<p>In a way, it's a bit of a matter of laziness. Specifying orbital elements at an arbitrary epoch time requires specifying the epoch time plus six additional items. If the epoch time is known to be time of perihelion passage, one only needs to specify five additional items because one of them is now known. For example, if for an orbital element set that comprises semi-major axis length, eccentricity, inclination, right ascension of ascending node, argument of periapsis, and mean anomaly, the final element can be omitted if the epoch time is specified to be time of periapsis passage because mean anomaly, eccentric anomaly, and true anomaly are by definition exactly zero at periapsis.</p>

CC BY-SA 4.0
2022-10-19T17:15:56.253Z
So why does NASA, for example, give planetary Mean Orbital Elements (including mean longitude) for the epoch J2000. See https://nssdc.gsfc.nasa.gov/planetary/factsheet/fact_notes.html
Peter
CC BY-SA 4.0
2022-10-19T17:35:36.590Z
@Peter It makes sense to treat small bodies differently to major bodies. JPL has data on over a million small bodies. You can read about how they're treated differently in the [Horizons manual](https://ssd.jpl.nasa.gov/horizons/manual.html).
PM 2Ring
CC BY-SA 4.0
2022-10-19T18:14:50.490Z
@Peter The epoch J2000 is a very specific point in time: 12:00:00 (noon) Terrestrial Time on 1 January 2000. The odds that any planet was exactly at perihelion at that specific point in time is zero. That means that a set of orbital elements with J2000 as the epoch time needs to specify all six orbital elements at that time as opposed to five if the planet-specific epoch time is chosen to be time of perihelion passage.
David Hammen
CC BY-SA 4.0
2022-10-19T14:44:39.193Z
2022-10-19T14:44:39.193Z
David Hammen
<p>In <em>Practical Astronomy With Your Calculator Or Spreadsheet</em> by Duffett-Smith and Zwart, the authors write (in relation to calculating cometary positions):</p>
<blockquote>
<p>The longitude of the comet is not usually specified at a particular
epoch. Rather, the epoch is given when the comet is at perihelion, the
point of its closest approach to the Sun.</p>
</blockquote>
<p>Why is this? Is time of perihelion easier to measure than longitude at a particular epoch?</p>

CC BY-SA 4.0
2022-10-19T15:40:23.600Z
FWIW, JPL has 30 entries for Halley's comet. https://ssd.jpl.nasa.gov/api/horizons.api?format=text&MAKE_EPHEM=NO&COMMAND=%27DES%3D1P%3B%27 Their epochs are *mostly* near perihelion, but the most recent one isn't https://ssd.jpl.nasa.gov/api/horizons.api?format=text&MAKE_EPHEM=NO&COMMAND=90000030
PM 2Ring
CC BY-SA 4.0
2022-10-19T17:09:29.457Z
@PM2Ring - Any idea what the significance is to those dates? What's so special about the most recent one, 1968?
Peter
CC BY-SA 4.0
2022-10-19T17:28:03.497Z
Sorry, I have no idea. I guess we'd have to ask JPL. 1968 is especially mysterious. ;) There's a little more info at https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1P
PM 2Ring
CC BY-SA 4.0
2022-10-19T17:38:02.860Z
@PM2Ring - "Hey Jude" was released in 1968. Maybe they're Beatles fans at JPL?
Peter
CC BY-SA 4.0
2022-10-19T20:49:25.443Z
Different databases will have different parameters specified, mostly for historic reasons.  But (as David said in his answer) using perihelion has the advantage of only specifying one additional parameter.  But it is also more accurate for the odd trajectories comets might have, e.g. a for highly inclined orbit a tiny error in longitude would result in a huge error in latitude.
Greg Miller
<observational-astronomy><comets><orbital-elements>
Time of comet at perihelion query
Peter
# Time of comet at perihelion query

(Asked by: username_0 on 2022-10-19)

In *Practical Astronomy With Your Calculator Or Spreadsheet* by Duffett-Smith and Zwart, the authors write (in relation to calculating cometary positions):

> The longitude of the comet is not usually specified at a particular
> epoch. Rather, the epoch is given when the comet is at perihelion, the
> point of its closest approach to the Sun.

Why is this? Is time of perihelion easier to measure than longitude at a particular epoch?

**Tags:** observational-astronomy, comets, orbital-elements
> **Comments:**
> **username_1**: FWIW, JPL has 30 entries for Halley's comet. https://ssd.jpl.nasa.gov/api/horizons.api?format=text&MAKE_EPHEM=NO&COMMAND=%27DES%3D1P%3B%27 Their epochs are *mostly* near perihelion, but the most recent one isn't https://ssd.jpl.nasa.gov/api/horizons.api?format=text&MAKE_EPHEM=NO&COMMAND=90000030  
> **username_0**: @PM2Ring - Any idea what the significance is to those dates? What's so special about the most recent one, 1968?  
> **username_1**: Sorry, I have no idea. I guess we'd have to ask JPL. 1968 is especially mysterious. ;) There's a little more info at https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1P  
> **username_0**: @PM2Ring - "Hey Jude" was released in 1968. Maybe they're Beatles fans at JPL?  
> **username_2**: Different databases will have different parameters specified, mostly for historic reasons.  But (as David said in his answer) using perihelion has the advantage of only specifying one additional parameter.  But it is also more accurate for the odd trajectories comets might have, e.g. a for highly inclined orbit a tiny error in longitude would result in a huge error in latitude.  

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## 1 answers

### Answer by username_3 on 2022-10-19. Score: 4 | Accepted answer

In a way, it's a bit of a matter of laziness. Specifying orbital elements at an arbitrary epoch time requires specifying the epoch time plus six additional items. If the epoch time is known to be time of perihelion passage, one only needs to specify five additional items because one of them is now known. For example, if for an orbital element set that comprises semi-major axis length, eccentricity, inclination, right ascension of ascending node, argument of periapsis, and mean anomaly, the final element can be omitted if the epoch time is specified to be time of periapsis passage because mean anomaly, eccentric anomaly, and true anomaly are by definition exactly zero at periapsis.

> **Comments:**
> **username_0**: So why does NASA, for example, give planetary Mean Orbital Elements (including mean longitude) for the epoch J2000. See https://nssdc.gsfc.nasa.gov/planetary/factsheet/fact_notes.html  
> **username_1**: @Peter It makes sense to treat small bodies differently to major bodies. JPL has data on over a million small bodies. You can read about how they're treated differently in the [Horizons manual](https://ssd.jpl.nasa.gov/horizons/manual.html).  
> **username_3**: @Peter The epoch J2000 is a very specific point in time: 12:00:00 (noon) Terrestrial Time on 1 January 2000. The odds that any planet was exactly at perihelion at that specific point in time is zero. That means that a set of orbital elements with J2000 as the epoch time needs to specify all six orbital elements at that time as opposed to five if the planet-specific epoch time is chosen to be time of perihelion passage.  

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