Stellarium

Accuracy of the Minor Planets

Asked by Matt Orsie

First off this is a great program! I wanted to double check the accuracy for the minor planets... in this case the fifth largest moon of Jupiter (Himalia). I entered a date time of (7-29-2020 05:00 UT). Stellarium shows the RA and Dec (on date) as:

19hr 24m 35s -21 52m 2s.

I went to the Minor Planet Center web site ( https://www.minorplanetcenter.net/iau/NatSats/NaturalSatellites.html ) and ran the RA Dec locations for Himalia for the same date/date and there is a mismatch. From the web site I get:

Jupiter VI (Himalia)
Perturbed ephemeris below is based on 41530-day-arc elements from MPC 111777. Last observed on 2018 Sept. 18.

[J006S ]
Date UT R.A. (J2000) Decl. Delta r r_J El. Ph. V Offset Sky Motion
...
2020 07 29 050000 19 27 38.0 -21 52 07 4.108 5.091 0.0664 164.0 3.2 14.8 71+ 1468+ 0.38 263.3
...

The 050000 is UT.

Which one is correct?

If it's the web site can I load the ephermeris into Stellarium?

Thanks so much,
Matt Orsie - Hedgesville, WV

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Alexander Wolf (alexwolf) said : 		#1

Both pairs of coordinates are correct. If you will get an ephemeris at JPL HORIZON service, then you will get another pair of correct coordinates and this pair will have other values. Explanation is very simple: different orbital elements from 3 different sources will give 3 different pairs of coordinate values for the same time.

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Matt Orsie (wvbirder) said : 		#2

Alexander,
  Thanks for getting back to me on this. Perhaps I don't understand why
there would be so much different between the different data sources?
Which one is the right one? They certainly are not close enough to each
other to find the moon. If I plug in the position of Himalia using the
expected location using the Minor Planets Center ephemeris, look how far
away it is from where Stellarium says it is at the same same date/time?

For 0500 UT on 7-29-2020

 From Minor Planet Center:

Just trying to figure out which one to trust as Himalia is in a totally
different location for said date and time.

Thanks,
Matt

On 9/6/2020 6:05 AM, Alexander Wolf wrote:
> Your question #692770 on Stellarium changed:
> https://answers.launchpad.net/stellarium/+question/692770
>
> Status: Open => Answered
>
> Alexander Wolf proposed the following answer:
> Both pairs of coordinates are correct. If you will get an ephemeris at
> JPL HORIZON service, then you will get another pair of correct
> coordinates and this pair will have other values. Explanation is very
> simple: different orbital elements from 3 different sources will give 3
> different pairs of coordinate values for the same time.
>

Revision history for this message
Matt Orsie (wvbirder) said : 		#3

I've contacted MPC and JPL/HORIZONS to see if there is an API interface available to obtain current orbital elements/ephemeris data. MPC doesn't offer an API interface at this time. I've not heard back from JPL. Both MPC and JPL's ephemeris MATCH for a specific date and time on Himalia and I'd expect for other irregular/outer moons of Jupiter and Saturn as well. Not critical as I can get the RA/DEC locations and manually mark the locations on a printed out star chart.

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Matt Orsie (wvbirder) said : 		#4

I've submitted possible sources of data from a programmatic standpoint to the developers. Not a bug. Will close.

Revision history for this message
Matt Orsie (wvbirder) said : 		#5

From JPL:

1)
There are a couple of methods for programmatically getting Horizons results. The first is via HTTPS using a CGI script https://ssd.jpl.nasa.gov/horizons_batch.cgi (which provides instructions). The second is via the main telnet interface using the "expect" scripting language. Jon Giorgini (Horizons Cog-E) can provide more details. We are working on adding a Horizons API to our API server which will be RESTful but that may be a few months away.

2)
There are also automation scripts here: ftp://ssd.jpl.nasa.gov/pub/ssd/SCRIPTS/
.. written in the Expect scripting language. However, you need to parse the returned output to extract numbers.