Method
Algorithm
Solar position is computed with the NOAA Solar Calculator formulation, which follows Meeus, Astronomical Algorithms, 2nd ed., ch. 25. Its stated accuracy is roughly 0.01° in declination and better than a minute of time in the equation of time, for 1800–2100.
Why not the simple formula
The sinusoidal approximation for declination is off by up to half a degree. In the tropics half a degree means a date that is days wrong — and the output still looks entirely plausible. That is why the engine was asserted against published values before a single pixel was drawn.
Why "zero" has a width
The sun’s disc is about half a degree across, so a real object’s shadow does not mathematically vanish; it shrinks to a minimum over a couple of minutes. This app reports a window, not an instant, and the window is derived from the sun’s apparent semi-diameter on the day rather than a fixed number.
A day is discrete; the crossing is continuous
The declination equals your latitude at some moment, which may fall at 03:00 local time. So the zero shadow day is found by minimising the culmination shadow across candidate days, rather than rounding a crossing time to a date.
What is neglected
Atmospheric refraction (under 0.01° for a sun above 45°, and every figure here is taken near culmination in the tropics), observer elevation, and ΔT of about 70 seconds. No timezone database: Indonesia’s zones are the 105°, 120° and 135°E meridians, and the country observes no daylight saving.
Location
Coordinates are resolved and used entirely in the browser. No network request after first load, no reverse geocoding, no analytics on coordinates.
The authoritative source
BMKG publishes official zero shadow day announcements for Indonesian cities each year, and is the authoritative reference.
Verification
- declination @ equinox · |δ| < 0.01°
- declination @ solstice · |δ| = ε ± 0.001°
- equation of time · 4 extrema, 4 zero crossings
- BMKG · 38 ibukota provinsi × 2 tabel · tanggal ± 1 hari
- BMKG · waktu kulminasi · < 5 detik
- tropic edge · converged, single date
- outside tropics · structured no-result, both hemispheres
- Eratosthenes · round trip recovers the generating radius
A personal educational project. Not an official product of any institution.