Sunrise and sunset set the limits of daylight, but a calculated time and the moment you see the Sun are not always the same. The result depends on the definition, local horizon and atmosphere. Airpult uses a geometric calculation for a level horizon and displays it in local time.
What counts as sunrise and sunset?
The U.S. Naval Observatory defines conventional sunrise as the moment when the upper edge of the Sun’s disc appears on a level, unobstructed horizon. Sunset is when that same edge disappears. The definition needs to mention the edge because the Sun is not a point: its disc is about half a degree wide, so several minutes can pass between the first and last contact with the horizon.
There is also a difference between a geometric and an apparent event. The atmosphere bends light near the horizon and makes the Sun appear higher than its geometric position. A conventional calculation places the Sun’s centre about 0.833° below the geometric horizon. Of that angle, about 16 arcminutes allow for the Sun’s apparent radius and about 34 arcminutes for average refraction. Those are standard assumptions, not a measurement of the atmosphere at that place and minute.
Airpult uses a different boundary. We calculate when the Sun’s centre crosses a geometric horizon of 0°. We do not adjust that result for the Sun’s radius, atmospheric refraction, observer elevation, buildings or terrain. With the same clear, level horizon, Airpult’s geometric sunrise is later and its geometric sunset earlier than conventional apparent times. The calculated day is therefore shorter.
Why the visible time can differ
Refraction changes with the air near the horizon. Temperature, pressure and unusual layers can shift the apparent Sun by more than a standard model predicts. NOAA notes that observed sunrise and sunset can differ from calculated values because atmospheric conditions vary. The uncertainty grows at high latitudes, where the Sun meets the horizon at a shallow angle.
Your horizon often makes a larger difference. A ridge or buildings can hide the Sun after the geometric event. From a higher viewpoint with a clear view over lower ground or sea, the visible horizon lies slightly below a horizontal plane, so sunrise may be seen earlier and sunset later. Height does not help if a nearby ridge blocks the Sun.
This matters in places with strong relief. On the Sion Airport forecast, for example, the daylight card provides a consistent time for the coordinates, but the surrounding Alpine horizon can change when the Sun itself becomes visible. For photography or an observation from a particular slope, check the bearing of the Sun and the terrain in that direction as well as the clock time.
Why day length changes through the year
Earth’s rotation axis is tilted by about 23.4° relative to its orbital plane. As Earth travels around the Sun, each hemisphere alternately leans towards and away from it. In summer the Sun takes a higher, longer route across the sky; in winter the route is lower and shorter. The seasons are caused by this geometry, not by Earth being nearer to the Sun during a particular hemisphere’s summer.
Latitude determines how large the change becomes. Near the equator, the geometric day stays close to 12 hours throughout the year. At middle latitudes, summer and winter day lengths separate markedly. Within the polar circles, there are dates on which the Sun never drops below the chosen horizon, producing polar day, and dates on which it never rises, producing polar night. The closer a place lies to a pole, the longer these periods can last.
The solstices mark the longest and shortest geometric days, but they do not normally bring both the earliest or latest sunrise and the corresponding sunset. Sunrise, solar noon and sunset move against civil clock time because apparent solar time is not perfectly uniform. The equation of time, which reflects Earth’s axial tilt and the changing speed along its slightly elliptical orbit, shifts those clock-time extremes to dates before and after the solstice. The amount of separation depends on latitude.
Civil, nautical and astronomical twilight
Daylight does not end when the Sun crosses the horizon. Sunlight scattered through the upper atmosphere continues to illuminate the sky. Astronomers divide this transition into three stages, using the geometric position of the centre of the Sun:
- Civil twilight runs between the horizon and 6° below it. There is often enough natural light for many outdoor activities, although cloud and the surroundings can make it feel much darker.
- Nautical twilight covers 6° to 12° below the horizon. Under good conditions, the horizon remains distinguishable while many stars are visible, which is the origin of the name.
- Astronomical twilight covers 12° to 18° below the horizon. Once the Sun is more than 18° below, its scattered light is below other natural sources in the night sky.
These angles define the stages; they do not promise a particular brightness. Thick cloud can make civil twilight gloomy, while snow cover, moonlight and artificial lighting can keep a place bright later. Twilight is brief where the Sun descends steeply and can persist for hours at high latitude. In summer, some places never reach nautical or astronomical darkness even though the Sun sets.
Airpult’s forecast data includes civil dawn and dusk at 6°. The daylight card shows sunrise, sunset and calculated day length, but not the nautical or astronomical boundaries.
Local time, date and daylight saving
Airpult calculates the events from the location’s coordinates, then formats each timestamp in that location’s named time zone. Where daylight saving time applies, the displayed offset is the one in force on the date of the event. A one-hour clock change therefore produces a one-hour jump in the listed time even though the Sun’s path changes smoothly from one day to the next.
The same instant can belong to different calendar dates in different time zones. Use the forecast for the place you will be rather than copying a time from a city in another zone. Near a time-zone boundary, nearby places can show different clock times for almost the same solar event.
How to use Airpult’s times
The displayed day length is the interval between Airpult’s calculated 0° sunrise and sunset. It will not exactly match a service that uses the conventional upper-edge and refraction definition. During polar day or polar night, no real crossing occurs. Airpult’s forecast pipeline may supply synthetic times near the boundaries of its forecast day so that day, night and duration calculations still work. Treat those clock values as technical placeholders, not observed sunrise or sunset.
For a walk, flight or outdoor job, the daylight card is a useful first check, but weather still decides how much light reaches the ground. Low cloud can obscure the Sun, while a clear gap at the horizon can reveal it beneath overcast. Check the hourly cloud forecast, allow for terrain, and never use a general forecast time as the sole basis for safety-critical navigation.
Use the explore page to find your destination. For the first visible ray from a precise viewpoint, compare Airpult’s geometric time with an astronomical service that states its sunrise definition, then account for the local horizon.
