<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/username_3/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](https://rhodesmill.org/skyfield/) and I get `alt: 15.736 az: 268.206`

Since Skyfield uses essentially the same type of [JPL ephemeris](https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_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:

[![JPL Horizons input setup](https://i.sstatic.net/dwnJ5.png)](https://i.sstatic.net/dwnJ5.png)

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 :-)  

---

