I would like to make a photo to Moon/Sun when it is very low on horizon, so if I make the shot far away from an object on Earth surface, it appears as big as the Sun.
But how can I calculate the right moment? Any site or app? Could NASA Horizons help? I made a GUI for it, but don't know if any quantity I can query could give me the information I need:
The Photographers Ephemeris is a very nice tool to give you the direction and time of both Moon and Sunset and -rise, made for exactly that purpose: plan and time shots to be at the right place and time for an awesome scenery.
Other programmes and tools to tell you rise and set times are
As noted above, many sites will give you sunrise and sunset times, but both refer to when the entire Sun is below the horizon.
You appear to be looking for when the Sun (or Moon) is touching the horizon, not when it goes completely below the horizon.
The Sun sets when its geometric position is 50 minutes of arc below the horizon (because of refraction and the Sun's angular diameter), so the Sun touches the horizon when its geometric position is 18 minutes of arc below the horizon.
When that happens, the object you want to photograph should have an angular diameter of 16 minutes of arc, just like the Sun.
One caveat: because of refraction, the Sun's diameter isn't exactly 16 minutes of arc when it starts setting, but should be fairly close.
How to determine when Sun/Moon is aligned to two location on Earth's surface?
First check out the many answers to our canonical Where can I find the positions of the planets, stars, moons, artificial satellites, etc. and visualize them?
Both Horizons and Skyfield below are based on NASA JPL Development Ephemerides and so are rock-solid accurate and reliable.
Horizons and the Spice infrastructure provide ways to search for specific events. You can read more about spice here in Astronomy ~40 posts and also in Space Exploration SE ~100 posts.
If you can use Python then Skyfield has several methods for finding events. See the Searching and Almanac headings. This package has been under active development for years and has all kinds of helpful methods and goodies!
Here's an old example of a photographer's sunset question based on a specific location on Earth that I answered using a Skyfield script. I think that today it would be written even more easily using the newer features described above! Sunset on [Mountain] from [Viewing Point]
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