<p>They're identical. See <a href="https://en.wikipedia.org/wiki/Epoch_(astronomy)#Julian_years_and_J2000" rel="noreferrer">https://en.wikipedia.org/wiki/Epoch_(astronomy)#Julian_years_and_J2000</a> &quot;J2000 = JD 2451545.0 (TT)&quot; - <a href="/users/16685">PM 2Ring</a></p>
<p>They are the same. It is possible for an epoch ending with a fractional amount, which is why some people put the .0 there. E.g. the Hipparcos catalog's epoch is J1991.25, which indicates a fraction of a year. - <a href="/users/25729">Greg Miller</a></p>

CC BY-SA 4.0
2023-01-09T16:27:37.437Z
2023-01-09T16:27:37.437Z
Mike G
<p>I've seen the standard epoch described as both J2000 and J2000.0. Is there any difference between the two designations?</p>

CC BY-SA 4.0
2023-01-09T14:20:05.783Z
They're identical.See https://en.wikipedia.org/wiki/Epoch_(astronomy)#Julian_years_and_J2000 "J2000 = JD 2451545.0 (TT)"
PM 2Ring
CC BY-SA 4.0
2023-01-09T15:24:53.383Z
They are the same.  It is possible for an epoch ending with a fractional amount, which is why some people put the .0 there.  E.g. the Hipparcos catalog's epoch is J1991.25, which indicates a fraction of a year.
Greg Miller
CC BY-SA 4.0
2023-01-09T16:10:28.677Z
Also some of the Gaia data releases have non .0 epochs e.g. DR2 has the reference epoch of J2015. 5
astrosnapper
<time><definition>
J2000 and J2000.0 difference?
Peter
# J2000 and J2000.0 difference?

(Asked by: username_0 on 2023-01-09)

I've seen the standard epoch described as both J2000 and J2000.0. Is there any difference between the two designations?

**Tags:** time, definition

> **Comments:**
> **username_1**: They're identical.See https://en.wikipedia.org/wiki/Epoch_(astronomy)#Julian_years_and_J2000 "J2000 = JD 2451545.0 (TT)"  
> **username_2**: They are the same.  It is possible for an epoch ending with a fractional amount, which is why some people put the .0 there.  E.g. the Hipparcos catalog's epoch is J1991.25, which indicates a fraction of a year.  
> **username_3**: Also some of the Gaia data releases have non .0 epochs e.g. DR2 has the reference epoch of J2015. 5  

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## 1 answers

### Answer by username_4 on 2023-01-09. Score: 8 | Accepted answer

They're identical. See <https://en.wikipedia.org/wiki/Epoch_(astronomy)#Julian_years_and_J2000> "J2000 = JD 2451545.0 (TT)" - [username_1](/users/16685)

They are the same. It is possible for an epoch ending with a fractional amount, which is why some people put the .0 there. E.g. the Hipparcos catalog's epoch is J1991.25, which indicates a fraction of a year. - [username_2](/users/25729)


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