<p>By a lucky coincidence I happened to have a great view of the conjunction of Venus and the Moon on September 5, 2024. Once the naughty cloud moved up and the pair moved down, they aligned with a third, terrestrial body - <a href="https://en.wikipedia.org/wiki/Taipei_101" rel="nofollow noreferrer">Taipei 101</a> which is green on Thursdays.</p>
<p>I noticed that the arrangement does not agree with <a href="https://in-the-sky.org/skymap.php" rel="nofollow noreferrer">in-the-sky.org's planetarium</a> prediction. It shows Venus and the Moon at the same elevation above the horizon at 18:45:42 (UTC+08) but my photo at that time shows them with about a 60 degree orientation with respect to horizontal, with the Moon lower and south of Venus.</p>
<p>The photo was taken from 25.0411 N, 121.61675 E, and I estimate Taipei 101's location to be 25.0341 N, 121.5644 E (both from eyeballing Google Maps).</p>
<p>It would be great to first double-check that theres a problem with in-the-sky, and then help <a href="https://in-the-sky.org/about.php#copyright" rel="nofollow noreferrer">Dominic</a> out by identifying if it's the Moon's position or the planet's that's off.</p>
<p>For reference, the thin red lines in the simulation crossing near the Moon are 8° of elevation and 262.5° azimuth, if I've counted correctly.</p>
<p>For the photogrammetrists out there, these are uncropped iPhone SE2 images (3024 x 4032 pixels) with digital zoom ratios of 2.467 and 5.008 (&quot;naughty cloud&quot;, 18:30:51 UTC+08).</p>
<p>Perhaps the Moon's position is shown for the Earth's Geocenter rather than Earth's surface? It's just a thought.</p>
<p><a href="https://i.sstatic.net/263xGfcM.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/263xGfcM.jpg" alt="Venus, the Moon, and Taipei 101 September 5, 2024" /></a></p>
<p><a href="https://i.sstatic.net/Lh7cATSd.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Lh7cATSd.png" alt="Venus and the Moon September 5, 2024 in-the-sky.org" /></a></p>
<p><a href="https://i.sstatic.net/f9cmYE6t.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/f9cmYE6t.jpg" alt="Venus, the Moon, and Taipei 101 September 5, 2024" /></a></p>
<p>What the rest of the &quot;third body&quot; looks like, for reference:</p>
<p><a href="https://i.sstatic.net/JppJtd82.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/JppJtd82.jpg" alt="Taipei 101 September 5, 2024" /></a></p>

CC BY-SA 4.0
2024-09-08T17:35:43.870Z
Looking at the source code, it looks like he's using a lot of low precision algorithms, so he's likely aware that it's accurate enough for pointing with your finger, but not much else.  It would be a good idea that the app inform the users of that fact though.
Greg Miller
CC BY-SA 4.0
2024-09-09T00:34:13.293Z
Or just update to either a better algorithm (e.g. [VSOP](https://stackoverflow.com/q/34487170/3904031) or an ephemeris proper. Thanks for that information - I'd seen disagreements with conjunctions before but never with this much documentation.
uhoh
CC BY-SA 4.0
2024-09-09T13:39:22.713Z
I tried to find a star in your image; that would be the best way to determine if Venus, the Moon, or both are off in position. I could not see any stars (just dust on my monitor ;-). You could use JPL's Horizon to calculate the azimuth and altitude of the Moon and Venus to check how accurately In-the-Sky matches.
JohnHoltz
CC BY-SA 4.0
2024-09-09T14:50:48.947Z
@JohnHoltz [Skyfield](https://rhodesmill.org/skyfield/) would be a more convenient way - it  uses basically the same [JPL ephemerides](https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris) as Horizons and has a model for the Earth's surface built in, so one can actually define the top of Taipei 101 and the top of the building I was on and generate the alignment of all three bodies (two celestial, one terrestrial) and figure it out exactly, a bit like [I did here](https://astronomy.stackexchange.com/a/30141/7982).
uhoh
<amateur-observing><photography><ephemeris><astrometry><conjunction>
In-the-sky.org's simulation of the September 5, 2024 Venus-Moon conjunction seems off, but how, and why?
uhoh
# In-the-sky.org's simulation of the September 5, 2024 Venus-Moon conjunction seems off, but how, and why?

(Asked by: username_0 on 2024-09-08)

By a lucky coincidence I happened to have a great view of the conjunction of Venus and the Moon on September 5, 2024. Once the naughty cloud moved up and the pair moved down, they aligned with a third, terrestrial body - [Taipei 101](https://en.wikipedia.org/wiki/Taipei_101) which is green on Thursdays.

I noticed that the arrangement does not agree with [in-the-sky.org's planetarium](https://in-the-sky.org/skymap.php) prediction. It shows Venus and the Moon at the same elevation above the horizon at 18:45:42 (UTC+08) but my photo at that time shows them with about a 60 degree orientation with respect to horizontal, with the Moon lower and south of Venus.

The photo was taken from 25.0411 N, 121.61675 E, and I estimate Taipei 101's location to be 25.0341 N, 121.5644 E (both from eyeballing Google Maps).

It would be great to first double-check that theres a problem with in-the-sky, and then help [Dominic](https://in-the-sky.org/about.php#copyright) out by identifying if it's the Moon's position or the planet's that's off.

For reference, the thin red lines in the simulation crossing near the Moon are 8° of elevation and 262.5° azimuth, if I've counted correctly.

For the photogrammetrists out there, these are uncropped iPhone SE2 images (3024 x 4032 pixels) with digital zoom ratios of 2.467 and 5.008 ("naughty cloud", 18:30:51 UTC+08).

Perhaps the Moon's position is shown for the Earth's Geocenter rather than Earth's surface? It's just a thought.

[![Venus, the Moon, and Taipei 101 September 5, 2024](https://i.sstatic.net/263xGfcM.jpg)](https://i.sstatic.net/263xGfcM.jpg)

[![Venus and the Moon September 5, 2024 in-the-sky.org](https://i.sstatic.net/Lh7cATSd.png)](https://i.sstatic.net/Lh7cATSd.png)

[![Venus, the Moon, and Taipei 101 September 5, 2024](https://i.sstatic.net/f9cmYE6t.jpg)](https://i.sstatic.net/f9cmYE6t.jpg)

What the rest of the "third body" looks like, for reference:

[![Taipei 101 September 5, 2024](https://i.sstatic.net/JppJtd82.jpg)](https://i.sstatic.net/JppJtd82.jpg)

**Tags:** amateur-observing, photography, ephemeris, astrometry, conjunction
> **Comments:**
> **username_1**: Looking at the source code, it looks like he's using a lot of low precision algorithms, so he's likely aware that it's accurate enough for pointing with your finger, but not much else.  It would be a good idea that the app inform the users of that fact though.  
> **username_0**: Or just update to either a better algorithm (e.g. [VSOP](https://stackoverflow.com/q/34487170/3904031) or an ephemeris proper. Thanks for that information - I'd seen disagreements with conjunctions before but never with this much documentation.  
> **username_2**: I tried to find a star in your image; that would be the best way to determine if Venus, the Moon, or both are off in position. I could not see any stars (just dust on my monitor ;-). You could use JPL's Horizon to calculate the azimuth and altitude of the Moon and Venus to check how accurately In-the-Sky matches.  
> **username_0**: @JohnHoltz [Skyfield](https://rhodesmill.org/skyfield/) would be a more convenient way - it  uses basically the same [JPL ephemerides](https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris) as Horizons and has a model for the Earth's surface built in, so one can actually define the top of Taipei 101 and the top of the building I was on and generate the alignment of all three bodies (two celestial, one terrestrial) and figure it out exactly, a bit like [I did here](https://astronomy.stackexchange.com/a/30141/7982).  

---

## 0 answers

