Day length calculator
Compare the time the Sun is above the standard sunrise threshold at two latitudes on the same date. Results use a midnight-to-midnight UTC window and show each crossing in UTC.
On 2026-06-21 in the UTC midnight-to-midnight window, 51.5° has 16.64 daylight hours (sunrise and sunset in the selected utc window); -51.5° has 7.83 hours (sunrise and sunset in the selected utc window). Both use longitude 0° and the same standard sunrise threshold.
Read this calculation
Daylight = sum of intervals with solar altitude ≥ −0.833°
The threshold puts the Sun’s centre 0.833° below the geometric horizon, combining a conventional sea-level refraction allowance with the apparent solar radius. Terrain and the actual atmosphere can shift a visible crossing.
The two locations share a date and longitude so their seasonal contrast is easy to compare. A polar-day or polar-night result has no crossing in this UTC window; it is not a missing sunrise time.