<p>Based on the <a href="https://ssd.jpl.nasa.gov/astro_par.html" rel="noreferrer">Astrodynamic Parameters</a> page, It appears to be the <a href="https://en.wikipedia.org/wiki/Julian_year_(astronomy)" rel="noreferrer">astronomical Julian year</a>, defined as exactly 365.25 days of 86400 SI seconds.</p>

CC BY-SA 4.0
2022-06-13T09:55:15.957Z
I think you're right, but I can't find a solid reference in the Horizons docs, tutorial, or batch-file docs. Apart from that Parameters page, other evidence that they're using Julian years are the various formulae in those docs which use Julian centuries, as do the formulae on https://ssd.jpl.nasa.gov/planets/approx_pos.html
PM 2Ring
CC BY-SA 4.0
2022-06-13T10:17:33.790Z
+1. In absence of more solid proof that sounds reasonable
planetmaker
CC BY-SA 4.0
2022-06-13T19:10:02.100Z
Also note that JPL uses what used to be called $\text{T}_\text{eph}$. This eventually became the latest revision to Barycentric Dynamical Time (TDB). (While the acronym is French, it's TDB in every language.) $\text{T}_\text{eph}$/TDB is a relativistic time frame that on average ticks at the same rate as an ideal clock at sea level (actually, the geoid) on Earth.
David Hammen
CC BY-SA 4.0
2022-06-13T09:49:08.230Z
2022-06-13T09:49:08.230Z
notovny
<p>I'm new to astronomy and therefore probably missing the obvious here, but what type of year (tropical, sidereal?) is used for the orbital periods in the JPL Small-Body Database Lookup? For example, the data for Halley's comet <a href="https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?des=1P" rel="noreferrer">here</a> gives the period as 75.91006173810142 years. But I can't see a note detailing their definition of &quot;year&quot;. Thank you</p>

CC BY-SA 4.0
2022-06-13T11:33:17.437Z
A problem with using a tropical or sidereal year is that they're variable, so you also need to specify the epoch. See https://en.wikipedia.org/wiki/Year#Sidereal,_tropical,_and_anomalistic_years
PM 2Ring
CC BY-SA 4.0
2022-06-13T13:21:27.190Z
If you hover over the 'y' for years, it says "Astronomical/Julian years: 365.25 days". Hovering of the 'd' for days says 86400 seconds
Barry Carter
CC BY-SA 4.0
2022-06-13T14:11:46.587Z
@barrycarter - Hidden in plain sight. Well done for spotting that.
Peter
<ephemeris><definition>
What unit of year in JPL Small-Body Database Lookup?
Peter
# What unit of year in JPL Small-Body Database Lookup?

(Asked by: username_0 on 2022-06-13)

I'm new to astronomy and therefore probably missing the obvious here, but what type of year (tropical, sidereal?) is used for the orbital periods in the JPL Small-Body Database Lookup? For example, the data for Halley's comet [here](https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?des=1P) gives the period as 75.91006173810142 years. But I can't see a note detailing their definition of "year". Thank you

**Tags:** ephemeris, definition
> **Comments:**
> **username_1**: A problem with using a tropical or sidereal year is that they're variable, so you also need to specify the epoch. See https://en.wikipedia.org/wiki/Year#Sidereal,_tropical,_and_anomalistic_years  
> **username_2**: If you hover over the 'y' for years, it says "Astronomical/Julian years: 365.25 days". Hovering of the 'd' for days says 86400 seconds  
> **username_0**: @barrycarter - Hidden in plain sight. Well done for spotting that.  

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## 1 answers

### Answer by username_3 on 2022-06-13. Score: 12 | Accepted answer

Based on the [Astrodynamic Parameters](https://ssd.jpl.nasa.gov/astro_par.html) page, It appears to be the [astronomical Julian year](https://en.wikipedia.org/wiki/Julian_year_(astronomy)), defined as exactly 365.25 days of 86400 SI seconds.

> **Comments:**
> **username_1**: I think you're right, but I can't find a solid reference in the Horizons docs, tutorial, or batch-file docs. Apart from that Parameters page, other evidence that they're using Julian years are the various formulae in those docs which use Julian centuries, as do the formulae on https://ssd.jpl.nasa.gov/planets/approx_pos.html  
> **username_4**: +1. In absence of more solid proof that sounds reasonable  
> **username_5**: Also note that JPL uses what used to be called $\text{T}_\text{eph}$. This eventually became the latest revision to Barycentric Dynamical Time (TDB). (While the acronym is French, it's TDB in every language.) $\text{T}_\text{eph}$/TDB is a relativistic time frame that on average ticks at the same rate as an ideal clock at sea level (actually, the geoid) on Earth.  

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