Analysis
Winter solstice sunset from Beltany Stone Circle. A geometric result says nothing about intention.
| Event | Winter solstice sunset |
|---|---|
| Epoch | 2500 BC |
| Observer | 54.8508° N, 7.6047° W (WGS 84) |
| Horizon altitude (assumed) | 0.00° |
| Disc reference | Upper limb (first/last gleam) |
| Obliquity at epoch | 23.974° |
| Solar declination | -23.974° |
| Geometric altitude of disc centre | -0.841° |
| Azimuth (from true north) | 226.76° |
| Algorithm | btb-solar/1.0.0 |
Method
- Solstice declination taken as ± mean obliquity of the ecliptic at the epoch (Laskar 1986).
- Refraction at the horizon from Bennett (1982) at 1010 hPa, 10 °C.
- Event defined by the upper limb (solar semidiameter 0.2666°).
- Azimuth from spherical astronomy: cos A = (sin δ − sin φ sin h) / (cos φ cos h).
Limitations
- Horizon altitude is an input, not measured here. A flat 0° horizon is rarely true inland.
- Refraction near the horizon varies with weather by tenths of a degree or more.
- Solar parallax and nutation are ignored (each < 0.01°).
- A geometric match is not evidence of intention. Archaeological evidence is required for that.
Full result as JSON
{
"azimuthDeg": 226.76385380930267,
"azimuthRangeDeg": null,
"declinationDeg": -23.974226182587525,
"obliquityDeg": 23.974226182587525,
"geometricAltitudeDeg": -0.8412255623877094,
"comparison": null,
"input": {
"latitude": 54.8508,
"longitude": -7.6047,
"epochYear": -2500,
"event": "winter_solstice_sunset",
"assumptions": {
"horizonAltitudeDeg": 0,
"limb": "upper"
},
"horizonAltitudeUncertaintyDeg": 0,
"testOrientationDeg": null,
"testOrientationUncertaintyDeg": null
},
"assumptions": {
"horizonAltitudeDeg": 0,
"limb": "upper",
"pressureHpa": 1010,
"temperatureC": 10
},
"algorithmVersion": "btb-solar/1.0.0",
"method": [
"Solstice declination taken as ± mean obliquity of the ecliptic at the epoch (Laskar 1986).",
"Refraction at the horizon from Bennett (1982) at 1010 hPa, 10 °C.",
"Event defined by the upper limb (solar semidiameter 0.2666°).",
"Azimuth from spherical astronomy: cos A = (sin δ − sin φ sin h) / (cos φ cos h)."
],
"limitations": [
"Horizon altitude is an input, not measured here. A flat 0° horizon is rarely true inland.",
"Refraction near the horizon varies with weather by tenths of a degree or more.",
"Solar parallax and nutation are ignored (each < 0.01°).",
"A geometric match is not evidence of intention. Archaeological evidence is required for that."
],
"calculatedAt": "2026-10-11T20:07:05.101Z"
}