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Mathematica has Sunrise[], but no Moonrise[]. But it has MoonPosition[], so I thought it would be trivial to calculate with something like

FindRoot[MoonPosition[DateObject[{2016, 9, 28, x}]][[2,1]], {x, 12}]

But the evaluation of DateObject fails in this context!

A Table works fine, but I cannot get it to evaluate the Dateobject in Solve, NSolve, FindRoot or similar.

Suggestions?

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  • $\begingroup$ It does not fail, it just complains and at the end I get {x -> 29.1625} $\endgroup$ – Kuba Sep 28 '16 at 8:33
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    $\begingroup$ Try with mp[x_?NumberQ] := MoonPosition[DateObject[{2016, 9, 28, x}]][[2, 1]] to not prompt any errors. $\endgroup$ – Kuba Sep 28 '16 at 8:34
  • $\begingroup$ Take a look here: How do I use ?NumericQ to affect order of evaluation? and User-defined functions, numerical approximation, and NumericQ which is likely a duplicate. $\endgroup$ – Kuba Sep 28 '16 at 8:36
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    $\begingroup$ Just an astronomical note: moon position varies up to a degree depending where on Earth you are, and you might want to consider refraction of 34 minutes at the horizon. If you're looking for super-accuracy (matching USNO's tables), it's possible but more difficult. $\endgroup$ – barrycarter Sep 28 '16 at 13:33
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f[x_?NumberQ] := MoonPosition[DateObject[{2016, 9, 28, x}]][[2, 1]]

FindRoot[f[x], {x, 12}]

{x -> 17.5595}

ListPlot[Table[{x, f[x]}, {x, 1, 24, 0.5}], Frame -> True, FrameLabel -> {"x", "f(x)"}]

enter image description here

So another root is at about 4:

FindRoot[f[x], {x, 4}]

{x -> 3.7208}

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  • $\begingroup$ Unfortunately it's extremely slow. Because MoonPosition itself is so slow that it isn't really usable for such tasks ... $\endgroup$ – Szabolcs Sep 28 '16 at 9:21
  • $\begingroup$ Thanks guys, also @Kuba! Curious, in Mathematica 10 I get {x -> 4.00933}, but I suppose this could be some rounding or insufficient resolution in MoonPosition[] (declination seem only given to 0.01 resolution) $\endgroup$ – HJensen Sep 28 '16 at 11:35
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    $\begingroup$ @HJensen MoonPosition[] uses \$GeoLocation and \$TimeZone to determine your location and time zone, so from your location, the Moon's position is different, and hence FindRoot[] finds a different time for Moon rise. $\endgroup$ – creidhne Sep 28 '16 at 14:23
  • $\begingroup$ @creidhne Good point! Anyway, the AngularDegree returned by MoonPosition[] seems to have a resolution of (only) 3-4 digits. $\endgroup$ – HJensen Sep 29 '16 at 16:27

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