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Is there any software which will tell me accurately at what moment any part of the solar disk will become from a specific point on the ground taking account of terrain. I feel like Google maps should have the terrain data I need, and the rest is geometry, but it looks like quite a bit of work, so I wondered if it already existed.

Specifically, I am interested in when a viewer 1.5m above local ground level at latitude 46.497300, longitude 7.714651 will first see direct sunlight on the morning of 1 August 2023, taking account of the surrounding mountains. Ideally I'd like to know how that varies with height above the ground (1-2m) and for locations within 10-20m around that location (in other words, how and when the shadow of the mountains moves off that area).

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  • $\begingroup$ You like to plan in advance :-) The following is not an answer, but can you watch and time the sunrise around Aug 1 on any of the next few years? From that, it would be relatively easy to correct for any day plus or minus a frw days for any year in the future. In other words, use the sunrise of July 28-Aug 4 to measure the horizon. $\endgroup$
    – JohnHoltz
    Sep 2, 2018 at 18:10
  • $\begingroup$ I started a "project" on this (github.com/barrycarter/bcapps/blob/master/DEM) but haven't actually done anything with it. Stellarium lets you create custom horizons, and there was some talk of using terrain/elevation maps to create an accurate horizon for any given location on Earth. Feel free to contact me directly (contact info in profile) if you want to work on this. $\endgroup$
    – user21
    Sep 2, 2018 at 19:14

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The PeakFinder app1 can do most of what you're looking for. It's available for Android and IOS as well as through a web browser. The app shows the paths across the sky for both the sun and moon2 and displays rise/set times taking into account the local topography and viewpoint elevation3. At least the Android version works without a network connection (nice for hiking). IIRC, it uses SRTM data4 for elevations so it's not super-accurate but does a good job over all.

I plugged in the location, elevation (ground + 1m), and date you gave and got sunrise at 05:09 local time, reaching the ridgeline at 08:37 local time. Here's a link to the page; screenshot is below.

1 I'm not affiliated with PeakFinder, just a satisfied user.

2 If using a web browser, click on the "sun" and "moon" icons on the page's toolbar.

3 It also shows what the surrounding terrain looks like and the names of visible peaks.

4 As does OpenStreetMap, IIRC.

Cropped screenshot of PeakFinder site Source: Cropped screenshot of PeakFinder site

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  • $\begingroup$ Do you know if the time is for first visibility of any part of the solar disk, or for the centre? Also is any adjustment made or needed for atmospheric refraction? $\endgroup$
    – Steve Linton
    Sep 3, 2018 at 6:46
  • $\begingroup$ That I couldn't say. To hazard a guess I'd say centre-point with no refraction compensation. I'd send an email to the app's author and see what they have to say. $\endgroup$
    – Alex Hajnal
    Sep 3, 2018 at 6:50
  • $\begingroup$ @SteveLinton If you get a response from the author could you post a comment here with their answer? I'm also interested in knowing what criteria they're using. $\endgroup$
    – Alex Hajnal
    Sep 3, 2018 at 7:12
  • $\begingroup$ @SteveLinton Given that the precision of the app is only to the minute and the sun moves across the sky at about one solar disc radius per minute, there usually isn't a measurable difference between centre and edge (with this app). In your case though the sun is closely following the ridgeline so it might matter. $\endgroup$
    – Alex Hajnal
    Sep 3, 2018 at 7:25
  • $\begingroup$ It seems to be centre-point, not edge. See e.g. peakfinder.org/… (note: This is not the location you specified) $\endgroup$
    – Alex Hajnal
    Sep 3, 2018 at 7:42

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