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I have tried to compare the Ephemeris data from Nasa's Horizons with ephemeris tables from astro.com. And they do not match. Example:

Date: 22th of July 2018. Body: Mars. Time 00:00 UT Horizons: 309 degrees (Ephemeris type=Observer) Astro.com: 305 degrees (http://www.astro.com/swisseph/ae/2000/ae_2018.pdf)

But some bodies are matching. Like Mercury and Saturn.

Am I doing something wrong?

Sources: Nasa Horizons: https://ssd.jpl.nasa.gov/horizons.cgi#top

Astro.com: http://www.astro.com/swisseph/swepha_e.htm

Martin Ingvar Kofoed Jensen
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  • astro-seek state that they get it from horizonz and theirs dont match either. Example: https://horoscopes.astro-seek.com/astrology-ephemeris-planetary-online-ephemerides – Martin Ingvar Kofoed Jensen Jul 22 '18 at 14:52
  • Astro-seek state that they get their data form Horizonz and theirs matches astro.com com, but not Horizonz (at least when I try to get it through Horizonz). Example: https://horoscopes.astro-seek.com/calculate-astrology-ephemeris-july-2018/?uzel_true=&lilith_true=&presnost=2&barva=p – Martin Ingvar Kofoed Jensen Jul 22 '18 at 15:02
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    What positions are you giving? I do not see 309 or 305 degrees in either set of data that you give. Actually, I have no idea what the astro.com site is giving. I need to consult an astrologer to know what they are giving, but I do not know any astrologers to ask. – JohnHoltz Jul 22 '18 at 15:38
  • Could you perhaps include some screenshots-- like @JohnHoltz I couldn't see 305 anywhere in your PDF file. –  Jul 22 '18 at 23:00
  • It is unclear what is being compared here. Normally at least 2 co-ordinate values such as Right Ascension, Declination or an Ecliptic longitude and latitude are needed to specify the position of anything – astrosnapper Jul 22 '18 at 23:52
  • I'm voting to close this question as off-topic because one of the two sites is non-scientific while the other is the preeminent site regarding the behavior of the solar system. – David Hammen Jul 23 '18 at 02:54
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    Astrology notation is archaic, but ecliptic longitude is ecliptic longitude. Voting to keep open. – Mike G Jul 23 '18 at 05:31
  • You got my "leave open" vote, but with an edit suggestion to remove the astrology link. – peterh Jul 23 '18 at 08:16

1 Answers1

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You might be comparing an ecliptic longitude to a right ascension. These are measured around different circles, the ecliptic and the equator respectively. Their values coincide at the solstices ($\lambda$ 90$^\circ$ or 270$^\circ$, RA 6h or 18h) but generally not elsewhere.

With target body Mars, a geocentric observer location, and output quantities 2 (apparent RA & Dec) and 31 (observer ecliptic lon & lat) selected, JPL HORIZONS gives:

Date__(UT)__HR:MN     R.A.__(a-apparent)__DEC    ObsEcLon    ObsEcLat
2018-Jul-22 00:00     20 38 23.96 -24 58 44.6 305.5424192  -6.2964464

RA 20h 38m 24s is 309.60$^\circ$, but the quantity to compare is the ecliptic longitude, 305.54$^\circ$.

The Astrodienst ephemeris table, based on JPL DE431, gives ♒ 5$^\circ$ 33', i.e. ecliptic longitude 300$^\circ$ + 5.55$^\circ$ = 305.55$^\circ$, for Mars on 2018-07-22.

Mike G
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  • Yep, you are right, I thought that the RA DEC from Horizons was in the ecliptic plane - which is used in Astrology, but they are not. The 31 (observer ecliptic long & lan) is exactly what Im looking for. Thanks! – Martin Ingvar Kofoed Jensen Jul 23 '18 at 12:13
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    Note that you can compare the quantities more directly by selecting 'decimal degrees' under the 'angle format' when choosing the output quantities – astrosnapper Jul 23 '18 at 14:14