Two spans, two divisions
Modern civil hours are normally sixty minutes. Traditional unequal hours use the Sun as the boundary: measure the time from sunrise to sunset and divide it by twelve, then measure sunset to the following sunrise and divide that by twelve. A day with fourteen hours of light yields planetary day hours of seventy minutes; its ten-hour night yields planetary night hours of fifty minutes. This arithmetic example ignores the exact minute of solar crossing, but it shows why a generic table headed “7–8 a.m.” is only a rough mnemonic.
Sunrise and sunset are not assigned to universal clock times. Their UTC instants depend on date, longitude, latitude, and the horizon convention. The displayed civil times then depend on a time zone. Astronomy Engine’s rise/set search includes the apparent solar disc and a standard refraction model. A local hill, building, sea horizon, or unusual atmospheric conditions can change observed first and last light. The calculator uses zero metres elevation and an unobstructed modeled horizon; it does not infer your street-level landscape.
Why noon is not the boundary
Some modern tables start their sequence at midnight or at a fixed 06:00. The sunrise-based rule in the Key of Solomon begins the named day’s first hour at dawn, even though the civil date began hours earlier. Its night half continues after civil midnight until the next sunrise. For a Tuesday night, an interval at 02:00 Wednesday still belongs to the planetary cycle that began at Tuesday sunrise. The next day’s ruler takes over at Wednesday’s sunrise, not at midnight.
Where the method breaks down
Near the polar circles, the Sun may stay above or below the horizon for an entire local date. Without a sunrise–sunset–next-sunrise sequence, the ordinary unequal-hour construction cannot provide twelve daylight and twelve night intervals. Different modern practitioners may substitute a nearby date, twilight, or an equal-hour scheme, but those are separate conventions. This calculator withholds a table rather than inventing a solar boundary. A fixed UTC offset is another limit: if daylight saving changes between the first and next sunrise, one offset cannot label both sides of the transition accurately.