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 gives the period as 75.91006173810142 years. But I can't see a note detailing their definition of "year". Thank you
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$\begingroup$ A problem with using a tropical or sidereal year is that they're variable, so you also need to specify the epoch. See en.wikipedia.org/wiki/… $\endgroup$– PM 2RingJun 13, 2022 at 11:33
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3$\begingroup$ 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 $\endgroup$– Barry CarterJun 13, 2022 at 13:21
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1$\begingroup$ @barrycarter - Hidden in plain sight. Well done for spotting that. $\endgroup$– PeterJun 13, 2022 at 14:11
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Based on the Astrodynamic Parameters page, It appears to be the astronomical Julian year, defined as exactly 365.25 days of 86400 SI seconds.
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$\begingroup$ 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 ssd.jpl.nasa.gov/planets/approx_pos.html $\endgroup$– PM 2RingJun 13, 2022 at 9:55
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1$\begingroup$ +1. In absence of more solid proof that sounds reasonable $\endgroup$ Jun 13, 2022 at 10:17
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$\begingroup$ 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. $\endgroup$ Jun 13, 2022 at 19:10