<blockquote>
<p>Am i doing something wrong?</p>
</blockquote>
<p>Yes I think so, probably some input to Horizons was incorrect.</p>
<hr />
<ul>
<li>EST or Eastern Standard Time is UTC-04, so the time &quot;24/07/23 21:00 pm EST&quot; is <code>2023-07-25 01:00:00</code></li>
<li>Lat/Long: [43.0000N, 79.0000W] is lat/lon = <code>(43.0, -79.0)</code></li>
</ul>
<p>Put these into <a href="https://rhodesmill.org/skyfield/" rel="nofollow noreferrer">Skyfield</a> and I get <code>alt: 15.736  az:  268.206</code></p>
<p>Since Skyfield uses essentially the same type of <a href="https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris" rel="nofollow noreferrer">JPL ephemeris</a> as Horizons, and Horizons is considered by many to be the &quot;gold standard&quot; since it is used by so many astronomers and space exploration professionals, I assume you must have entered some data incorrectly.</p>
<p>However, we won't know how or what until you at least post a screen shot of your entered data.</p>
<p>If I set it up like this:</p>
<p><a href="https://i.sstatic.net/dwnJ5.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/dwnJ5.png" alt="JPL Horizons input setup" /></a></p>
<p>I get an azimuth of <code>268.206</code> and altitude of <code>15.736</code> which agrees with Skyfield.</p>
<hr />
<pre><code> Date__(UT)__HR:MN     R.A._____(ICRF)_____DEC  Azi____(a-app)___Elev    APmag   S-brt             delta      deldot     S-O-T /r     S-T-O  Sky_motion  Sky_mot_PA  RelVel-ANG  Lun_Sky_Brt  sky_SNR
<span class="math-container">$$SOE
 2023-Jul-24 01:00 Cm  10 40 04.17 +09 29 47.4  268.205789  15.735799    1.694   4.403  2.33907874798369   8.8764023   37.3941 /T   21.9361   1.5409375   112.93846   11.511078         n.a.     n.a.
 2023-Jul-25 01:00 Cm  10 42 22.58 +09 15 25.1  268.303009  15.275338    1.691   4.396  2.34398745433334   8.7759797   37.0564 /T   21.7632   1.5426111   113.01694   11.348959         n.a.     n.a.
$$</span>EOE
</code></pre>
<p>Here's the Skyfield script I used:</p>
<pre><code>from skyfield.api import Loader, Topos
import numpy as np
import matplotlib.pyplot as plt

load = Loader('~/Documents/fishing/SkyData') # avoid multiple copies of large files
ts = load.timescale()
eph = load('de421.bsp')

earth, mars = [eph[x] for x in ('earth', 'mars')]

topo = Topos(latitude_degrees = 43.0000,
                  longitude_degrees = -79.0000)

time = ts.utc(2023, 7, 24, 1)  # Date: 24/07/23 21:00 pm EST

mars_astrometric = (earth + topo).at(time).observe(mars)

alt, az, d = mars_astrometric.apparent().altaz()

print('alt: ', round(alt.degrees, 3), ' az: ', round(az.degrees, 3))
</code></pre>

CC BY-SA 4.0
2023-07-25T01:37:15.980Z
Ah it appears I didn't realize the timings were in UTC and not EST, Im also getting the right values now 

Thanks
warherolion
CC BY-SA 4.0
2023-07-25T08:00:04.717Z
@warherolion that's great! Thanks for the feedback :-)
uhoh
CC BY-SA 4.0
2023-07-24T23:15:37.327Z
2023-07-24T23:15:37.327Z
uhoh
<p>I am trying to use NASA's Horizons systems to create a tracker for planets but I am running into a problem related to the returned ephemeris data, particularly with the altitude and azimuth. When comparing the values of a planet like Mars from Horizons to other star maps like theskylive I get values that are incredibly different.</p>
<pre><code>Planet: Mars 

Lat/Long: [43.0000N, 79.0000W]

Date: 24/07/23 21:00 pm EST

Horizons Az&amp;Alt: [223.53, 48.51]

theskylive Az&amp;alt: [268.8, 14.6]
</code></pre>
<p>Am i doing something wrong?</p>

CC BY-SA 4.0
2023-07-24T23:19:50.230Z
I think your input to Horizons may have had some error (see my answer) Can you show a screenshot or your input data?
uhoh
CC BY-SA 4.0
2023-07-25T01:05:59.443Z
Can reproduce OP's HORIZONS result with inputs of -79°E 43°N, 2023-07-24 21:40 UT.
Mike G
CC BY-SA 4.0
2023-07-25T08:13:41.913Z
For future reference: on the Horizons Web Application, just above the "Generate Ephemeris" button, it says "the batch-file corresponding to the settings above can be viewed by using this link." You can paste that batch file data into your question, inside a code block. That will show us *exactly* what data you submitted to Horizons. FWIW, I have Python code to fetch Horizons data using batch file data here: https://gist.github.com/PM2Ring/b1fec75e78cc08f6fc28c6f6c43529c3
PM 2Ring
<observational-astronomy><ephemeris>
Mismatch between Nasa Horizons Alt & Az data and theskylive and other star maps
warherolion
# Mismatch between Nasa Horizons Alt & Az data and theskylive and other star maps

(Asked by: username_0 on 2023-07-24)

I am trying to use NASA's Horizons systems to create a tracker for planets but I am running into a problem related to the returned ephemeris data, particularly with the altitude and azimuth. When comparing the values of a planet like Mars from Horizons to other star maps like theskylive I get values that are incredibly different.
Planet: Mars 

Lat/Long: [43.0000N, 79.0000W]

Date: 24/07/23 21:00 pm EST

Horizons Az&Alt: [223.53, 48.51]

theskylive Az&alt: [268.8, 14.6]

Am i doing something wrong?

**Tags:** observational-astronomy, ephemeris

> **Comments:**
> **username_1**: I think your input to Horizons may have had some error (see my answer) Can you show a screenshot or your input data?  
> **username_2**: Can reproduce OP's HORIZONS result with inputs of -79°E 43°N, 2023-07-24 21:40 UT.  
> **username_3**: For future reference: on the Horizons Web Application, just above the "Generate Ephemeris" button, it says "the batch-file corresponding to the settings above can be viewed by using this link." You can paste that batch file data into your question, inside a code block. That will show us *exactly* what data you submitted to Horizons. FWIW, I have Python code to fetch Horizons data using batch file data here: https://gist.github.com/PM2Ring/b1fec75e78cc08f6fc28c6f6c43529c3  

---

## 1 answers

### Answer by username_1 on 2023-07-24. Score: 2 | Accepted answer

Am i doing something wrong?

Yes I think so, probably some input to Horizons was incorrect.


EST or Eastern Standard Time is UTC-04, so the time "24/07/23 21:00 pm EST" is 2023-07-25 01:00:00
Lat/Long: [43.0000N, 79.0000W] is lat/lon = (43.0, -79.0)

Put these into Skyfield and I get alt: 15.736  az:  268.206
Since Skyfield uses essentially the same type of JPL ephemeris as Horizons, and Horizons is considered by many to be the "gold standard" since it is used by so many astronomers and space exploration professionals, I assume you must have entered some data incorrectly.
However, we won't know how or what until you at least post a screen shot of your entered data.
If I set it up like this:

I get an azimuth of 268.206 and altitude of 15.736 which agrees with Skyfield.

 Date__(UT)__HR:MN     R.A._____(ICRF)_____DEC  Azi____(a-app)___Elev    APmag   S-brt             delta      deldot     S-O-T /r     S-T-O  Sky_motion  Sky_mot_PA  RelVel-ANG  Lun_Sky_Brt  sky_SNR
$$SOE
 2023-Jul-24 01:00 Cm  10 40 04.17 +09 29 47.4  268.205789  15.735799    1.694   4.403  2.33907874798369   8.8764023   37.3941 /T   21.9361   1.5409375   112.93846   11.511078         n.a.     n.a.
 2023-Jul-25 01:00 Cm  10 42 22.58 +09 15 25.1  268.303009  15.275338    1.691   4.396  2.34398745433334   8.7759797   37.0564 /T   21.7632   1.5426111   113.01694   11.348959         n.a.     n.a.
$$EOE

Here's the Skyfield script I used:
from skyfield.api import Loader, Topos
import numpy as np
import matplotlib.pyplot as plt

load = Loader('~/Documents/fishing/SkyData') # avoid multiple copies of large files
ts = load.timescale()
eph = load('de421.bsp')

earth, mars = [eph[x] for x in ('earth', 'mars')]

topo = Topos(latitude_degrees = 43.0000,
                  longitude_degrees = -79.0000)

time = ts.utc(2023, 7, 24, 1)  # Date: 24/07/23 21:00 pm EST

mars_astrometric = (earth + topo).at(time).observe(mars)

alt, az, d = mars_astrometric.apparent().altaz()

print('alt: ', round(alt.degrees, 3), ' az: ', round(az.degrees, 3))

> **Comments:**
> **username_0**: Ah it appears I didn't realize the timings were in UTC and not EST, Im also getting the right values now   Thanks  
> **username_1**: @username_0 that's great! Thanks for the feedback :-)  

---

