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2934188296
* fix(suncalc): parenthesise the sunrise-equation denominator
`sunHourAngle` computed
cos(z) / cos(lat) * cos(dec) - tan(lat) * tan(dec)
but the sunrise equation is
cos(H) = cos(z) / (cos(lat) * cos(dec)) - tan(lat) * tan(dec)
In Go `a / b * c` is `(a / b) * c`, so `cos(dec)` multiplied where it should divide.
The new test asserts a property rather than a formula: feeding the returned hour
angle back through the definition of the zenith angle,
cos(z) = sin(lat)*sin(dec) + cos(lat)*cos(dec)*cos(H)
has to reproduce the zenith angle that was requested. It fails on the old expression
and passes on the corrected one, over a grid of latitudes from the equator to the
polar circle, declinations across a full year, and both zenith targets the callers
use (elevTwilight -6, elevDaylight 3).
Worst-case error before the fix, in clock time:
Singapore lat 1.35 285 s
Mexico City lat 19.43 312 s
Cairo lat 30.04 354 s
New York lat 40.71 439 s
Trieste lat 45.69 512 s
London lat 51.51 671 s
Oslo lat 59.91 654 s
Tromso lat 69.65 1443 s
The error grows with latitude and with |declination|, and is largest at the civil
twilight zenith — which is where the gamma ramp starts and ends.
It also moves the boundary at which Acos leaves its domain, i.e. where the code
decides midnight sun / polar night. Swept over 505062 (lat, dec, zenith) points, the
classification differs at 961 of them (0.190%), all beyond 88 degrees of latitude.
The existing suncalc tests keep passing: they assert sunrise and sunset to the hour,
which is two orders of magnitude coarser than this error.
* fix(suncalc): classify each solar threshold independently
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Co-authored-by: Mark2Mac <[email protected]>
Co-authored-by: bbedward <[email protected]>