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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

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  • $\begingroup$ astro-seek state that they get it from horizonz and theirs dont match either. Example: horoscopes.astro-seek.com/… $\endgroup$
    – Martin Ingvar Kofoed Jensen
    Jul 22, 2018 at 14:52
  • $\begingroup$ 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: horoscopes.astro-seek.com/… $\endgroup$
    – Martin Ingvar Kofoed Jensen
    Jul 22, 2018 at 15:02
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    $\begingroup$ 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. $\endgroup$
    – JohnHoltz
    Jul 22, 2018 at 15:38
  • $\begingroup$ Could you perhaps include some screenshots-- like @JohnHoltz I couldn't see 305 anywhere in your PDF file. $\endgroup$
    – user21
    Jul 22, 2018 at 23:00
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    $\begingroup$ Astrology notation is archaic, but ecliptic longitude is ecliptic longitude. Voting to keep open. $\endgroup$
    – Mike G
    Jul 23, 2018 at 5:31

1 Answer 1

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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.

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  • $\begingroup$ 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! $\endgroup$
    – Martin Ingvar Kofoed Jensen
    Jul 23, 2018 at 12:13
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    $\begingroup$ Note that you can compare the quantities more directly by selecting 'decimal degrees' under the 'angle format' when choosing the output quantities $\endgroup$
    – astrosnapper
    Jul 23, 2018 at 14:14

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