlling a clock. Instead, a highly stable crystal oscillator whose output is automatically multiplied and compared with the frequency of the atomic system controls the clock. Errors in the oscillator frequency are then automatically corrected. Time is usually displayed by an atomic clock with digital or other sophisticated readout devices.

The first atomic clock, invented in 1948, utilized the vibrations of ammonia molecules. The error rate was typically about one second in three thousand years. In 1955 the first cesium-beam clock was placed in operation at the National Physical Laboratory at Teddington, England. The cesium-beam clock is the most accurate standard of atomic time currently in use; it is estimated that such a clock would gain or lose less than a second in three million years.

Many of the worlds nations maintain cesium clocks at standards laboratories, the time kept by these clocks being averaged to produce a standard called international atomic time (TAI).

Highly accurate time signals from these standards laboratories are broadcast around the globe by shortwave-radio broadcast stations or by artificial satellites, the signals being used for such things as tracking space vehicles, electronic navigation systems, and studying the motions of the earths crust.

As of January, 2002, NIST’s latest primary cesium standard was capable of keeping time to about 30 billionths of a second per year. Called NIST-F1, it is the 8th of a series of cesium clocks built by NIST and NIST’s first to operate on the “fountain” principle.

Prototypes of atomic clocks using other kinds of atoms, such as hydrogen or beryllium, could be thousands of times more accurate even than todays cesium clocks. Current atomic clocks using the hydrogen atom maser can attain an error rate of about one part in 2 quadrillion. Such an extremely low error rate allowed their use in an experiment confirming an important prediction of Einsteins theory of relativity.

National Institute of Standards and Technology, the U.S. Naval Observatory, and the International Bureau of Weights and Measures in Paris assist the world in maintaining a single, uniform time system.

NIST Time and Frequency Services
NOTE: The following is reprinted from the NIST Time and Frequency Division Web site at: http://tf.nist.gov

Since 1923, NIST radio station WWV has provided round-the-clock shortwave broadcasts of time and frequency signals. WWV’s audio signal is also offered by telephone: dial (303) 499-7111 (not toll-free). A sister station, WWVH, was established in 1948 in Hawaii, and its signal can be heard by dialing (808) 335-4363 in Hawaii.

Broadcast frequencies are 2.5 MHz (megahertz), 5 MHz, 10 MHz, and 15 MHz for both stations, plus 20 MHz on WWV. The signal includes UTC time in both voice and coded form; standard carrier frequencies, time intervals and audio tones; information about Atlantic or Pacific storms; geophysical alert data related to radio propagation conditions; and other public service announcements. Accuracies of one millisecond (one thousandth of a second) can be obtained from these broadcasts if one corrects for the distance from the stations (near Ft. Collins, Colorado, and Kauai, Hawaii) to the receiver. The telephone services provide time signals accurate to 30 milliseconds or better, which is the maximum delay in cross-country telephone lines.

In 1956, low-frequency station WWVB, which offers greater accuracy than WWV or WWVH, began broadcasting at 60 kilohertz. The broadcast power for WWVB was increased in 1999 from about 10 kilowatts to 50 kilowatts, providing much improved signal strength and coverage to most of the North American continent. This has stimulated commercial development of a wide range of inexpensive radio-controlled clocks and watches for general consumer use.

Time signals are an important byproduct of the Global Positioning System (GPS), and indeed this has become the premier satellite source for time signals. The time scale operated by the USNO serves as reference for GPS, but it is important to note that the time scales of NIST and USNO are highly coordinated (that is, synchronized to well within 100 nanoseconds, or 100 billionths of a second). Thus, signals provided by either NIST or USNO can be considered as traceable to both institutions. The agreements and coordination of time between these two institutions are important to the country, since they simplify the process of achieving legal traceability when regulations require it.

Official U.S. Government time, as provided by NIST and USNO, is available on the Internet at http://www.time.gov. NIST also offers an Internet Time Service (ITS) and an Automated Computer Time Service (ACTS) that allow setting of computer and other clocks through the Internet or over standard commercial telephone lines. Free software for using these services on several types of popular computers can be downloaded there.


Approximately 5000 to 6000 years ago great civilizations in the Middle East and North Africa began to make clocks in addition to calendars to regulate time more efficiently. No doubt before the dawn of civilization, primitive humans had some need for breaking up the day and night into more manageable increments.

Sun Clocks
As early as 3500 BCE, the Egyptians used obelisks (4-sided monuments) to divide the day into smaller parts. Obelisks were like sundials, allowing the day to be divided into morning and afternoon by watching shadows. They also showed the longest and shortest days of the year when the shadow at noon was the shortest or longest of the year.

Egyptian sundials (shadow clocks) came into use around 1500 BCE. The sundial divided the day into 10 parts plus two “twilight hours” in the morning and evening. Shadows were cast based on the Suns movement East to West across the sky. The merkhet, the oldest known astronomical tool and introduced around 600 BCE, was used to to establish a north-south line (or meridian) by aligning them with the Pole Star (North Star). Nighttime hours were marked when certain other stars crossed the meridian. By 30 BCE, sundials became quite sophisticated.

Clock Components
All clocks have two basic components:

1) A regular, constant or repetitive process or action to mark off equal increments of time. Examples in early times included the suns movement across the sky, candles marked in increments, oil lamps with marked reservoirs, sand glasses (hourglasses), and in the Orient, knotted cords and small stone or metal mazes filled with incense that would burn at a certain pace. Modern clocks use a balance wheel, pendulum, vibrating crystal, or electromagnetic waves associated with the internal workings of atoms as regulators.

2) A means of keeping track of the increments of time and displaying the result. Modern clocks track the passage of time by the position of clock hands and digital time displays.

Water Clocks
Water clocks did not depend on the observation of celestial bodies. One of the oldest was found in the tomb of the Egyptian pharaoh Amenhotep I, buried around 1500 BCE. The Greeks started using them around 325 BCE and called them clepsydras. They were stone vessels with sloping sides that allowed water to drip at a fairly constant rate from a small hole at the bottom of the vessel. Other clepsydras slowly filled with water and markings on the inside surfaces marked the passage of time.

More elaborate and mechanized water clocks were developed between 100 BCE and 500 CE by Greek and Roman horologists and astronomers. The flow of water became more constant by regulating the pressure. Some water clocks rang bells while others opened doors and windows to show little figures of people, or moved pointers, and dials.

Mechanical Clocks
It wasnt until the 14th century in Italy when mechanical clocks came into being. The clocks were weight-driven and regulated by a verge-and-foliot escapement. These kinds of clocks lasted for more than 300 years. Like water clocks before them, regulating the rate with accuracy was difficult because oscillations of the escapement depended heavily on the amount of driving force and the amount of friction in the drive.

Peter Henlein of Nuremberg introduced spring-powered clocks shortly after 1500. Spring-powered clocks were less heavy than mechanical clocks and allowed for mobility. However, the clocks ran slower as the mainspring unwound.

In 1656, Dutch scientist Christiaan Huygens made the first pendulum clock, regulated by a mechanism with a “natural” period of oscillation. Galileo was studying pendulum motion back in the late 1500s. Huygens’ earliest pendulum clock had an error of less than 1 minute a day, which eventually he reduced to an error rate of less than 10 seconds a day. Around 1675 he developed the balance wheel and spring assembly, still found in some of today’s wristwatches. In London in 1671, William Clement built clocks with a new “anchor” or “recoil” escapement, a substantial improvement over the verge because it interferes less with the motion of the pendulum.

In 1721, George Graham improved the pendulum clock’s accuracy to 1 second per day by compensating for variations in temperature. Later, John Harrison, refined Graham’s temperature compensation techniques and developed new methods for reducing friction. By 1761 Harrison built a marine chronometer with a spring and balance wheel escapement. It kept time on board a rolling ship to about 1/5 of a second a day, approaching the accuracy of pendulum clocks on land.

Around 1889, Siegmund Riefler’s pendulum clock achieved an accuracy of a 1/100th of a second a day and became the standard in many astronomical observatories. R.J. Rudd and W.H. Shortt refined pendulum clocks later. In 1921, Shortts clock replaced Riefler’s clock in many observatories. Shortts clock had two pendulums, one a slave and the other a master. The slave pendulum pushed the motion of the master clock and drove the clocks hands. The master pendulum was then free from mechanical disturbances.

Quartz Clocks
In the 1920s, quartz crystal oscillators and clocks vastly improved on accuracy beyond pendulum and balance-wheel escapements. Quartz clock operation is based on the piezoelectric property of quartz crystals. In an electronic circuit, the interaction between mechanical stress and electric field causes the crystal to vibrate and generate an electric signal of relatively constant frequency. Quartz crystal clocks had no gears or escapements to disturb their regular frequency. However, quartz clocks still relied on mechanical vibrations with frequencies dependent on a crystals size, shape and temperature. No two crystals are exactly alike. Quartz clocks dominate the current market largely because of price.

Atomic clocks are next.


A calendar is a system of reckoning time for the purpose of recording past events and planning for the future. A calendar is used to indicate a series of scheduled events such as court cases, bills before congress, appointments, holidays, etc.

In the new millennium, we take the calendar for granted. There are 12 months in a year and each year is numbered. For practical purposes, thats all we need to know. Our lives march from birth to death in a linear fashion, with both dates marking the beginning and ending of our existence. In between are the regularly occuring birthdays, anniversaries, holidays and other arbitrary events of our lives.

Weve mapped out our existance on the calendar of time. Every aspect of our lives has a mark on the calendar. We served in a war between 1944 and 1946. Usually we know the month. The most memorable events of our lives we remember down to the day and time of day. Our work histories are marked by start and end dates, interpolated with the years we went to college.

Just how in sync our made made calendars are with nature doesnt really seem to be much of a concern. Seasonal changes have little bearing on industry and business. Auto manufacturers pump out just as many cars in the winter as they do in the summer. Certain foods might be seasonal, but import/export is the way around that delimma. Factory farming, along with genetic engineering in the future, is continous throughout the year.

Indoor stadiums allow for sports events to take place without worry of rain or snow. And the fall season is more likely to be important in terms of TV programming schedules than when foods are ready for market.

Calendars got their start through ancient observations of the Sun, Moon, stars and planets. The earths rotation, orbit around the sun, phases of the moon and other celestial observations were eventually calculated in terms of season, year, month and day cycles. It took 365 cycles of day and nightgive or take an hour or secondfor the earth to orbit the sun (called the Solar year). A day is the time it takes for the earth to rotate once on its axis. A Lunar year is the total of the 12 times the moon passes through its phases (a Lunar month).

The sundialand eventually clocksfurther divided the day into hours, minutes and seconds. Now, with atomic clocks, the cycles of nature are calculated in terms of billionths and trillionths of nanoseconds (a nanosecond being one billioneth of a second).

Today, a worldwide resource for fundamental astronomical data is the Astronomical Almanac, a joint publication of the U.S Nautical Almanac Office and Her Majesty’s Nautical Office in the UK. It contains technical and general astronomical data supplied by many scientists from around the world. Information includes: seasons, phases of the Moon, planet configuration, eclipses, time-scales and Coordinate systems, universal and sidereal times, detailed earth, sun, moon and other celestial sphere rotation and orbital positions, stars and steller systems, x-ray and gamma ray sources, quasars, pulsars, observatories, instrumentation, astronomical tables and data, and much more.

Exploring the history of ancient and modern calendars can be confusing and convoluted. How did we arrive at 365 days in a yearor is it 364 or 366give or take an hour or second? Why do some months have 31 days and some have 28? Why do we need a leap year? How do we compensate for the differences between the Solar Year and Lunar Year?

There are six principal calendars in current use: Gregorian, Hebrew, Islamic, Indian, Chinese, and Julian Calendars. Religion and politics have played important roles in determing how calendars came to be, regardless of astronomical obervations. Adjustments were made for sacred dates and extending the length of official times for rulers.

Mathematically, the fact that months and years cannot be divided exactly by days and that the years cannot be easily divided into months led to intercalation: the insertion of extra days or months into a calendar to make it more accurate. This meant making some months shorter and other months longer.

Solar years and Lunar years dont quite match up. Seasons and astronomical events do not repeat at an exact number of days, so a calendar that had the same number of days in each year would over time drift with respect to astronomical changes. A leap year (or intercalary year) is a year containing an extra day or month in order to keep the calendar year in sync with an astronomical or seasonal year.

The history of Egyptian, Roman, Greek, Chinese, Jewish, Mayan and other calendars is really a journey in how each calendar compensated for astronomical changes. Give or take a few leap seconds or leap years, its unlikely the astronomical changes that gave birth to calendars will change much in far into the future. It would take a castrophic event to knock the earth off its axis or change the speed of orbit around the sun. Are days getting longer and nights getting shorter? Will a meteor strike? Will continents divide and new oceans form? Will these changes have anything to do with how we get from one day to another?

Roman Calendar
The earliest versions of the Roman calendar had 10 months (using Roman names): March (31 days), April (29 days), May (31 days), June (29 days), Quintilis (31 days), Sextilis (29 days), September (29 days), October (31 days), November (29 days), and December (29 days). The Romans used intercalary days and the occassional month to maintain 365 days a year.

Julian Calendar
Prior to Julius Caesar, the Romans had so abused the calendar that January started in autumn. Julius Caesar, advised by astronomer Sosigenes, added 90 days to the year 46 B.C. (67 days between November and December, 23 at the end of February). This caused the spring of 45 B.C. to begin in March. He changed the length of other months as well, added a few days to other months, and changed Sextilis to July, named after himself.

In the Roman calendar three days in the month were used for counting the date. These three were the Kalends (1st day of the month), the Nones (the 7th day in March, May, July, and October, the 5th in the other months), and the Ides (the 15th day in March, May, July, and October, the 13th in the other months).

Gregorian Calendar
The Julian year is 365 days and 6 hoursa little too long. By the 16th century the accumulation of surplus time had displaced the vernal equinox to Mar. 11 from Mar. 21, the date set in the 4th century. To compensate, Pope Gregory XIII suppressed 10 days in the year 1582 and ordained the years ending in hundreds should not be leap years unless they were divisible by 400.

The year 1600 was a leap year under both systems, but 1700, 1800, and 1900 were leap years only in the unreformed calendar. The reform was gradually accepted throughout most Roman Catholic and Protestant countries, and in the Eastern Church the Julian calendar was used well into the 20th century. Slightly modified from the Julian calendar, the Gregorian calendar has become the internationally accepted civil calendar.

Solar Calendars
The Julian calendar introduced by Julius Caesar in 45 BCE added a simple leap year rule: insert an extra day every four years. The Julian calendar standardized March 21 as the date of the vernal equinox. However, the leap year rule did not precisely match the solar year. Over the centuries the date of the astronomical vernal equinox slowly drifted away from March 21. Ecclesiastical rules were used to compute the date of Easter matching March 21 as the date of the vernal equinox.

The leap year rule for the Gregorian calendar differs slightly from one for the Julian calendar. The Gregorian leap year rule states that every year divisble by four is a leap year, except for years that are exactly divisible by 100. The year 1900 is not a leap year; the year 2000 is a leap year. Years divisible by 400 are still leap years.

Lunar Calendars
The Islamic Calendar is a lunar calendar with months corresponding to the lunar phase cycle. The twelve months of the Islamic calendar systematically shift with respect to the months of the international civil calendar.

A lunar calendar bases each month on a full cycle of the Moon’s phases (called a lunation or synodic month). Lunar calendars usually start each month with a New Moon or the first visible crescent moon after New Moon. Since the solar year does not contain an integral number of days or an integral number of lunar months, many calendars (called lunisolar calendars) adjust the length of their years and months. Without such adjustments the seasons will steadily drift through the months.

Lunisolar Calendars
The Hebrew Calendar is a lunisolar calendar based on calculation rather than observation and its current form dates from around 359 CE. It is the official calendar of Israel, although variations on this calendar exist. Passover dates are computed from a set of defined rules.

The National Calendar of India is a lunisolar calendar with leap years coinciding with those of the Gregorian calendar. Tabulations of the religious holidays are prepared by the India Meteorological Department and published annually in The Indian Astronomical Ephemeris.

The Chinese calendar is also a lunisolar calendar based on calculations of the positions of the Sun and Moon.

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.