How is time defined in astronomy?

How is time defined in astronomy?

I was amazed when my teacher told me that in space, time is define as a coordinate. It means we can define something with a four coordinate system with "negative time" $-t$ as easily as we can say $-x$. Assuming that we can, what will this suggest?

Edit:) yesterday I saw so many answer on it, create one more doubt in me, that how do I define a origin in space, because there nothing to which I compare, suppose I have two events A and B, how can I say A occur before B, if I say this, from which reference I am comparing it…

I'd like to pick up the point you ask about in your edit.

Suppose we have a homogeneous cube and we want to construct some axes so we can locate points in the cube. The obvious choice is to put the origin at the centre and use axes that are like this:

Suppose we have two points $A$ and $B$ then we can locate them by their position vectors $mathbf a$ and $mathbf b$ where $mathbf a$ would be something like $mathbf a = (a_x, a_y, a_z)$ and likewise for $mathbf b$. Then the interval between the events would just be $mathbf a - mathbf b$.

If we chose a different origin then the position vectors $mathbf a$ and $mathbf b$ would change but their difference $mathbf a - mathbf b$ would not, so the interval between the points does not depend on where we place our origin.

Now suppose we have an infinite universe instead of a finite cube. An infinite universe doesn't have a centre so there isn't an obvious choice of where to put our origin. But as explained above it doesn't matter where we put the origin because the interval between the points is independent of the choice of origin. So we can put the origin anywhere we find convenient.

So far my axes have been spatial, but in relativity time is just another axis and the universe is a four dimensional manifold. That is, we choose four axes $t, x, y, z$ and identify points by their position vector $mathbf a = (a_t, a_x, a_y, a_z)$. So the interval between our two points will be:

$$ mathbf a - mathbf b = (a_t - b_t, a_x - b_x, a_y - b_y, a_z - b_z) $$

And as before this interval vector does not depend on our choice of where to put the origin. We can move the origin in space and in time and the interval stays the same. So when you ask:

suppose I have two events A and B, how can I say A occur before B

you just look at the time component of the interval, $a_t - b_t$. If this is negative then $A$ happened before $B$ and if it's positive then $A$ happened after $B$. And this result doesn't depend on what point in space or time we put the origin.

I'll mention one last point just for interest. In the above I have assumed we stay in the same inertial frame, and in that case what I've said is true that the time component of our interval is always the same. And likewise the $x$, $y$ and $z$ components of the interval don't change when we move our origin. However if we transform to a different inertial frame, i.e. do a Lorentz transformation, this causes our interval vector to rotate in the new axes. It's still the same vector, so it's length (technically its norm) is not changed by the transformation, but the individual components $(t,x,y,z)$ will be different in the new coordinates. This rotation happens in the time direction as well, so the time component $a_t-b_t$ will change. If the two points $A$ and $B$ are spacelike separated then the time component can even change sign. That is for spacelike separated points a Lorentz transformation change the time order of the events.

$t$ signifies time; see the Wikipedia article for spacetime, and then the subsection for 4-vectors.

The basics are pretty natural to understand. Suppose something happens, an event, like an apple falling off of a tree.

In order to tell someone else about it you need the three space coordinates $x, y, z$ and the time coordinate $t$. Without all four, you won't be able to see it happen.

update: The question was clarified to focus on the meaning of "negative time" or $-t$. That's no big deal. There's no problem with $-x$ for example. We don't think of that as negative distance or negative space. It's just a coordinate relative to some origin $(0, 0, 0)$, and we can put that anywhere, arbitrarily.

So you should really thing of the time coordinate as $t-t_0$ where $t_0$ is some arbitrary origin in time. It might be midnight yesterday, or midnight next Tuesday. That would make $t_{Now}$ positive in the first case and negative in the second case, but either way it's still Now.

A negative time coordinate means nothing more or less that

"this happened before the clock read zero".

This is exactly the same as years "BC" (which, stipped of cultural baggage means "before the time we've assigned as calandar zero", right?).

How is time defined in astronomy? - Astronomy

The world is divided into a number of standard time zones. Roughly speaking, there are 24 time zones spaced at intervals of 15° in longitude. (Practically, due to geographic and political factors, the boundaries of time zones are more circuitous. In addition, a few time zones are offset by an odd number of half hours from the Greenwich and U. S. time zones.) Within the confines of each time zone, the hour and minute of the day is defined to be the same. Time zones eliminate the problem that local noon (defined according to the elevation of the Sun) actually occurs at a different time for nearby towns at slightly different longitudes, so that each town's clocks differ by a few minutes from those of neighboring clocks. This problem was already encountered in making up timetables for long distance train travel, but became completely unmanageable with the advent of modern air travel. Defining time zones means than watches need only be adjusted in one hour steps upon crossing of a time zone boundary, as opposed to continuously along any east-west journey.

Time zones are usually specified by the number of hours they differ from Greenwich mean time. Greenwich, England is defined as the 0 of longitude, and is the center of the Greenwich time zone, relative to which other time zones are usually referenced. For example, U. S. Eastern Standard Time (EST) is UT - 5 hours, U. S. Central Time (CST) is UT - 6 hours, U. S. Mountain Time (MST) is UT - 7 hours, and U. S. Pacific Time (PST) is UT - 8 hours.

Almost all time zones differ an integral number of hours from GMT, but there are a number, the most famous of which in North America is Newfoundland, which differ by an odd number of half-hours. Other examples are Iran, Afghanistan, India, Nepal, Myanmar, and central Australia.

Unfortunately, the system is further complicated by daylight saving time, which is seasonally inserted in some (but not all) time zones. Daylight saving time is in effect during summer, and is usually one hour ahead of Standard Time. Therefore, U. S. Eastern Daylight Time (EDT) is UT - 4 hours, U. S. Central Daylight Time (CDT) is UT - 5 hours, U. S. Mountain Daylight Time (MDT) is UT - 6 hours, and U. S. Pacific Daylight Time (PDT) is UT - 7 hours. In the United States, the only states which do not use daylight saving time are Hawaii, Arizona, and most of Indiana. (The situation in Indiana is particularly complicated since while most of Indiana remains on Eastern Standard Time year-round, some portions near borders maintain the same time as the neighboring state and therefore do shift to daylight saving time.)

The following table gives the conversions between universal time (UT) and standard and daylight saving time in the United States.

standard time zone hours to add to UT daylight saving time zone hours to add to UT
Eastern standard time (EST) -5 Eastern daylight time (EDT) -4
Central standard time (CST) -6 Central daylight time (CDT) -5
Mountain standard time (MST) -7 Mountain daylight time (MDT) -6
Pacific standard time (PST) -8 Pacific daylight time (PDT) -7

A construct related to time zones is the international date line at the 180° meridian, which occurs (mostly) in the middle of the time zone offset 12 hours from Greenwich.

HiLink Communications. "Local Times Around the World." .

Seidelmann, P. K. (Ed.). "Time Zones." §2.27 in Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books, pp. 56-58, 1992.

Advice from people who have been there

“Cusco, the object of my desire“

“I went for the 3rd time in December of 2016 and the truth would return a thousand times more. When you see Cusco for the first time you fall in love, it is something indescribable. It's worth the time. The center of the city was built to imitate the stars of the sky That is, Cusco was built in the form of the groups of stars above it. But you can not go to Cusco and not see Machu Picchu. I went in the rainy season and not when it is normally recommended, but I had an awesome sun. The only thing I can say: It is unpredictable to visit Machu Picchu if you are in Cusco.“

Astronomy, Charlemagne And The Mystery Of Phantom Time

In my last post, I talked about how historians can help us understand aspects of astronomy, such as the rate at which Earth's rotation is slowing. The same thing can happen in reverse, where astronomy can confirm aspects of history. Take, for example, the Early Middle Ages, and the theory of phantom time.

One of the wilder things that has been proposed in Medieval history is the idea that about 300 years in the Middle Ages was simply made up. Gap of history and phantom time. Proposed by Heribert Illig in 1991, the idea is that the Catholic Church, led by Pope Sylvester II, rewrote the calendar to put themselves at 1000 AD, rather than around 700 AD. As a result, the years 614 - 911 are simply made up. If this were true, historical figures such as Charlemagne were simply made up.

The idea has never been accepted by mainstream Medievalists, but it raises an interesting aspect about the challenges of reconstructing history, namely that that contemporary authors of history don't always tell the truth. Victors of a battle may overplay the strength of their enemies to make their success all the more glorious. Or the case of the Egyptian Pharaoh Thutmose III, who tried to expunge his Aunt and predecessor Hatshepsut from the records.

Usually such revisionist history is unsuccessful, since we can compare different historical accounts to get an accurate view of events. One of the biggest criticisms of the phantom time is that simply adding three centuries to European history would make it disagree with other historical regions, such as the Islamic expansion and the Tang Dynasty of China, and we see no such discrepancies. But another way to disprove phantom time is to look at astronomical records.

Throughout history, humans have recorded major astronomical events. We have, for example, observations of solar eclipses from both before and after the early Middle Ages. Pliny the Elder mentions a solar eclipse in 59 AD, which agrees with our current dates. Astronomical observations of the Tang Dynasty also confirm our current date.

So both mainstream historians and astronomy agree that phantom time is an idea that simply doesn't hold up. If 300 years of history were simply added to the record, the forgery would be written in the stars, and this simply isn't the case.

Astronomy Terms

Astronomy terms are used to describe the various phenomena in space. In this section you can learn what every astronomy term means and how it helps us to better understand the cosmos.

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Interferometer, an instrument that uses the interference patterns formed by waves (usually light, radio, or sound waves) to measure certain characteristics of the waves themselves or of materials that reflect, refract, or transmit the waves.

Magnitude, in astronomy, a unit of measurement of the brightness of stars. The scale of magnitude extends from negative numbers (for example, the minus first magnitude) for very bright stars to positive numbers (for example, the fourth magnitude) for dimmer ones.

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The revised second edition of this established dictionary contains over 4,300 up-to-date entries covering all aspects of astronomy. Compiled with the help of over 20 expert contributors under the editorship of renowned author and broadcaster Ian Ridpath, A Dictionary of Astronomy covers everything from space exploration and the equipment involved, to astrophysics, cosmology, and the concept of time. The dictionary also includes biographical entries on eminent astronomers, as well as worldwide coverage of observatories and telescopes. Supplementary material is included in the appendices, such as tables of Apollo lunar landing missions and the constellations, a table of planetary data, and numerous other tables and diagrams complement the entries.

The entries have been fully revised and updated for this edition, and new entries have been added to reflect the recent developments within the field of astronomy, including magnetic reconnection, Fornax cluster, luminosity density, and Akatsuki. The content is enhanced by entry-level web links.

Bibliographic Information


Ian Ridpath is an author and broadcaster on stars and planets for a general audience. He is the editor of, among other titles, Norton's Star Atlas and The Monthly Sky Guide.

1.1 The Nature of Astronomy

Astronomy is defined as the study of the objects that lie beyond our planet Earth and the processes by which these objects interact with one another. We will see, though, that it is much more. It is also humanity’s attempt to organize what we learn into a clear history of the universe, from the instant of its birth in the Big Bang to the present moment. Throughout this book, we emphasize that science is a progress report—one that changes constantly as new techniques and instruments allow us to probe the universe more deeply.

In considering the history of the universe, we will see again and again that the cosmos evolves it changes in profound ways over long periods of time. For example, the universe made the carbon, the calcium, and the oxygen necessary to construct something as interesting and complicated as you. Today, many billions of years later, the universe has evolved into a more hospitable place for life. Tracing the evolutionary processes that continue to shape the universe is one of the most important (and satisfying) parts of modern astronomy .

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    Astronomy Without A Telescope – Time Freeze

    There is a story told about traveling at the speed of light in which you are asked to imagine that you begin by standing in front of a big clock – like Big Ben. You realize that your current perception of time is being informed by light reflected off the face of the clock – which is telling you it’s 12:00. So if you then shoot away at the same speed as that light – all you will continue to see is that clock fixed at 12:00, since you are moving at the same speed that this information is moving. And so you discover that at the speed of light, time essentially stands still.

    While there are a number of things wrong with this story – as it happens, one correct thing is that if you were able to travel at the speed of light you would experience no passage of time – although there are several reasons why this is probably an impossible situation to find yourself in.

    But nonetheless, if you were able to travel at light speed and not experience the passage of time – then you would have no time available to reassess your situation – indeed there would be no time available for your neurons to fire. So, you might well leave Earth with the image of the clock fixed on your retina, but since your brain has stopped working, this has nothing to do with the information carried in the light beam you are moving along with. Your retina is never refreshed with a new image as long as you stay at the speed of light.

    Some insight into special relativity is gained by considering the context of someone who stayed back on Earth. If your light speed trip was aimed at a mirror at Alpha Centauri (4.3 light years away) – then from their perspective, it takes you 8.6 years to go there and bounce back. This is true even though you leave and return with an image of 12:00 stuck on your retina and rightly announce that (from your perspective) no time has passed since your departure.

    But moving at light speed and experiencing no passage of time is probably an impossible scenario for we mass-challenged beings. Relativity has it that you possess a proper mass, a proper length and a proper time – which persist regardless of your velocity. If you could survive the G forces to get up to such speeds, then you could happily coast at 99.95% of the speed of light and check your pulse against your watch to find your heart still beating at 72 beats per minute – just like it did back on Earth.

    It’s only when you check back with Earth that you see that something remarkable is happening. Moving at 99.5% of the speed of light gives you a time dilation factor of around 10. So while someone back on Earth will still measure your trip duration at about 8.6 years – for you it will only be around 10 months. And with a remarkably good telescope you might look back to Earth and see a distorted Big Ben, red-shifted and running slow on the way there and then blue-shifted and running very fast on the way back.

    At speeds of less than 10% of the speed of light (0.1c or 30,000 km/sec) time dilation is miniscule, but from 99% speed of light up it increases asymptotically towards infinite.

    One of the reasons that probably makes the experience of light speed/time freeze unobtainable is that time dilation keeps increasing the faster you move. For example, at a speed of 99.99995% of the speed of light you get a time dilation factor of about 1,000. So even if you have a spacecraft with an infinite power source capable of seemingly infinite velocities – you will keep arriving at your destination before your speedometer makes it all the way from 99.99999(etc)% of the speed of light to c = 1.0.

    This is perhaps how we will populate the universe – using difficult-to-imagine investments of energy, coupled with the principle of time dilation to cross vast distances. The trick is not to get homesick, because after covering such distances you can never go back – unless it is to meet your very, very, very great grandchildren.

    (I have cheated a bit by ignoring any periods of acceleration and deceleration within the journeys described here).

    The Telescope Drive Master

    To take truly inspirational images of the cosmos you need three astronomical tools a telescope (optical tube assembly), an imaging device (normally a CCD or DSLR camera), and a mount. All three components should be of good quality in order to achieve the results desired during your journey to the beginning of space and time.

    Some amateur astronomers may believe the imaging device and optical tube assembly are more important than the mount. This might make a good mount less important in their eyes, but a safe, steady, well-made mount is just as important in taking great astroimages.

    What is the difference between Astrology and Astronomy?

    • Astronomy and astrology are similar studies of movement of celestial bodies.

    • While astrology is a set of beliefs and thoughts that planetary motions have bearings on life of human beings, astronomy merely records movements of heavenly bodies and is considered a science.

    • Astrology is supposed to have given birth to astronomy.

    • The data collected through astronomy is verified with astrophysics, but modern day astrologers do not make verifications with the knowledge that was gathered by astrologists thousands of years ago.

    • Both astronomy and astrology study heavens, but astronomy does not make any predictions whereas astrology makes predictions about future events on earth and in particular in an individual’s life based upon this study.

    Images Courtesy: Stars and astrology text via Wikicommons (Public Domain)