# Why does Astronomy still use the Anno Domini system for Time Synchronization

Since Time is such a valuable variable in the calculation of Astronomic movement why does the Astronomy Community still rely on the the 6th Century Anno Domini system developed by Dionysius Exiguus to establish Time on Earth. Obviously it is not 2013 on Earth, wouldn't it be more technically accurate to determine the actual time on Earth or get as close as possible to it than to use a time that is clearly wrong and developed by someone 15 centuries ago?

What makes you think that it is "obviously not 2013 on Earth"?

In actual calculations, astronomers use the Julian day, which is a decimal representation of time. A Julian year is exactly 365.25 days of 86,400 seconds each. Astronomical coordinates are usually written in the J2000 epoch, which allows us to compensate for Earth's axial precession.

Our ability to measure and record time is limited only by the accuracy of our most precise clocks. There's nothing fundamentally wrong or imprecise about it.

• Most estimates put the age of Earth at approximately 4.5 billion years. What ever that actual number is is the actual time on Earth. By saying it is the year 2103 that would be like saying that a significant moment in your life determines your age not the starting time of your life. So Jesus's birth is the starting time on Earth not the actual start of Earth. This may seem trivial but if we cannot calculate the time on our own Planet how can we say with certainty how time works or what the age of anything else is? Seems important to me but maybe I am being to technical. Oct 24, 2013 at 2:47
• There are 3 age systems in use here, the astronomical Julian age as described by Moriarty, the AD/BC used in history (not all the time) and for greater ages, 'millions of years ago' etc.
– user8
Oct 24, 2013 at 5:04
• @alphadev: You adapt scale to measured entities. Just like you use meters or kilometers to describe size of neutron star, instead of picoparsecs, you use julian years to describe dates of cosmic events that take place within our lifetimes, instead of basing time on a point offset by 4.5bln years back.
– SF.
Oct 24, 2013 at 8:56
• Any chosen time base is arbitrary. The point of calling this year "2013" is not that it's been 2013 years since the birth of Christ (historical evidence suggests that's not correct anyway), it's to establish a consistent numbering system that with an agreed meaning. Describing moments as durations since the origin of the Earth would not be any better, both because we don't know it with any precision and because such large numbers (4+ billion years) would be inconvenient. Oct 25, 2013 at 20:20
• Someone should really put in a mention of ephemeris time
– user21
Jan 7, 2016 at 3:39

To consider this question we must consider four things:

1. Humans work on a much smaller scale than what you are suggesting (i.e. seconds, days, years, centuries and maybe millennia) meaning that it is harE for us to comprehend the time scale of the existence of earth (billions of years). We also cannot get an accurate enough time for the age of the earth, creating the problem of when did it start. Would this year be 4 500 000 000 or would this year be 4 500 000 013 or would it be 4 487 575 864? With the current system we have defined a point in time from which to start our calendar (about four years after the real person called Jesus was born).

2. Most things that we do now operate on the BCE/CE system (BC/AD: new names, same system), meaning that everything would need to be updated to the new system. All past observations, all past reports, all computer systems, all systems that control telescopes, EVERTHING. It would cost hundreds of millions, if not billions or even trillions of dollars.

3. What do you consider as the point at which Earth became Earth? The formation of a planet spans over millions of years and we cannot simply define a point at which Earth was Earth.

4. When writing the long version of a date instead of simply writing eight characters, you would potentially have to write 14. it would become a lot harder to read and write and become impractical and produce more room for transcription errors and create confusion.

So all in all it becomes expensive and impractical to change the system and the system we would change to has no benefit worth the extra expense or effort.

Astronomy is done over huge orders of magnitude. To make things manageable, we try to keep numbers under a thousand - it's both more practical and more intuitive. This is reason #1.

The second reason why we don't set our zero point to the beginning of the Earth is because it's a measured quantity with an error. If you set your clocks to the 'beginning' of the Earth (which by the way is not a well-defined statement since the time of the formation of the Earth is actually an inclusive range of times), you would have to report the year as: $$4,540,002,013 \pm 50,000 \text{ years}$$ Though the zero-point is certainly arbitrary, it at least defines a point in time where we can use clocks to measure from.

Lastly, why do you think the age of the Earth is the relevant zero-point? Why not use the beginning of the Universe?

• The age of Earth is the relevant zero point for Earth. The Solar System has its own Time and zero point, The Galaxy has its own Time and zero point etc... The Universe is a theory that has not been proven to be correct, it is just the most popular explanation for our encapsulating system currently. The key is to understand Time and how to measure it accurately in your own system first so that you can then understand it and apply measuring it to other systems, whether yours is inclusive to them or not. Someone knows exactly what year it is on Earth, just not the Earthlings. Oct 25, 2013 at 22:41
• I disagree completely. As I and others (@damned truths) have stated, the "beginning" of systems like the Earth are poorly defined temporally speaking and would make for absolutely silly $t=0$ points. Literally any other point, though arbitrary, would make for a much better zero-point. You claim "The Universe is a theory", I'm not sure what this statement means so I don't know how to respond to that. "Someone knows exactly what year it is on Earth" -- not necessarily true. Oct 26, 2013 at 11:50
• The theory of Special Relativity (and later General Relativity) has proven that the time coordinate in space is a 'relative' quantity for different inertial observers -- this is huge! For somebody traveling at a high speeds as seen by another, or somebody in a different place within a gravitational field will measure time differently. The ONLY relevant zero-point is the beginning of the universe, and unfortunately it is a measurement whose error bars are substantial when compared to the average human lifetime. This is why an arbitrary zero-point is a necessity. Oct 26, 2013 at 11:52
• This is a nice link having to do with what I'm talking about: nist.gov/public_affairs/releases/… Oct 26, 2013 at 11:54

...wouldn't it be more technically accurate to determine the actual time on Earth or get as close as possible to it than to use a time that is clearly wrong...

No, the time system you're talking about is quite as accurate as any other, and there's nothing wrong about "I identify this particular point in time as 2022-12-02 03:35:33 UTC."

Every time system needs two things: a unit of measurement (seconds is a typical one) and an epoch, or an identified point to use as a reference from which any other given point in time is identified. (Time "zero" is the epoch point; positive times identify times after the epoch; negative times identify times before the epoch.)

Astronomers have chosen 0 CE as their particular point, but that's certainly not the only one in common use, and in fact you yourself use systems that have chosen a different epoch, though you have probably not seen this directly. In Unix systems (including probably the server that served you this page) the epoch is January 1st, 1970, 00:00 UTC. Windows uses January 1st, 1601. These are usually converted to the CE epoch when displayed to you.

As an example of how the idea of an epoch works, let's consider a line on which Alice, Bob and Carol are all standing one metre apart. We can designate Alice's position at the epoch and say that they're standing at points 0, 1 and 2, or just as well we can designate Bob's position as the epoch and say that they're standing at points -1, 0 and 1. A little more mathematically we might express this as: Both are equally accurate, both are equally correct, and they actually say the same thing. In a slightly more precise notation, we might say:

{ epoch: Alice;  positions: { Bob:    1; Carol: 2; } }
{ epoch: Bob;    positions: { Alice: -1; Carol: 1; } }

Once you have an epoch and can express times before and after it in at least one standard unit, you can convert between that time system and any other. So which one you want to use is a matter mainly of convenience. In computers we tend to express everything in seconds (or nanoseconds) before or after an epoch relatively close to our own era, because that's really easy to deal with in programs, and convert to some format more amenable to humans reading it when we need to display it. For a lot of astronomical work it's most convenient to use the epoch commonly used in other fields and even a bunch of additional derived units (such as hours and days), so that's what they do.

† I think; I'm not so familiar with Windows internals, and this may have changed over time.