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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 which is green on Thursdays.

I noticed that the arrangement does not agree with in-the-sky.org's planetarium 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 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

Venus and the Moon September 5, 2024 in-the-sky.org

Venus, the Moon, and Taipei 101 September 5, 2024

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

Taipei 101 September 5, 2024

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    $\begingroup$ 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. $\endgroup$
    – Greg Miller
    Commented Sep 8 at 17:35
  • $\begingroup$ Or just update to either a better algorithm (e.g. VSOP or an ephemeris proper. Thanks for that information - I'd seen disagreements with conjunctions before but never with this much documentation. $\endgroup$
    – uhoh
    Commented Sep 9 at 0:34
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    $\begingroup$ 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. $\endgroup$
    – JohnHoltz
    Commented Sep 9 at 13:39
  • $\begingroup$ @JohnHoltz Skyfield would be a more convenient way - it uses basically the same JPL ephemerides 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. $\endgroup$
    – uhoh
    Commented Sep 9 at 14:50

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