<p>According to Wikipedia and a few other trustable sources, the current Julian Period started in 4713 BCE and will end in 3268 CE. I remember seeing an online document (though maybe it was a scan of a printed document?) which used an initial 9 for all Julian day numbers before 4713 BCE. I presume it also used a way to differentiate dates after 3268 CE.</p>
<p>So my question, in line with <a href="https://astronomy.stackexchange.com/questions/32022/is-there-a-maximum-julian-day">Is there a maximum Julian day?</a>, is: What will happen at the end of the current Julian Period? Will we start numbering Julian days at 1? I mean, in theory, <em>we should</em>, as it will be a new Julian Period…</p>
<p>(Accessorily, if someone knows about the document I mentioned above, that used a 9 for pre-4713BCE dates, it would be appreciated.)</p>

CC BY-SA 4.0
2024-07-10T23:39:03.553Z
No, the Julian Days astronomers use is strictly a count of days since 4713.  There are already data sets that cover large intervals where this would come in to play (e.g. JPL's Development Ephemeris) and they do not make any special cases for a new Julian Period.
Greg Miller
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2024-07-11T08:15:29.483Z
Thank you @GregMiller. For some reason, I never thought of looking at JPL data.
Pierre Paquette
CC BY-SA 4.0
2024-07-12T12:32:37.490Z
How could you even prefix a number by a 9?   How do you know whether 990 is negative 90 or positive 990?
asdfex
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2024-07-12T13:17:38.457Z
@asdfex: I wholeheartedly agree with you. I found this extremely weird when I saw that publication, and it took me a while to figure out that they meant the previous Julian Period, as it was not mentioned explicitly in the text.
Pierre Paquette
CC BY-SA 4.0
2024-07-12T15:25:39.587Z
I suspect the 9 prefix notation is related to the old way of handling negative numbers back in the day when we used log tables. See https://math.stackexchange.com/q/719616/207316
PM 2Ring
CC BY-SA 4.0
2024-07-20T04:09:50.013Z
@GregMiller it would be great if you could turn that helpful comment into a helpful *answer*. The OP has what they need, but the question may be relevant for someone else in the future, and comments aren't permanent... :-)
Chappo Hasn't Forgotten
<time>
The Julian Day Bug
Pierre Paquette
# The Julian Day Bug

(Asked by: username_0 on 2024-07-10)

According to Wikipedia and a few other trustable sources, the current Julian Period started in 4713 BCE and will end in 3268 CE. I remember seeing an online document (though maybe it was a scan of a printed document?) which used an initial 9 for all Julian day numbers before 4713 BCE. I presume it also used a way to differentiate dates after 3268 CE.

So my question, in line with [Is there a maximum Julian day?](https://astronomy.stackexchange.com/questions/32022/is-there-a-maximum-julian-day), is: What will happen at the end of the current Julian Period? Will we start numbering Julian days at 1? I mean, in theory, *we should*, as it will be a new Julian Period…

(Accessorily, if someone knows about the document I mentioned above, that used a 9 for pre-4713BCE dates, it would be appreciated.)

**Tags:** time

> **Comments:**
> **username_1**: No, the Julian Days astronomers use is strictly a count of days since 4713.  There are already data sets that cover large intervals where this would come in to play (e.g. JPL's Development Ephemeris) and they do not make any special cases for a new Julian Period.  
> **username_0**: Thank you @GregMiller. For some reason, I never thought of looking at JPL data.  
> **username_2**: How could you even prefix a number by a 9?   How do you know whether 990 is negative 90 or positive 990?  
> **username_0**: @username_2: I wholeheartedly agree with you. I found this extremely weird when I saw that publication, and it took me a while to figure out that they meant the previous Julian Period, as it was not mentioned explicitly in the text.  
> **username_3**: I suspect the 9 prefix notation is related to the old way of handling negative numbers back in the day when we used log tables. See https://math.stackexchange.com/q/719616/207316  
> **username_4**: @GregMiller it would be great if you could turn that helpful comment into a helpful *answer*. The OP has what they need, but the question may be relevant for someone else in the future, and comments aren't permanent... :-)  

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